Download system description

SYSTEM DESCRIPTION
SIOX BUS HARDWARE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Design Criteria . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Physical Bus Signals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Cable Selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Communication Speed vs Transfer Quality . . . . . . . . . . . . . . . . . . .
Bus Lengths over 1 000 m . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Modems and Half Duplex Echo . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Typical Module Dimensions and Connectors . . . . . . . . . . . . . . . . . .
2
2
4
5
6
7
7
8
SIOX - A COMMUNICATION PROTOCOL . . . . . . . . . . . . . . . . . . . . . . . . 9
Basics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Master - Slave . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Characters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Message Types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Data Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
String Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
String Address . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Global Address . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
String Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
String End . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Inhibit Answer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
String Answer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Slave is Busy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
Group Address Expansion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
Meta Characters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
Repeated Characters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
Setup Messages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
Enquiry Help Texts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
Selective Type Access . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
Bit Modify . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
Sequential Addressing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
Message Types Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
FAULT TRACING IN A SIOX SYSTEM . . . . . . . . . . . . . . . . . . . . . . . . . .
Checking the Supply Voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
LED Diagnostic System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Bus Listeners . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Oscilloscope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
TELEFRANG AB
S:\Allmänt\Man\SIOXsys4_uk.wpd/2005-05-11\TF
23
23
23
25
25
26
SIOX SYSTEM DESCRIPTION p 2
SIOX BUS HARDWARE
SIOX - Serial I/O Xchange - is the name of a system of remote I/O modules
that communicate through the standard serial port of a computer. Up to 63
modules may be connected in parallel to a two-wire communication line, the
SIOX bus.
SIOX modules may be regarded as a range of I/O cards for the computer,
including analog and digital inputs and outputs, operator interface terminals,
card and bar code readers, serial and parallel ports etc. The difference
between SIOX modules and traditional I/O cards is that the backplane bus of
the computer with its 50-100 strips carrying multiplexed I/O information has
been replaced by a two-wire bus. This may, in a typical application, be up to
several kilometres long.
Design Criteria
The SIOX bus is principally intended for use in industrial environments,
requiring high noise immunity, ease of installation and robustness. The
following features should be noted in this connection:
*
The two-wire bus can, via a simple adapter, be connected to the RS232
serial port of any computer capable of transmitting and receiving
asynchronous characters at moderate speed.
*
SIOX I/O modules are typically opto-isolated from the bus, permitting each
module to be referred to a local ground. Other communication systems
without galvanic isolation are very susceptible to electrical noise caused by
differential voltages between various grounding points.
*
Any type of cable may be used for the SIOX bus, including telephone and
intercom wires, signal cables, twisted pairs, screened communication lines
and power cables. The few limitations relating to maximum wiring
resistance and length are explained on page 6-7.
*
The SIOX bus needs no terminators at the ends of the cable and may be
branched off at will. This is due to the moderate communication speed
selected, typically permitting a total wire length of 1 000 m including stubs.
*
The bus supply voltage may vary between 15 V and 35 V to match typical
power supplies already available. Equally flexible, the power supplied to the
SIOX modules may generally vary between 12 V and 35 V DC and in many
cases also 12 V and 25 V AC. Certain modules even run directly off the bus
voltage and therefore need only two connecting wires.
SIOX SYSTEM DESCRIPTION p 3
*
The bus connection of the modules is tolerant to installation errors, since
it may be short-circuited or connected to e.g. 24 V DC or AC without any
damage.
*
The two wires are interchangeable, further simplifying installation and later
modifications.
*
Thanks to the relatively high signal levels and the fact that the bus wires are
balanced, the bus can be run near high tension voltage cables. Also, the
low-pass filter characteristic of each bus receiver eliminates externally
generated spikes.
SIOX SYSTEM DESCRIPTION p 4
Physical Bus Signals
Essentially, the SIOX bus is a current loop similar to an ordinary point-to-point
20 mA current loop. In such a loop, a "1" is indicated by current flowing into the
receiver and a "0" by no current being available. The SIOX bus current,
normally generated by the computer adapter, may be anything up to 200 mA
depending on the number of modules. Each module's receiver section
consumes 1 - 1,5 mA regardless of variations in the supply voltage. Hence,
with 62 modules connected to the bus, consumption is approximately 100 mA,
which is the standard bus generator current.
When the computer or a SIOX module transmits a "0", the bus is effectively
short circuited, inhibiting the current flow in all optocoupler receivers. For the
duration of each such "0" bit, the bus will carry the maximum generator current
(100 mA).
Note that such modules that are supplied directly off the bus draw all their
current while the bus is transferring a "1", storing current in a reservoir capacitor for "0" transfers. The average supply current for such modules must
therefore be increased by a factor of 3 when calculating the total bus current
drawn by all modules.
