CHAPTER 3: INSTALLATION DIRECT INPUT AND OUTPUT COMMUNICATIONSB30 BUS DIFFERENTIAL SYSTEM – INSTRUCTION MANUAL 3-453Figure 3-50: Clock and data transitions3.4.5.4 Receive timingThe RS422 interface utilizes NRZI-MARK modulation code and therefore does not rely on an Rx clock to recapture data.NRZI-MARK is an edge-type, invertible, self-clocking code.To recover the Rx clock from the data-stream, an integrated digital phase lock loop (DPLL) circuit is utilized. The DPLL isdriven by an internal clock, which is 16-times over-sampled, and uses this clock along with the data-stream to generate adata clock that can be used as the serial communication controller (SCC) receive clock.3.4.6 RS422 and fiber interfaceThe following figure shows the combined RS422 plus fiberoptic interface configuration at 64 K baud. The 7L, 7M, 7N, 7P,and 74 modules are used in two-terminal with a redundant channel or three-terminal configurations where channel 1 isemployed via the RS422 interface (possibly with a multiplexer) and channel 2 via direct fiber.AWG 20 to 24 twisted shielded pair wiring is recommended for external RS422 connections. Ground the shield only at oneend. For the direct fiber channel, address power budget issues properly.Figure 3-51: RS422 and fiber interface connectionWhen using a laser interface, attenuators can be necessary to ensure that you do not exceedmaximum optical input power to the receiver.
