Series 2000 Communications Handbook - Soliton

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Digital Communications HardwareSeries 2000 Communications Handbook2. CHAPTER 2 DIGITAL COMMUNICATIONSHARDWAREThis chapter defines the differences between the RS232, RS422 and RS485 digitalcommunications standards. Details of configuration, cabling and termination will help toestablish basic communications.2.1. RS232, RS422 AND RS485 TRANSMISSION STANDARDSThe Electrical Industries Association, (EIA) introduced the Recommended Standards, RS232,RS422 and RS485. These standards define the electrical performance of a communicationsnetwork. The table below is a summary of the different physical link offered by the threestandards.EIA Standard RS232C RS422 RS485Transmission mode Single ended Differential DifferentialElectrical connections 3 wire 5 wire 3 wireNo. of drivers and receiversper line1 driver,1 receiver1 driver,10 receivers32 drivers,32 receiversMaximum data rate 20k bits/s 10M bits/s 10M bits/sMaximum cable length 50ft, (15M) 4000ft, (1200M) 4000ft, (1200M)Note: RS232C has been abbreviated to RS232. The RS232 standard allows a singleinstrument to be connected to a PC, a Programmable Logic Controller, or similar devicesusing a cable length of less than 15M.The RS485 standard allows one or more instruments to be connected (multi-dropped) usinga two wire connection, with cable length of less than 1200M. 31 Instruments and one‘master’ may be connected in this way. The balanced differential signal transmission is lessprone to interference and should be used in preference to RS232 in noisy environments.RS422/RS485 is recommended for plant installation. Although RS485 is commonly referredto as a ‘two wire’ connection, a ground return/shield connection is provided as a ‘common’connection for Series 2000 Instruments, and in general this should be used in installations toprovide additional protection against noise.2-2 Series 2000 Communications Handbook Part No HA026230 Issue 2.0 Feb-00
Series 2000 Communications HandbookDigital Communications HardwareStrictly speaking, RS422 is a standard permitting ‘point to point’ connection of two pieces ofequipment using a full duplex, differential signal on two pairs of wires. In principle,therefore, an RS422 link only allows a single instrument to be connected to a PC. However,Series 2000 instruments provide an enhanced version of RS422 that also meets the fullelectrical requirements of RS485 described above. This allows up to 31 instruments to beconnected on the same network, but only with a 5 wire electrical connection. Thetransmission and reception of data use two pairs of twisted cable, with a separate cableprovided for common. The optional screen will provide additional noise immunity.The 2 wire RS485 should be used where possible for new installations where multi-dropcapability is required. RS422 is provided for compatibility with existing instruments.Some earlier instruments use a terminal marking that is different from that used in theRS422/RS485 standards. The table below compares this marking.RS422/RS485A'B'ABC & C'InstrumentRX+RX-TX+TX-CommonUsing RS232 or RS422/485, the Series 2000 instruments operate in a half duplex mode thatdoes not allow the simultaneous transmission and reception of data. Data is passed by analternating exchange.Most PC's provide an RS232 port for digital communications. The KD485 Comms Interfaceunit is recommended for interfacing to RS422/485. This unit is also used to buffer anRS422/485 network when it is required to communicate with more than 32 instruments onthe same bus, and may also be used to bridge 2 wire RS485 to 4 wire RS422 networksegments.. Wiring information for this unit is given at the end of this chapter., or refer toKD485 Installation and Operation Manual, available on request from your supplier, for a fulldescription.2.2. SELECTING RS232 OR RS422/485Changing between RS232, RS422, and RS485 is possible for 2400 Series instruments byreplacing the plug-in ‘H’ Module with a communications module of the required type.2200 Series communications hardware is a fixed build and must be specified when theinstrument is being ordered.Series 2000 Communications Handbook Part No HA026230 Issue 2.0 Feb-00 2-3
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