SIOX SYSTEM DESCRIPTION p 5
Cable Selection
As mentioned above, the current flowing in the bus is typically 100 mA. The
wire resistance must therefore be low enough to carry this current without any
excessive voltage drop. With a 15 V power supply, the permissible voltage drop
from the central current generator to any module is 2 x 2 V, increasing with the
supply voltage. At the maximum bus length of 1,000 m, a wire area of 1 mm2
= AWG 17 is required, decreasing to 0,2 mm2 = AWG 24 at the maximum bus
voltage of 35 V.
In some cases, twisted and shielded wire may be used. Using twisted wires
reduces possible magnetic interference from nearby high-current cables,
motors etc. Shielding reduces capacitively coupled noise from high-voltage
cables running parallel with the SIOX bus, but may for long lines and uncertain
grounding create new problems.
SIOX SYSTEM DESCRIPTION p 6
Communication Speed vs Transfer Quality
Maximum communication speed for a SIOX bus is 19 200 bits/s, which permits
software handling of the communication protocol. A SIOX module therefore
needs no power-hungry high-speed preprocessing IC's and communication
front-ends or bus interfaces with separate, isolated power supplies. The result
is increased life expectancy and lower cost.
The normal communication speed used with SIOX bus is 4800 bits/second,
corresponding to a bit time of 208 µs. As already mentioned, this moderate
speed has definite advantages with respect to noise immunity and ease of
cabling. In fact, the opto-isolated design yields a rise and fall time of 20 µs,
limiting the maximum practical communication speed, but at the same time effectively cancelling high-frequency noise:
These rise and fall times also match behaviour of a 1 000 m unterminated
communication line.
A typical wire pair will exhibit a characteristic impedance of 170 with a wire wire capacitance of 180 pF/m. Termination with 170 at each bus end, and
possibly at the end of each branching stub, is impossible due to the excessive
driver currents required. Keeping the communication speed low permits, to a
degree, rise and fall times of 20 µs and ringing caused by mismatch.
Each SIOX module connected to the bus also contributes to the damping of
electrical interference and ringing. These effects can best be studied on a 1000
m bus with one single module placed at the remote end. In this extreme case,
an extra termination resistor in the 500 - 1000 range across the end of the
bus has proved effective in minimizing the number of communication misses
and hence the number of time-consuming retransmissions.
SIOX SYSTEM DESCRIPTION p 7
Bus Lengths over 1 000 m
Communication speeds of lower than 4800 bits/s may be necessary if the bus
length is increased above 1 000 m. The general rule is 2 000 m at 2400 bits/s,
4 000 m at 1200 bits/s and so on. However, the limitation caused by the bus
resistance will quickly become a problem and must be overcome either by
using heavier wires or by reducing the maximum current from the generator.
Note that lower currents increase the rise time in the remote end of the bus,
necessitating a further decrease in the communication speed. Hence, a 4 000
m bus must in most cases be run at 300 bits/s, using a 50 mA current
generator.
Shorter buses than 1 000 m or low capacitance cable permit modules to
communicate at up to 19 200 bits/second.
Modems and Half Duplex Echo
In general, the SIOX bus may be extended using a modem, either short-haul
or full dial-up. However, the half-duplex nature of the SIOX bus, where
information is transferred alternately in both directions, must be borne in mind.
If a modem pair is used to interface between two SIOX buses, each with its
own current generator, a "0" (short-circuit) on either bus is transferred to the
other and then back again, causing both buses to lock up. This situation can
be resolved by using an echo cancellation technique or intelligent bus
extenders that automatically switch repeating direction. Such repeaters may
also be used as routers to expand a serial port to address more that 63
modules through selective repetition:
SIOX SYSTEM DESCRIPTION p 8
Typical Module Dimensions and Connectors
SIOX modules come in many sizes to suit various installation needs, but one
standard housing is suitable for installation in electrical cabinets, using the clip
on the bottom to hook onto a 35 mm DIN 50022 mounting rail. It has the
dimensions given in the figure below, and conforms with the IP 41
encapsulation class. Alternatively, it can be mounted using the four corner
holes.
The screw connectors used are plug-in Combicon MSTB 2,5 mm2 from Phoenix
Contact, to which several second sources exist.
SIOX SYSTEM DESCRIPTION p 9
SIOX - A COMMUNICATION PROTOCOL
SIOX is the name of a system for communication that is primarily intended for
use in industrial environments. Continued automatisation increases the need
for communication with analog and digital sensors and controls, the transmission of data between distributed computers and a better information
man - machine.
In the SIOX system information is transferred between different units through
a standardised link, the SIOX bus, consisting of a two-wire cable. To this bus
a broad spectrum of cost-effective hardware modules can be connected with
a minimum of work effort.
The following description of the communication principles is intended for
system designers and service staff.
The SIOX system has been optimised to provide ease of installation, flexibility
and expansion opportunities, and to ensure high noise immunity and low cost.
Efficient message transfers decrease the need for high communication speeds,
in turn leading to improved noise immunity and reduction in the cost of
computers and modules to be connected to the SIOX bus.
Basics
All messages are transmitted asynchronously, one character at a time, at a
speed of between 300 and 19200 bits/s. This permits nearly all computers to
communicate via a SIOX bus through a simple adapter connected to their serial
ports.
