Download Transmission of Serial Data Using Inductive

Transmission of Serial Data
Using Inductive Data Transfer
by
Mikhail Dembicki
4/27/04
Table of Contents
Introduction:
Specifications
Parts List
Construction method:
Testing
Theory of Operation
Conclusion
3
3
4
6
9
11
12
Figure list
Circuit schematic
Finished board
Coil construction
Hyper Terminal setup
Hyper Terminal operation
Circuit logic levels
Correct coil alignment
Coil alignment
6
7
8
9
10
10
11
13
Introduction:
Sending data between two electronic devices that are separated by some distance is
usually as simple as running a cable between the two devices, except that this direct electrical
connection isn’t always possible. This is when inductive data transfer becomes an important tool
in moving data from one device to another. Inductive data transfer sends data by magnetic
induction not a direct connection. This allows the devices sending and receiving data to be
completely isolated from one another, the only requirement is that the devices be near enough for
magnetic induction to occur.
The system that was designed to fulfill the requirements of being able to send and receive
data using magnetic induction, without the use of costly parts, is easy to construct, operate, can
be adapted to be used in an underwater environment, and can be connected to standard serial data
communications bus, such as the serial port on a computer. All of theses requirements were met
in the design and construction of this device.
Specifications:
Size:
Board
Coil
3 in. long by 2 in. wide, by ¾ in. tall
2 inches diameter by 3/4inch tall cylinder
Power:
Voltage
Current draw in idle and receiving state
Average current draw in sending state
12V
20mA
100mA
Communication:
RS232 serial
Female 9-pin connector
Maximum data rate
4800 Baud
Flow control
None
Note: Start bits, stop bits, and parity bits, along with all data error checking needs to be handled
by external devices. Higher data rates are not recommended because data accuracy drops below
acceptable limits. Lower data rates are possible and will not affect system function.
Cost:
2 complete boards and coils
$20
Parts List
Parts
Resistors
330 ohm 5%
470 ohm 5%
1,000 ohm 5%
2,200 ohm 5%
10,000 ohm 5%
22,000 ohm 5%
2
1
3
1
3
1
Capacitors
.001 micro Farad
.1 micro Farad
10 micro Farad
2
4
2
Integrated circuits
MAX232A
555 timer
7805 voltage regulator
1
1
1
Transistors
NPN 2222A
NPN 3904
TIP31C
1
1
1
Diodes
1N4001
1N4148
1
4
Other parts
2”x3” solder board
3-pin screw terminal
5-pin screw terminal
Female DB9 connector
18 AWG magnet wire
30 AWG magnet wire
24 AWG hook up wire
8 pin chip socket
16 pin chip socket
4-40 metal machine screw
4-40 metal nut
60-40 tin/lead solder
100 grit sand paper
.5 inch SCH 40 PVC straight coupling
.1 thick Plexiglas
Super glue
1
1
1
1
5 feet
30 feet
2 feet
1
1
2
2
2 feet
10 square inches
1
2-inch diameter circle
1 tube
Quantity
Recommended tools for construction
Wire strippers
Wire cutters
Soldering iron
Vise
Needle nose pliers
Solder flux
Flux remover
Drill & 1/8 inch drill bit
Recommended tools for testing completed boards
12 Volt current limiting power supply
Multi meter that can measure voltage, and resistance
200Khz or faster oscilloscope
This figure shows the schematic of the board with the coil attached to it.
Construction method:
An estimated 3 hours is needed to assemble & test one board and coil
Board:
1) Select location on board for large components
2) Drill holes for screwing down of 7805 and TIP31C
3) Place and solder in all large components such as screw terminals and chip sockets
4) Screw down and then solder in 7805 and TIP31C
5) Place and solder in other components one at a time
6) Cut off any extra wire or component leads
7) Using hook up wire make all necessary connections between components
8) Place chips in chip sockets
These two figures show the completed board with all the connectors labeled. The send coil must
be connected to the TX terminals, the receive coil to the RX terminals, +12V and ground to the
power terminals, and a serial cable to the 9-pin serial port.
Note: Two capacitors are mounted on the under side of the board, future versions of this board
should mount these capacitors on the top to the board.
Coil:
1) Cut a 2 inch diameter circle form Plexiglas
2) Drill 2 holes near one end of PVC straight coupling
3) Super glue PVC straight coupling to center of Plexiglas circle, allow to dry
4) Pull about 6 in of 18AWG magnet though one of the holes in the coupling
5) Wind 30 turns of wire around coupling and pull remaining wire though other hole
6) Twist the two leads together to prevent unraveling
7) Repeat this process with 30AWG magnet wire with 100 turns around the coupling
8) Tack down with super glue as needed
9) Remove enamel from the last 1/4in of each magnet wire lead with sandpaper
(1) Drilled holes should be near
one end of coupling and opposite
each other
(1)
(2) Super glue coupling to
Plexiglas making sure drilled holes
are on far end of coupling
(2)
(3) Begin winding coil by
threading 6 inches of wire though
one of the holes
(3)
(4) Wind 30 turns of Magnet wire
then thread another 6 inches
through opposite hole, supper glue
coils down as needed
(4)
(5)
(6)
(7)
(5) Repeat process with 30 AGW
magnet wire, winding 100 turns
around the coupling, super glue as
needed
(6) (7) Photographs of a completed
and functional coil
Testing:
All resistors should be tested to be sure that they are within giving tolerances before they
are instilled on the board, all diodes should be tested as well to be sure that they are functional.
After the board is assembled it should be connected to the coil and powered up with a current
limiting power supply and the current load should be measured, if it is substantially higher than
the 20mA idle current draw there is a problem that needs to be resolved before further testing can
occur.
