Envision Peripherals manual NV3128 RS-422A Machine-Control Data Switch

Because commands are broadcast on one balanced pair and responses received on another, the systems crosspoint count, wiring density and complexity are inherently double that of a video or audio router. Fig. 1.2 shows the two- level connection required to implement a route through a conventional data switch. To pass through the switch, the connection must be separated into two components: the command, which travels from source to destination, and the response, which returns through the switch from destination to source. Cables are split such that transmitting pins from both machines are hooked to switch input blocks, and receivers to output blocks. Additionally, the router-control system must be highly specialized, for it now must address two connections with every connect command it issues.

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