An asynchronous character always consists of 1 start bit, 7 or 8 data bits
containing information, 1 optional parity bit and 1 or 2 stop bits.
SIOX uses 8 data bits, 1 even parity bit and 1 stop bit.
The start bit always has the value 0 to synchronize the character between
transmitter and receiver.
SIOX SYSTEM DESCRIPTION p 10
The eight bits of information can each be a 1 or 0, together forming one of 256
different characters. The position in the character decides the weight of the bit;
the first called D0 has a weight of 1, the second called D1 has a weight of 2,
the third D2 a weight of 4 and so on until the last bit, D7 whose weight is 128.
Each hexadecimal figure /
0 - 9 or A-F consists of a combination of 4 bits of information as follows:
D3
0
0
0
0
0
0
0
0
1
1
1
1
1
1
1
1
D2 D1 D0
0 0 0
0 0 1
0 1 0
0 1 1
1 0 0
1 0 1
1 1 0
1 1 1
0 0 0
0 0 1
0 1 0
0 1 1
1 0 0
1 0 1
1 1 0
1 1 1
represent:
=
0+ 0+ 0+
=
0+ 0+ 0+
=
0+ 0+ 2+
=
0+ 0+ 2+
=
0+ 4+ 0+
=
0+ 4+ 0+
=
0+ 4+ 2+
=
0+ 4+ 2+
=
8+ 0+ 0+
=
8+ 0+ 0+
=
8+ 0+ 2+
=
8+ 0+ 2+
=
8+ 4+ 0+
=
8+ 4+ 0+
=
8+ 4+ 2+
=
8+ 4+ 2+
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
Hexadecimal
0
1
2
3
4
5
6
7
8
9
A
B
C
D
E
F
Decimal
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
D7
0
0
0
0
0
0
0
0
1
1
1
1
1
1
1
1
D6 D5 D4
0 0 0
0 0 1
0 1 0
0 1 1
1 0 0
1 0 1
1 1 0
1 1 1
0 0 0
0 0 1
0 1 0
0 1 1
1 0 0
1 0 1
1 1 0
1 1 1
represent:
=
0+ 0+ 0+ 0
=
0+ 0+ 0+16
=
0+ 0+32+ 0
=
0+ 0+32+16
=
0+64+ 0+ 0
=
0+64+ 0+16
=
0+64+32+ 0
=
0+64+32+16
= 128+ 0+ 0+ 0
= 128+ 0+ 0+16
= 128+ 0+32+ 0
= 128+ 0+32+16
= 128+64+ 0+ 0
= 128+64+ 0+16
= 128+64+32+ 0
= 128+64+32+16
Hexadecimal
00
10
20
30
40
50
60
70
80
90
A0
B0
C0
D0
E0
F0
Decimal
0
16
32
48
64
80
96
112
128
144
160
176
192
208
224
240
The parity bit checks that the other bits in the character have been transferred
correctly. The number of bits set at 1, of the eight information bits and the parity
bit, must be an even number.
The stop bit is always a 1, serving as the shortest pause before the start bit of
the next character.
SIOX SYSTEM DESCRIPTION p 11
Master - Slave
In every SIOX bus one of the units connected must be a master or "central"
and any number of units up to 63 are slaves or station modules. Each
exchange of information is initiated by the master transmitting a message,
comprising a number of characters, via the SIOX bus. One of the slaves
recognizes the sequence of characters as a message to itself and sends a
response back to the master. All other slaves must remain silent. This
exchange of information constitutes one complete communication. If a
message should be disrupted by electrical noise so that the slave cannot
recognize it, no reply is sent. Alternatively, the master may find a fault in the
reply from the slave. In both cases the communication is repeated.
After a successful communication the master can communicate with another
slave and so on. Slaves cannot send messages to each other without the help
of the master.
Slaves are singled out through a unique address between 1 - 63 assigned to
them at installation time. In large systems it is possible to group up to 63
stations by means of a Router to coexist with other such groups. A single serial
port can thereby access nearly 4000 stations. See also page 7.
Characters
Of the 256 different characters that can be sent, the first 128 (hex 0/0/ - 7F) are
data characters. The last information bit (D7) of each such character is
cleared to 0.
Data characters have different meanings depending on the type of message
and type of station receiving or returning a message. For a station with digital
I/O (Inputs and Outputs) each one of the bits D0 - D6 may represent one active
or passive I/O. For measurement stations, D0 - D6 may represent an analog
value, and for stations with a text display they may represent one of 128 letters
or figures and so on.
The remaining 128 characters (hex 80/ - FF) with D7 set at 1 are special
characters. No messages containing two special characters in succession are
valid.
SIOX SYSTEM DESCRIPTION p 12
Message Types
In order to optimally transfer the various types of information, the current SIOX
II specification defines two basic types of message; data mode and string
mode. A complete two-way communication in data mode includes 4 or 5
characters, while a communication in string mode can vary between 6 and
(theoretically) 326 characters.
