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

To further complicate matters, the sense of transmit and receive connections in many modern machines is switchable. That is, internal circuitry within the machine reassigns the interface pin assignments to allow it to operate in either controlled or controlling mode. Fig. 1.3 illustrates the physical pin swapping within each of two directly connected devices. On most units the changeover is a physical switch, but on some, such as the Sony D1 VTR, the pin flip is under software control. In either case, the traditional routing switch cannot adapt to a reversal of data direction, and thus severely compromises a systems flexibility. Referring again to Fig. 2, note that the physical connection fixes the operating mode - controlled or controlling - of the device. If pin pair 3/8 from a machine is connected to a router input, then that machine is a controlling device from now until someone gets out a crimping tool and rewires the connection.

Yet another frustration in implementing a switched machine-control system: one-to-many connections have historically been problematic. Sending a command down a daisy chain of slaved devices is not difficult, and many machines provide a second D connector to accommodate the cable loop through. But it is difficult for the controlling device to discern which of the slaved machines is responding, and - worse- there is a strong probability of bus contention. Again, the user must get out of his chair, find his trusty crimping tool, and disable the responding transmit connection on all but one of the controlled machines. While keeping plant technicians gainfully employed, this activity defeats the purpose of a routing switch.

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