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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