The master (central computer) decides which type of message to use at each
new communication. Most slaves (stations) can receive messages in both data
mode and string mode. The answer mode from the station must be identical to
the mode sent by the central. It is not permitted for a station e.g. to respond in
data mode to a message in string mode.
Data Mode
Because of its compact form, data mode is used mainly for transfer of digital
or analog I/O signals.
The central first transmits an address character (hex C1 - FE) to alert the
station concerned. Since many SIOX buses have fewer than 63 stations connected, certain modules may accept several address characters. The program
overhead in the central is minimised if, for example, four analog channels in a
SIOX module can be directly identified using four different addresses. The
central may treat the module as four separate slaves with unrelated data.
The data characters (0/0/ - 7F) following the address are then received by the
station. Either 1 or 2 such characters are accepted, depending on station type,
which must be known by the central beforehand.
Once the correct number of characters has been duly received, the station
may, for example, let the 7 or 14 information bits (excluding the D7's = 0)
control digital outputs, convert the bits to an analog signal or show a character
on a display.
As an answer the station immediately transmits two data characters, via the
SIOX bus, containing 14 information bits with, for the station, specific meaning.
These characters are received by the central for further processing.
SIOX SYSTEM DESCRIPTION p 13
The reply from the station is checked by the central for any parity or format
errors, in which case the communication is repeated. This is also the case if the
station does not answer at all within a timeout period due to the station being
disconnected or the central's message being disturbed. Typically, the central
makes three attempts to communicate with the module before it gives up and
starts with the next station.
Hence a complete data mode communication consists of 1 address character
and either 1 or 2 data characters from the central with 2 data characters in
reply from the station. At the normal speed of 4800 bits/s a communication of
this type takes 10 - 13 ms.
Typical communication sequence between a central and three modules (data mode):
Call
Reply
C1
Address of slave 1 (a digital I/O module).
0/3
Description
Data which activates two of seven possible outputs in the slave.
0/0/ Data from the module showing that seven inputs are passive.
0/8 Data from the module showing that one out of seven further inputs is active.
C3
Address of slave 3 (here part of an analog module with four inputs and four outputs).
0/0/??
Electrical noise on the wire causing slave 3 to reject the message from the central.
C3
The central retransmits the message after a timeout.
0/0/
Seven data bits of a D/A value.
0/0/
Another seven bits of D/A value 0/0/0/0/ which sets the analogue output of the module to
zero. Both the central and the analogue slave module are defined for data mode
communication using 2 data characters to the slave.
0/0/ Slave 3 answers with seven bits of an A/D input value.
0/0/ Last seven bits show that the analogue input was set to 0 V.
C4
Address of slave 4 (same hardware module as slave 3).
40/
The least significant seven bits of the value 0/140/ = 0000001.
0/2
Remaining seven bits of the value 0/140/ = 0010000.
7F Slave 4 answers with seven bits of A/D value 0/1FF.
0/3 The most significant part of 0/1FF.
SIOX SYSTEM DESCRIPTION p 14
String Mode
Compared with the compactness of the data mode message, a string mode
communication offers more flexibility through its variable message length. The
inclusion of a checksum for the message in addition to the parity check offers
vastly improved reliability in the transmission. Up to 80 data characters, each
with eight data bits, can be transmitted in each direction.
Two types of string mode messages are possible, namely setup and text.
Setup strings are used for reading or modifying parameters and other functions
in a station. Text strings may contain text characters to be shown on a display
or a printer, and return keyboard codes or strings of numbers from magnetic
or bar code readers. Text strings may also represent digital or analog bit
patterns or contain program sequences in binary code.
String Address
All string mode communications start with the character C0/ from the central.
By analogy with data mode this would represent an address of an invalid slave
number 0. To identify one individual slave, the C0/ is directly followed by a data
character (0/0/ - 7F). The data characters 0/1 - 3F or 41 - 7F serve here as
equivalents to the data mode addresses C1 - FF for the 63 slaves available.
The difference between e.g. 0/1 and 41, both representing an address to slave
number 1, at the same time defines whether the communication is meant to be
a setup (0/1 - 3F) or text string (41 -7F).
Global Address
The slave number 0 is not allowed. Instead, when the C0/ is followed by an
address character 0/0/ or 40/, this indicates a call to all modules irrespective of
their address 1-63. Data or text can consequently be sent to all modules,
although the overlapping answers will probably be unreadable. A typical station
also terminates its answer as soon as it identifies that the bus is interfered with.
Another use is for the central to quickly find out if a bus is inhabited. Older
modules cannot identify global addresses and will not answer.
String Data
Following the two-character string address, up to 80 data characters (0/0/ - 7F)
can be sent. More than 80 characters will not be accepted by the selected
station.
SIOX SYSTEM DESCRIPTION p 15
String End
Since the number of characters cannot be defined in advance, a special
character must end the string. A reserved character (BC - BF) is used for this
purpose. The ending character is, in turn, followed by an extra character (0/0/ 7F), containing the checksum for the whole message (excluding the start
character C0/). All characters including the end character (BC - BF) are added
and the sum is complemented (1-complement, i. e. every bit is changed from
0 to 1 and vice versa). The eighth bit must, however, always be cleared to 0 so
that the checksum cannot be mistaken for a data mode address or other
special character.
