Envision Peripherals NV3128 - Manual (Page 16)

NV3128 RS-422A Machine-Control Data Switch 1-6
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.