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 Understanding RS422 and RS485
 Those familiar with RS232 will know that the standard
 defines how RS232 lines should  be driven electrically:
 what they are called, what information they carry and
 even the connectors and pin numbers to use (see
 <span style="color: #990099;">[[]]</span> if you are
 interested in details of the RS232 standard).  RS422
 and RS485, by contrast, define only the electrical
 characteristics of the driver.
 Advantages or RS485 and RS422
 RS485 and RS422 drive lines in a differential fashion:
 two wires are required for each signal.  If you wish to
 transmit four signals then eight wires are needed.  The
 figure below shows a single RS485 / RS422 signal being
 transmitted. To transmit a logic 1, line B is high and
 line A is low.  To transmit a logic 0, line B is low and
 line A is high.  The advantage of this arrangement is
 that signals can be transmitted faster and over greater
 distances than is possible with a single wire.

external image rs4851.gif

<span style="font-family: 'courier new'; font-size: 10pt;">
 Diffences Between RS422 and RS485
 In both RS422 and RS485, one driver can drive many
 receivers. In RS485 each driver can be switched off
 allowing multiple units to send data over a single
 pair of wires. This is not possible in RS422. This
 extra feature causes extra problems for RS485 units.
 Other differences are small so hereafter I will refer
 to units as RS485 but the remarks apply to RS422 also.
 Interface units often describe themselves as RS422/RS485
 because a RS485 unit which is not using its extra
 capability is RS422 compatible.
 When instruments are described as having an RS485 interface
 this tells you nothing for sure about the signals being
 transmitted. Usually though only the Transmit Data (TX)
 and Receive Data (RX) of a normal serial port are converted
 to RS485 or RS422. The other signals of the serial port are
 not used. Three arrangements are commonplace: Write only,
 4-wire (full duplex) and 2-wire (half duplex).
 Write Only System: Sending Data Only
 In this arrangement the computer sends data out over a
 single RS485 pair of wires to many instruments. It never
 reads any data back. An example of this is the remote
 control of  CCTV cameras. The operator can send out
 messages to move the camera. His feedback is the TV
 picture so no return data is needed. This requires
 only conversion of the RS232 TX signal to RS485.
 Full Duplex System: Sending and Receiving Data over 4 Wires
 Full Duplex means that data can pass simultaneously
 both to and from the instruments. This requires
 4 wires - one pair to Transmit and one pair to
 Receive.  The computer alone controls the TX
 transmit line. It sends out a message which includes
 an Address. The addressed instrument alone replies by
 driving the RX line.
 This does mean that all the connected instruments must
 use the same software protocol - otherwise there will
 be confusion as to which instrument is being addressed.
 A simple conversion of RS232 to RS485 will suffice
 for this arrangement and no special demands are
 made on general purpose software such as ComDebug.

external image rs4852.gif

<span style="font-family: 'courier new'; font-size: 10pt;">
 Half Duplex System: Sending and Receiving Data over 2 Wires
 Half Duplex means that data can only pass in one
 direction at a time. In this arrangement both TX
 and RX signals share a single pair of wires.  This
 can save in installation costs.
 The computer drives the lines to send out its
 request message but must turn its driver off to
 allow the instrument to reply.  This is the tricky
 bit.  Many RS232-to-RS485 converters recruit the
 RTS line of the serial port to switch the
 RS485 driver On and Off.  However Windows has never
 made more than a half hearted attempt to support
 such an arrangement.
 The multi-tasking nature of Windows means that it
 is always difficult to make actions occur at exact
 times - in this case there can be a significant
 delay between the request being sent out and the
 driver being switched off. If the instrument starts
 its reply before the switch-off then the start of
 the reply will be missed. Instrument manufacturers
 are forced to find their own solutions to this
 problem. This may include special software, special
 hardware or both.
 In recent years most Serial Ports have been
 implemented as USB-to-Serial arrangements.  These
 units always include a microprocessor and this can
 perform the task of controlling the RS485 driver
 very well. If you need to implement a 2-wire
 RS485 system you should always obtain a USB-to-RS485
 converter that advertises this special feature.  An
 example type that we use is Moxa 1130.  This unit
 automatically switches off the driver when there
 is no more data to send. Such a unit can work
 with general purpose software such as ComDebug or
 HyperTerminal. Beware though that it may not work
 with the special software supplied by instrument
 manufacturers which may assume a particular RS232-
 to-RS485 converter and may produce errors when
 used with any other unit.

external image rs4853.gif

<span style="font-family: 'courier new'; font-size: 10pt;"> Preventing Voltage Reflection ============================= When a signal is sent down a cable there is always a reflection voltage which returns up the cable. This reflection is bigger as signals' edges get faster and lines get longer. It can be minimised by terminating the ends of the line with the characteristic impedance of the system. This is 120 ohm for RS485. How to Arrange the RS485 Wires ============================== Ideally an RS485 system consists of a single linear cable (no branches) with 120 ohm resistors connected across the 2 wires at each end of the cable. RS485 can handle speeds of over 10 Mbits per second and line lengths of over 1 km. If you are operating anywhere near these values you must arrange your wiring close to the ideal. For many applications where baud rates are slow (say 9600 baud), and lines are only tens of meters long, this is not essential. The wiring requirements can then be relaxed to allow cables to run in star arrangement from the computer and the terminating resistors become optional. Line Pull Ups: Avoiding Serial Errors ===================================== In many RS485 multidrop systems there are times when no unit is driving the RS485 lines. They can then go to any voltage and may produce serial errors. To avoid this it is often necessary to fit resistors which pull one line high and the other line low putting the line into the passive state. The RS485 converters usually include such resistors which can be connected if needed. Signal Names: Caution Required ============================== You will find that your RS485 signal names for a line pair are either A,B or +,-. In an ideal world you would simply connect A on the computer to A on the instrument. However these names are not consistently applied by manufacturers and you may need to connect A on the computer to B on the instrument.</span>