Inhibit Answer
This string end character can be modified by the central into B8-BB by clearing
bit 2. This indicates to the slaves that they shall inhibit their answer, but still
carry out the order in the message, typically a global call.
String Answer
When the station has received and accepted the whole string from the central,
it answers with a similar string, but where the address part has been left out.
The answer is terminated by the same end character (BC-BF) as in the call,
followed by the checksum for this message.
Example text string from central:
C0/ 41
Station address
54
T
45
E
53
S
54
T
BE
End
40/
Checksum
Example string answer from station:
4F
O
4B
K
BE
End
27
Checksum
Note that the central or the station can send "empty" messages (without data
characters). For example if the central transmits:
C0/ 41
Station address
BC
End
0/2
Checksum
this is only a request for a reply from station 1. If the station does not have a
message to present, it will answer with:
BC
End
43
Checksum
As already mentioned, the central can transmit one of eight different end
SIOX SYSTEM DESCRIPTION p 16
characters. BC (and BD, B8 and B9) means unnumbered calls, suitable for
most messages when a retransmission of the same message does not lead to
any ambiguity. A different case is when transferring e.g. a text, where sending
the message twice would cause the text to double on the display. If the central
sends BE and BF in alternate transmissions, the messages are numbered.
Should the station receive two consecutive numbered messages with the same
end character, and has already accepted the first message, the second one will
be discarded and the first response repeated.
Slave is Busy
The station must reply using the same end character (BC - BF) as transmitted
by the central. The station can also inform the central that it is busy, or that
there is no space for further messages. It does so by clearing bit 2 in its end
character, replacing BC - BF with B8 - BB. The central can then either wait a
while or transmit empty messages until the slave once again answers with BC
- BF. Note that the central also uses the end characters B8-BB to request
inhibition of the answer, but in that case the slave is not supposed to answer.
Group Address Expansion
Router modules like the R30 (page 7) permit expanding the number of modules
to be addressed in a bus by interpreting its received string address (C0/) 41 (C0/) 7F as a group address. The next consecutive character received, 0/1-3F
or 41-7F, is then the real module’s address in string mode for one of up to 63
modules connected to the R30's secondary side. R30 retransmits the whole
message except the character after C0/ (its own address), changing the
checksum to reflect the missing character. Any answer from the secondary bus
is copied, character by character, to the calling master side. The total delay will
be one character in each direction, which must be accounted for. Note that a
router cannot simultaneously accept text strings due to the address ambiguity.
Some modules include options to handle both the group and station address
without an intermediary router module. This increases the number of stations
on a single bus, but care must be taken not to consume too much bus current.
A group address = 40/ is accepted as a global group call. Each router
retransmits the rest of the message on its secondary bus. Hereby, data can
simultaneously be sent to a station with a specific address in each group.
Meta Characters
Since only data characters (0/0/ - 7F) may form a string message, whole bytes
or eight-bit ASCII text characters (80/ - FF) cannot be included in the message.
Such "meta characters" are therefore sent as two characters, first the special
character B0/ and then the desired character with the eighth, the most significant, bit cleared to 0. A string of 80 bytes can therefore theoretically be
SIOX SYSTEM DESCRIPTION p 17
formed by 164 transmitted characters.
If e.g. 4 bytes, 0/0/ 40/ 80/ C0/, shall be sent from the central to the station, the
whole string will look like this:
C0/ 41
0/0/
Address
40/
1st
B0/ 0/0/
2nd
B0/ 40/
-- 3rd --
BC 22
-- 4th -End Check
Repeated Characters
In transferring e.g. text, many equal characters may appear in succession.
Such characters can automatically be compacted to save communication time.
The transmitting central or station counts the number of equal characters in
succession, adds 80/ and sends a special character (82 - AF), followed by the
repeated character. The text:
N
a
m e
:
_
_
_
_
_
_
_
_
_
_
_
_
_
_
_
4E 61 6D 6E 2A 5F 5F 5F 5F 5F 5F 5F 5F 5F 5F 5F 5F 5F 5F 5F
may at the transmission be reduced to:
4E 61 6D 6E 2A 8F 5F
In this way up to 47 characters can be compacted down to only two. The meta
characters (80/ - FF), which already have received special treatment, cannot be
compacted.
The receiver expands the double character at arrival, recreating the original
text.
Setup Messages
String Setup messages include several particulars to let it handle data in the
module. Many SIOX modules contain a non-volatile EEPROM memory and/or
a RAM area used for storing setup parameters, station addresses and working
modes. Each parameter contains 16 data bits and may typically contain 16
digital information bits or a 16-bit integer. At power-up, many of the parameters
are copied from EEPROM to RAM to create default values that are active until
they are modified by communications.
As mentioned earlier, a range of string mode addresses, C0/ 0/1 - C0/ 3F for
station numbers 1 - 63, are set aside for setup messages to read and modify
such parameters. In this way, they cannot be mistaken for ordinary text
information using string addresses C0/ 41 - C0/ 7F.
