3-32 B90 Low Impedance Bus Differential System GE Multilin3.3 DIRECT INPUT/OUTPUT COMMUNICATIONS 3 HARDWARE33.3.5 RS422 INTERFACEa) DESCRIPTIONThere are two RS422 inter-relay communications modules available: single-channel RS422 (module 7T) and dual-channelRS422 (module 7W). The modules can be configured to run at 64 kbps or 128 kbps. AWG 20-24 twisted shielded pair cableis recommended for external connections. These modules are protected by optically-isolated surge suppression devices.The shield pins (6a and 7b) are internally connected to the ground pin (8a). Proper shield termination is as follows:• Site 1: Terminate shield to pins 6a or 7b or both.• Site 2: Terminate shield to COM pin 2b.Match the clock terminating impedance with the impedance of the line.Figure 3–35: RS422 INTERFACE CONNECTIONSThe following figure shows the typical pin interconnection between two single-channel RS422 interfaces installed in slot W.All pin interconnections are to be maintained for a connection to a multiplexer.Figure 3–36: TYPICAL PIN INTERCONNECTION BETWEEN TWO RS422 INTERFACESb) TWO-CHANNEL APPLICATION VIA MULTIPLEXERSThe RS422 interface can be used for single channel or two channel applications over SONET/SDH or multiplexed systems.When used in single-channel applications, the RS422 interface links to higher order systems in a typical fashion observingtransmit (Tx), receive (Rx), and send timing (ST) connections. However, when used in two-channel applications, certain cri-teria must be followed since there is one clock input for the two RS422 channels. The system functions correctly when thefollowing connections are observed and your data module has a terminal timing feature. Terminal timing is a common fea-ture to most synchronous data units that allows the module to accept timing from an external source. Using the terminaltiming feature, two channel applications can be achieved if these connections are followed: The send timing outputs fromthe multiplexer (data module 1), connects to the clock inputs of the UR–RS422 interface in the usual fashion. In addition,the send timing outputs of data module 1 is also paralleled to the terminal timing inputs of data module 2. By using this con-figuration, the timing for both data modules and both UR–RS422 channels are derived from a single clock source. As aresult, data sampling for both of the UR–RS422 channels is synchronized via the send timing leads on data module 1 asshown below. If the terminal timing feature is not available or this type of connection is not desired, the G.703 interface is aviable option that does not impose timing restrictions.~~~~~~~~~~~~~~ShieldShieldCOMTx +Tx +Tx –Tx –Rx –Rx –Rx +Rx +3b5b2a4a6a7b8b ClockRS422channel 1RS422channel 2Surge3a5a4b6b7a2b8aInter-relay communications 7W842776A3.CDRDual-channel RS422 module~~~~~ShieldTx +Tx –Rx –Rx +3b2a6aRS4223a4b~~~~COM8b ClockSurge7a2b8aInter-relay comms. 7TSingle-channel RS422 module~ indicates the slot position
