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RS-485 Harmonic Spectra Input Output 1st Sum 3rd 5th INCOM Because different frequencies are delayed and attenuated by different amounts, a waveform rich in harmonics is distorted after passing through an LC circuit. On the contrary, a pure sinusoid, while delayed and attenuated, keeps its original waveshape after passing through an LC circuit. Detecting RS-485 1’s and 0’s 11 00 11 01 0? 1? Receivers than convert serial bit streams into parallel data are called UARTs (Universal Asynchronous Receiver Transmitters). A UART determines whether the serial stream contains a 1 or a 0 by sampling at the bit rate (9600, 19.2 k, etc.) For better error immunity it typically samples 2 or 3 times per bit. Two bit readings in a row must be the same for the UART to declare a 1 or a 0. With distorted wave shapes, the UART will typically be observing a data stream where the bit value changes from within a bit time. This can cause the UART to declare various errors (such as framing or parity) and discard the byte. INCOM Decoding 0 1 0 1 Since the waveshape is not changed, it is still easy to detect the difference between a 1 and 0 from an Frequency Shift Keyed (FSK) signal. INCOM signals are not as affected by impedance mismatches (taps, T’s, Y’s, etc.). Consequently the cabling rules are more relaxed than with RS-485. Excerpt (Page 33) from RS-485 Based Installation Manual Noise Immunity RS-485 S1 + S2 S2 Delay S1 + S2 S1 Since RS-485 decides if a signal is a 1 or a 0 by examining the amplitude, external events can mask the true value. INCOM S1 + S2 S2 Delay S1 S1 + S2 INCOM FSK uses a bandpass filter to eliminate signals substantially above or below 100 kHz. Therefore large amplitude interfering signals removed INCOM Vs RS-485 Companies using RS-485 claim an advantage of higher bit rates (19.2K or faster) compared to INCOM (9.6K). One consultant tested the distance constraints for a leading vendor using RS-485 at 19.2K and found reliable communication could be guaranteed only if the network length was less than 4 feet. In addition, response time is based on the latency of the device. One leading vendor’s motor protective relay requires 1 second to formulate a response to a message. A network with 10 of these relays would update no faster than once every 10 seconds! INCOM device latency varies from 0.003 seconds to approximately 0.01 seconds. 485 networks cannot be run nearby power conductors and as such require their signal wire to be installed in conduit. FSK does not. That’s a minimum $6.65/LF installation cost adder per the 1999 Means Electrical Cost Data Manual. Also, 485 networks have a distance constraint of 4000 feet, then a repeater/signal booster of some type must be used. INCOM has a 10,000 foot limit. When you combine 485’s need to have a data concentrator for every 32 devices (PowerNet supports up to1000, unlimited taps, and 5 branches without using additional hardware) you have further reduced installation costs, opportunities for failure and additional maintenance overhead for hardware and software. IQ devices use an isolation transformer between the network and its circuitry. 485 devices can’t, as they must be DC coupled. Otherwise there would be severe distortion of the shape of their square wave. Typically they use an electrolytic capacitor as the isolation device. Electrolytic capacitors are liquid filled devices that eventually dry out. The result is limited life and low voltage and temperature rating. C-H provides 1500V network isolation; RS-485 provides little or none. A B C 1 Install in PC RJ-45 D CONI E RJ-11 F G H I J 1 Cutler-Hammer IMPCABLE or Belden 9463 (or equivalent in Plenum) This design places brings all network (Lighting Control and Power Monitoring) back to the Central Plant. IN 1 IP 2 Shield 3 Ethernet Hub Phoenix Contact RJ-11 to Terminals 2 Ethernet 10BASE-T RS-232 2 Belden 9207 (or equivalent in Plenum) RS-485/ 232 200’ max Typical Switchboard Tap RS-485 Device To PDS PONI 130 3 IED IED IED PONI 131 Typical 3 Typical IQ Analyzer IQ Analyzer 4 4 PONI 131 5 5 IED Typical IQ Analyzer 6 Terminal RS-485 Network DFTR DATE APPD DATE 1 7 RS-485 INCOM Network Network Parking Garage Parking Lot These taps are legal with INCOM wiring. These are not legal with RS485 DGL 11-DEC-98 THE INFORMATION ON THIS DRAWING IS THE PROPERTY OF EATON CORPORATION. IT IS DISCLOSED IN CONFIDENCE AND IS NOT TO BE REPRODUCED, USED OR DISCLOSED EXCEPT FOR THE PURPOSE FOR WHICH FURNISHED. CUTLER-HAMMER APPD DATE ENG. REF. DATE TITLE NEXT REF. DATE REVISION POWER MANAGEMENT PRODUCTS CENTER SHEET NO 8 1 OF 1 A B C D E F SIZE SCALE A NONE DIRECTORY DWG NO 6 RS-485 and INCOM Wiring Rules 200 feet 200 feet 200 feet 200 feet 200 feet 200 feet 200 feet Total Cable Length to = 13 x 200’ = 2600 feet 200 feet RS-485 Node RS-485 Node RS-485 Node RS-485 Node Conduit space required = sufficient for 2 twisted pairs RS-485 Node 200 feet 200 feet 200 feet 200 feet 200 feet 200 feet 200 feet 200 feet Total Cable Length to = 9 x 200’ = 1800 feet (70% of the RS-485 system) IQ Node IQ Node IQ Node IQ Node IQ Node Conduit space required = sufficient for 1 twisted pairs Excerpts (Page 34) from RS-485 Based Installation Manual