RS 485Problems by B66XBa6O


									Physical Signaling Differences
                              Harmonic Spectra
                              Input         Output

RS-485           Sum



                             Because different frequencies are

INCOM                        delayed and attenuated by
                             different amounts, a waveform
                             rich in harmonics is distorted
                             after passing through an LC

                             On the contrary, a pure sinusoid,
                             while delayed and attenuated,
                             keeps its original waveshape
                             after passing through an LC
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
                                                                      S1 + S2


     Delay                            S1            S1 + S2

             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


                                      S1            S1 + S2

              INCOM FSK uses a bandpass filter to eliminate signals
              substantially above or below 100 kHz. Therefore large
              amplitude interfering signals removed
 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               D                               E                        F               G                                    H                               I                    J

1                                                       Install         CONI       RJ-11                      Cutler-Hammer IMPCABLE or Belden 9463                                                                                                        1
                                                         in PC                                                         (or equivalent in Plenum)                                         This design places brings all network (Lighting Control and
                              RJ-45                                                            IN 1                                                                                      Power Monitoring) back to the Central Plant.
                                                                                               IP 2
                                                                                           Shield 3

                                                                                     Phoenix Contact
                                                                                    RJ-11 to Terminals
               Ethernet Hub

                              10BASE-T                                  RS-232
2                                                                                                                  Belden 9207                                                                                                                             2
                                                                                                            (or equivalent in Plenum)
                                                                                                                                                                                              200’ max

                                                                                                                                        RS-485                                                Typical Switchboard Tap
                               To PDS                                                                                                   Device
                                                                                                                                                                                                      PONI                 PONI
3                                                                                                                                                                                                     130                  131                             3

                                              IED             IED                                     IED
                                                                                                                                                                                          Typical                Typical
                                                                                                                                                                                       IQ Analyzer             IQ Analyzer

4                                                                                                                                                                                                                                                          4

5                                                                                                                                                                                                                                                          5
                                                                                                                                                                                                               These taps are
                                                                                                                                                                                                               legal with INCOM
                                                                                                                  Typical                    RS-485 INCOM                                                      wiring. These are
                                                                                                               IQ Analyzer                                                                                     not legal with RS-
                                                                                                                                             Network Network                                                   485                                         6

                                                    Parking Lot                                         Garage                                                                                Terminal

                                                                                                                                                           DFTR                 DATE    THE INFORMATION ON THIS DRAWING IS THE PROPERTY OF

                                                                                                                                                                                        EATON CORPORATION. IT IS DISCLOSED IN CONFIDENCE
                                                                                                                                                           APPD                 DATE    AND IS NOT TO BE REPRODUCED, USED OR DISCLOSED
                                                                                                                                                                                        EXCEPT FOR THE PURPOSE FOR WHICH FURNISHED.
                                                                                                                                                              DGL          11-DEC-98
                                                                                                                                                           APPD                 DATE                                CUTLER-HAMMER
                                                                                                                                                                                                              POWER MANAGEMENT PRODUCTS CENTER

                                                                                                                                                           ENG. REF.            DATE TITLE

                                                                                                                                                           NEXT REF.            DATE

                                                                                                                                                           SHEET NO
8                                                                                                                                                                                      SIZE           SCALE       DIRECTORY            DWG NO
                                                                                                                                                                       1 OF 1                 A           NONE
                  A                       B                         C               D                               E                        F
RS-485 and INCOM
Wiring Rules

                200             200             200             200
                feet            feet            feet            feet
                                                                                    Total Cable Length  to 
               200             200             200             200                 = 13 x 200’
                feet            feet            feet            feet                = 2600 feet
                                                                                    Conduit space required =
       RS-485          RS-485          RS-485          RS-485          RS-485
        Node            Node            Node            Node            Node        sufficient for 2 twisted pairs

                200             200             200             200
                feet            feet            feet            feet
                                                                                    Total Cable Length  to 
               200             200             200             200                 = 9 x 200’
                feet            feet            feet            feet                = 1800 feet (70% of the RS-485 system)
                                                                                    Conduit space required =
         IQ              IQ             IQ              IQ               IQ
        Node            Node           Node            Node             Node        sufficient for 1 twisted pairs
Excerpts (Page 34) from RS-485
Based Installation Manual
Reduce Total Cost of Ownership

   Reduce initial installed cost
   Lower operating cost
   Reliability & Safety
   Maximize capital utilization
   Better customer service

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