Setup strings normally contain 2, 4, 6 or 8 data characters in addition to the
address, end and checksum characters. Each data character is used to
transfer 4 bits of information to/from the station, using the ASCII characters 0 9 and A - F (hex 30/ - 39 and 41 - 46).
SIOX SYSTEM DESCRIPTION p 18
The first two or four data characters transmitted from the central to a station
form a combined command/parameter number. To identify the length, the
module looks at the whole message length. Four or eight characters define the
first four as a number. Any other length defines the shorter, two-character
number as follows:
Hex
30/ 30/ - 33 46 form a command/number:
0/0/ - 3F
Read one of the 64 first RAM parameters.
Hex
34 30/ - 37 46 form a command/number:
40/ - 7F
Read one of the first 64 EEPROM parameters.
Hex
38 30/ - 42 46 form a command/number:
80/ - BF
Write one of the first 64 RAM parameters.
Hex
43 30/ - 46 46 form a command/number:
C0/ - FF
Write one of the first 64 RAM parameters
and the corresponding EEPROM parameter.
A four-character number defines:
Hex
30/ 30/ 30/ 30/ - 33 46 46 46 form a command/number:
0/0/0/0/
- 3FFF Read one of 16384 RAM parameters.
Hex
34 30/ 30/ 30/ - 37 46 46 46 form a command/number:
40/0/0/
- 7FFF Read one of first 64 EEPROM parameters.
Hex
38 30/ 30/ 30/ - 42 46 46 46 form a command/number:
80/0/0/
- BFFF Write one of 16 384 64 RAM parameters.
Hex
43 30/ 30/ 30/ - 46 46 46 46 form a command/number:
C0/0/0/
- FFFF Write one of 16 384 RAM parameters
and the corresponding EEPROM parameter.
As can be seen, the first 64 parameters can be accessed either way. The two
bits defining Read/Write and RAM/EEPROM are always contained in the first
character.
After the two/four characters may follow four more characters, 30/ 30/ 30/ 30/ - 46
46 46 46 = 0/0/0/0/ - FFFF, which constitute the 16-bit data in write commands.
A Read discards them, while a Write with missing data is terminated. Following
example shows a write to EEPROM address 63 (hex 0/0/3F) in compact and
expanded form:
46 46 31 32 33 34
F F 1 2 3 4
or
43 30/ 33 46 31 32 33 34
C 0/ 3 F 1 2 3 4
The string is terminated by an end character, B8-BF, and a checksum
character.
SIOX SYSTEM DESCRIPTION p 19
The answer from the station always contains the 16 bit parameter data
expressed as 4 data characters (same as the write data from the central). As
usual, the string is terminated by an end character and a checksum.
Example:
1) Writing 0/70/1 to the first RAM+EEPROM parameter in station 1:
From central:
C0/ 0/1 43 30/ 30/ 37 30/ 31 BE 0/5
C 0/ 0/ 7 0/ 1
From station:
30/ 37 30/ 31 BE
0/ 7 0/ 1
79
2) Reading EEPROM parameter number 64 (the 65th, hex 40/) in station 1:
From central:
C0/ 0/1 34 30/ 34 30/ BF 77
4 0/ 4 0/
From station:
43 31 30/ 30/ BF 6C
C 1 0/ 0/
Enquiry Help Texts
Setup strings can, in most SIOX modules, access help texts stored in the
module. Two types of help information are available,
1) A Type Enquiry, to which the module answers with a text string describing
the module type, software version and special option numbers selected.
2) Parameter Help texts describe, for each parameter, the function of the
particular parameter, simplifying setup of the module.
A Type Enquiry consists of a setup string containing only one data character
from the central:
C0/ 0/1 0/5 BC 3D
ENQ
A typical station answer can be:
53 34 35 20/ 56 65 72 20/ 30/ 2E 30/ 35 3A 30/ 30/ 30/ BC 0/D
S 4 5
V e r
0 . 0 5 : 0 0 0
A Parameter Help enquiry is similar to the Type Enquiry string containing the
data character hex 0/5, but with the addition of a two/four-character parameter
number. Since the answer string from the station may contain a maximum of
80 data characters and some parameters may need more explanatory
characters, a secondary answer string can be requested by setting the 2nd
highest bit in the parameter address. This bit normally defines selection
between RAM and EEPROM.
SIOX SYSTEM DESCRIPTION p 20
Example: First Help enquiry for parameter number 8:
From central:
C0/ 0/1 0/5 30/ 38 BE 53
ENQ 0/ 8
Answer from station:
41 6E 61 6C 6F 67 20/ 43 68 61 6E 6E 65 6C 20/ 31 BE 45
C h a n n e l
1
A n a l o g
From central:
C0/ 0/1 0/5 34 38 BE 4F
ENQ 4 8
Answer from station:
53 63 61 6C 69 6E 67 BF 7F
S c a l i n g
Selective Type Access
A setup string may include the letter “T” (hex 54) anywhere in the call. Most
modules can collect the following three letters and compare them with its own
Type definition. If a full match is found, the module accepts the call, else not.