Once this first phase of testing has been completed, the board should be connected to a
computer running Windows 95 or better. Connect to one on the COM ports on the computer
though a serial cable. Once the board is connected to the computer it should then be giving
power. The application HyperTerminal which is included with the Windows operating system
should then be used to communicate with the board. Hyper Terminal should be set up the
following way:
Bits per second
Data bits
Parity
Stop bits
Flow control
4800
8,
None
1
None
In the properties
window of
Hyper Terminal
the Connect
using box should
be set to the com
port you will be
using to connect
to the board, the
port settings
window should
be set to be
identical to the
one pictured
Once all of this set up is done, the board can now be tested. During correct operation the
board will echo and data sent to it. This means that any data going out will also be received. In
simple terms this means that anything you type into the Hyper Terminal window should appear
on the screen, if you do not see any characters appearing or if they are not the same characters
you typed in, there is a problem. Start by checking for loose connections and then check for
correct logic levels in the board using the oscilloscope. Chart of correct logic level values can be
found on the next page.
The figure that shows the correct logic level operation of the board should be used in
diagnosing problems. If there is no problems are detected this way, try using a different
computer or a different serial port on the same computer. Try a different serial port or change the
COM port settings used in HyperTerminal if there is no data at all being echoed.
These figures represent the correct operation of the board on the left, the message “ABCD1234
Correct operation will echo characters” was typed in. The figure on the right simulates what you
might see if the board if not working properly.
These figures shows the voltage levels you
would expect to see in the board while a single
bit is being sent. The labeled locations in the
schematic above correspond to the logic level
chart in the figure on the left. Letters A, B, and
C correspond to the transmitter of the board,
while letters D, E and F correspond to the
receiver side of the board.
Once a pair of boards and coils have been constructed and tested individually, they can be
tested with each other. The two coils must be aligned to for data transfer to occur, both boards
must be powered, and connected to the COM ports of computers. They can be connected to the
same computer if the computer has more than one external COM port, or two different
computers if only one COM port is available.
Correct aligned of the two coils in important
for proper data transfer to occur. If a
misalignment does occur data quality will be
lost
During correct operation one board will be able to send and the other board receive. Both
boards cannot send data at the same time, trying to do this will result in data loss.
Theory of operation:
The system sends data under the principle of magnetic induction. Magnetic induction is
when a current passing though a length of wire causes a magnetic field to be formed. This
concept also states that a magnetic field will cause a current to from in a wire. Hence this system
works by inducing magnetic flux in one coil that also induces magnetic flux in a coil near it. This
is convenient, as it does not need to be any physical connection between the two coils for
magnetic induction to occur.
The basic function of the board is as follows: the transmitter side of the circuit functions
when the RS-232 bus coming into the board goes to a high state, this causes the output from the
MAX232 chip to go to a low state. The transistor between the MAX232 chip and the 555 timer
inverts this low output from the MAX232, and enables the 555 timer which then begins to output
a square wave at 132 kHz. When the output of the 555 timer goes high state, it enables the
TIP31C, which drives the primary coil in the circuit causing magnetic induction to occur though
it.
The receiver side of the Circuit functions mush the same way as the transmitter side only
it picks up magnetic flux instead of creating it. Magnetic flux is picked up in the secondary coil
and induces current; this current it is rectified in the full bridge rectifier and then inverted using
that transistor between the full bridge rectifier and the MAX232 chip. This pulls the input to the
MAX232 chip low from its normally high state. This causes the RS-232 bus to go to a high state,
which sends data to an external device.
Two boards are needed to make the system work, one to transmit data and one to receive
it. The system is designed to run in bi-directional half duplex mode. This means that while both
boards can send and receive data, only one can be sending at a time, if both boards were to try to
send at the same time that data would become garbled.
For communication with external devices, RS-232 Serial through a DB-9 female plug is
used. The system can function up to 4800 Baud with reasonable reliability. No flow control is
used, all data flow rates need to be managed by external devices. It is recommended that a cyclic
redundancy check or other data integrity check be used to assure that the data passing though the
system is correct.
When the system is powered up or shut off, there are several junk byte will be seen by the
receiver, any external systems should wait at least one second for voltage levels in system to
stabilize before data transfer can begin. These junk bytes will also appear on the receiver of the
device if its mate is turned off or on while the two coils are in close proximity.
For good data transfer the coils of the sender and receiver should be as close together as
possible. Data can still be sent with up to .3-inch or 7 degree misalignment, though this increases
the possibility of errors in the data stream. Many misalignment situations were tested and they
appear in a figure on the next page. The system proved to be rather robust in that it can handle a
significant misalignment and was still able to transmit accurate data at maximum data rate.
Conclusion:
This system fulfilled its requirements of being able to send data from one computer to
another without a direct-wired connection using magnetic induction. Construction and operation
of this board was done in a simple matter that anyone with basic electronics knowledge and
computer usage skills should be able to construct and use with a minimum for effort. It also
meats the design requirement of being low cost, there are systems currently on the market that
cost ten times what this system does and have the same functionality.
Further work that needs to be done to this system it a modification of the coils to make
them waterproof and attachment of a standard connector to the coils so a screw terminal will no
longer be needed. With the use of a printed circuit card and surface mount components to would
be possible to reduce the size of this board to approximately half of its current size. This may be
useful in situations such as in an Autonomous underwater vehicle where space is at a premium.
Coil, as should be assembled for
best performance
For optimal signal transfer two
coils should be placed face to face
as close as possible and aligning
the coils whenever possible
A horizontal misalignment of up
to .4 inches can be experienced
before signal loss starts to become
apparent
Coil separation of up to ¼ inch
can be experienced before signal
loss starts to become apparent
Misalignment of up to 5 degrees
before signal loss occurs. This
type of misalignment caused the
most signal loss during testing