3-32 B90 LOW IMPEDANCE BUS DIFFERENTIAL SYSTEM – INSTRUCTION MANUALDIRECT INPUT AND OUTPUT COMMUNICATIONS CHAPTER 3: INSTALLATION33.4.5 RS422 interface3.4.5.1 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 connected internally 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 2bMatch 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 interconnect between two RS422 interfaces3.4.5.2 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 way, observingtransmit (Tx), receive (Rx), and send timing (ST) connections. However, when used in two-channel applications, certaincriteria must be followed since there is one clock input for the two RS422 channels. The system functions correctly whenthe following connections are observed and your data module has a terminal timing feature. Terminal timing is a commonfeature to most synchronous data units that allows the module to accept timing from an external source. Using theterminal timing feature, two-channel applications can be achieved if these connections are followed: the send timingoutputs from the multiplexer (data module 1) connects to the clock inputs of the UR RS422 interface in the usual way. In