Older modules scrap the whole call. This is useful to select a specific module
on the bus if two or more stations are installed with the same address.
Together with a Global call it can modify a specific parameter exclusively in one
type of station.
From central:
C0/ 0/0/ 38 32 30/ 30/ 30/ 31 54 53 34 35 B8 0/C
Global Write0/2 = 0/0/0/1 Type S45 No answer
SIOX SYSTEM DESCRIPTION p 21
Bit Modify
Some parameters include functional bits that may change independently of one
another. For that purpose, e.g. the parameter(s) controlling digital outputs
typically include masking functions to protect local bits when a write parameter
is carried out. Still, it may be necessary to send a setup command to modify
only one bit. This is identified by including in the setup string a letter”S” or “R”,
followed by a letter 0 - F (hex 30/ - 46 ). This will set or reset the bit, appointed
by the second letter, in the parameter defined by the usual two or four
characters. Any 16-bit write data is scrapped, and a read command is
disregarded. The answer will contain the usual 16-bit data value of the modified
parameter.
Sequential Addressing
The address in several installed modules can be modified without accessing
each module individually if the modules have an addressing button or a magnetically accessible reed switch. The central sends repeated setup messages
which include the letter”Q” and an address character from 0/1 to 3F. No module
may answer, even if the Group, Station Address and Type are correct, unless
the operator presses the button on one module. This module will then send an
empty answer and save the received address character in its EEPROM.
The central can now choose to send another addressing command, typically
with the next sequential address, and the operator presses the button on next
module. The EEPROM address is only valid if no jumper address is set in the
modules.
Example:
From central:
C0/ 0/0/ 54 53 34 35 51 0/2 BC 60/
Global Type S45 Addr.02
Answer from module with button pressed and new address 0/2:
BC 43
Message Types Summary
The SIOX protocol uses asynchronous characters (even parity, one stop) with
hex 80/ - FF as addresses/special flags and hex 0/0/ -7F as data. Each "address"
must be followed by at least one "data", or both "addresses" will be considered
illegal by the receiver.
SIOX SYSTEM DESCRIPTION p 22
Function
Addr
Data
80/-81
Description
Reserved for future use.
RptCount
82-AF
0/0/-7F Multiple quantity 2 - 2F of the data character
(string mode).
Meta
B0/
0/0/-7F Convert data 0/0/-7F into 80/-FF (string mode).
B1-B7
Sign-Off
Reserved for future use.
B8-BF 0/0/-7F End of string mode message. The data character
contains the complemented checksum (7 bits) for
the whole string excluding the C0/ start character.
Address bit 2 = 0: No answer required / Not
ready for more data (station answer only).
Address bit 1 = 0: Unnumbered message.
Address bit 0 = 0/1: 1st/2nd message.
String
Start
C0/
0/1-3F String mode setup message header from central
to station number 1 - 63.
String
Start
C0/
41-7F String mode data message header from central
to station number 1 - 63.
Data
Message
C1-FF 0/0/-7F Data mode message from central to station number 1 - 63. An extra data character is optional.
--
0/0/-7F Data mode answer (2 characters) from
0/0/-7F station.
SETUP EXTENSIONS
Enquiry
0/5
Answer with module type / parameter help text.
Type
54 xx yy zz
Only modules of the type xx yy zz are affected.
Set bit
53 xx
Set bit number xx in the appointed parameter.
Reset bit
52 xx
Clear bit number xx in the appointed parameter.
SeqAddr
51 xx
Set the address xx in a physically selected
module.
SIOX SYSTEM DESCRIPTION p 23
FAULT TRACING IN A SIOX SYSTEM
A SIOX system consists of a central computer, typically a PC with a serial port,
a converter module of type K32 or U32, a two wire SIOX bus and up to 63 SIOX
modules, stations, containing different I/O functions.
The central can also be a custom computer or a modem adapter with a built-in
SIOX port, eliminating the need for the K32.
The converter isolates the central from the SIOX bus and translates the
central's RS232 signals to the special signal levels used for transferring serial
information to and from the stations.
Checking the Supply Voltage
Communication typically requires a SIOX bus supply of at least 15 V/100 mA.
It is easy to check both voltage and current on the bus using a universal
instrument. Note, however, that the voltage measured will be lower on a
communicating bus, since each transferred bit = 0 pulls the voltage down to
nearly 0 V. A current measurement between the bus wires also interrupts the
communication between the central and the stations during the measurement,
possibly triggering application dependent alarms.
If a communication interrupt can be allowed, the RS232 connector may be
pulled during the measurement. The K32 module will then supply a steady
signal to the bus, so that voltage and current may be checked.
LED Diagnostic System
Most SIOX modules are equipped with two LED's for supervising the
communication, one green and one red.
The green LED, when lit, indicates that sufficient voltage is being applied to the
SIOX bus. If it is flickering - at a rate corresponding to the communication
speed of the central - it indicates that the central is transmitting information.
