Envision Peripherals NV3128 User Manual download pdf (Page 17)

NV3128 RS-422A Machine-Control Data Switch 1-7

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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Envision Peripherals NV3128 User Manual Page 17