SIOX SYSTEM DESCRIPTION p 24
Several causes to the green LED being off exist, most of them self-evident:
*
The bus power supply is out of order or not connected.
*
The wrong COM-port in the PC is specified, and the current port
transmits a Break. Check by unplugging the K32 adapter from the
PC. If the LED lights up, this was the problem.
*
The SIOX bus has been disconnected somewhere between the
central and the module in question. Check the green LED's of
other modules for a rough indication of the break position.
*
The SIOX bus is short-circuited somewhere along the line, usually
at a branch-off point or a connector. Disconnecting the bus from
the central permits measuring with an ohmmeter. The reading will
be below 50 .
*
The bus adapter components in a module have failed, shorting
the bus. However, this is very unusual, since modules are
designed in such a way that most errors just disconnect the failing
module.
The red LED of a SIOX module flashes each time the station transmits a
response to the central. Should the LED never flash in spite of the fact that the
green LED is flickering, then the central is unable to communicate with this
particular module. Several reasons for this are possible:
*
The central transmits using a different speed. All standard
modules are supplied set to 4800 bits/s, while the central may
communicate using e.g. a 1200 bits/s modem. Modern modules
are typically set to automatic bit rate detection from 1200 to 19200
bps.
*
The address of the module is incorrect. Most modules are set to
address 1 at delivery, since this is available in most SIOX
systems. The address is set either by jumpers or in an EEPROM,
as described in the respective manual. If the address is
inadvertently set to 0, the station will switch to number 63, which
is not normally used by the central. Furthermore, most systems
communicate with less than the permitted 62 stations or may,
under program control, elect not to communicate with specific
stations for some time.
*
The module may be set up to, in data mode, receive two data
characters from a central set to transmit only one to this module.
This cannot occur when a standard SIOX protocol is used, since
this automatically adapts to the station type.
*
The supply voltage to the SIOX bus is too low (15 V - 35 V is
recommended) or not enough current can be supplied (typically
> 120 mA).
SIOX SYSTEM DESCRIPTION p 25
*
The SIOX bus may be continuously affected by intense electrical
interference, e.g. through involuntary connection to other signal
or power networks. It may also have been grounded at more than
one point, so that differences in ground potential are carried
through the bus, overlaying the bus signal. Such problems are
best identified using an oscilloscope (please see below).
*
Should two modules inadvertently be set to the same address, the
red LED's of both will flash when they answer a call from the
central. In almost every case, their overlapping answers will
enable the central to identify an error, retransmit and finally define
this address at a lower priority. This is identified by the red LED's
in the two doubly addressed modules flashing at a slower rate.
Bus Listeners
In the SIOX system several different terminal modules with keyboard and
display are available, which also may be used to "spy" on the bus communications (M10, M11, P10). On the display is shown a slice of the
communications, either to all stations or only to a specific address. For details,
please refer to the manuals of M10, M11, P10. Since the SIOX bus is
bidirectional, all characters to and from the modules are shown, specially indicating faults due to disturbances. Such a bus listener may be connected to the
bus at any point, since it doesn't require a separate power supply.
A standard PC may also be used as a bus listener through a K32 or U32
converter set at 0 mA bus current and the SIOX-Spy program. The advantage,
as compared to SIOX terminal modules, is that a larger slice of the
communications can be shown, but with less portability of the equipment.
Oscilloscope
An oscilloscope can be connected to the bus to check for interference etc. If
the oscilloscope has a grounded power cord, start by connecting only the probe
tip to either bus wire to check which lead may eventually be grounded and
which carries the positive voltage. The safest way to proceed is to use a fully
isolated instrument.
The levels for "1", typically 24 V, and "0" can now be checked. Note, that "0" bit
levels transmitted from a station measure approximately 4 V, while "0" bits from
the central drop to 1 V from ground. When long buses are employed, the "0"
bit levels may be considerably higher, depending on where along the bus that
the connection is made. The levels for "1" may also deviate, either if the supply
is poorly filtered, causing the level to fluctuate a few volts, or if the bus is
heavily loaded so that each "1" slants upwards to the right on the screen.
SIOX SYSTEM DESCRIPTION p 26
A check should also be made that each rise and fall time is adequately short,
less than 20% of the bit time (208 µs at 4800 b/s). It is difficult to obtain a
stationary picture for a complex communication, but the first few bits after the
trigger are usually identifiable.
Intense hum causing both the "0" and "1" levels to sway up and down more
than some 5 V indicate a grounding error. Tall, sharp spikes indicate that the
bus is subjected to capacitively or inductively coupled noise from other wiring
or magnetic fields Although the SIOX bus features much greater noise immunity than other communication buses, such problems should be corrected by
screening, twisting or relocating the bus wires.
Conclusion
Be aware that most errors have trivial grounds such as lacking supply or
shorted or disjoint bus. These errors can almost always be singled out by
checking the two LED's on the modules.
Telefrang AB, Varbergsgatan 8, S-412 65 Gothenburg, Sweden
Tel +46 31 403060, Fax +46 31 402025, E-mail [email protected]