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Line Tristating Issues

Last Modified: September 15, 2017

During device initialization, all terminals on the I/O connector and trigger buses are tristated. Tristated means the terminal is floating or at high impedance. For the terminal to be driven from the device, the tristate buffer associated with the terminal must be enabled.

For instance, assume that you have a device with a single bidirectional terminal on the I/O connector. The terminal on the I/O connector is called the trigger terminal for reference purposes. Also, the trigger terminal of the device is bidirectional because it can accept an external trigger signal or export the internal trigger signal. The exported internal trigger signal could be different from the external trigger signal.

Scenario Usage and Consequences
The trigger terminal is being driven by an external trigger signal only. This is a common case for triggering an operation from an external source. As a result of this operation, you must disable the tristate buffer associated with the trigger terminal so that the internal trigger signal does not drive the trigger terminal, too.
The trigger terminal is being driven by the internal device trigger only. In this case, an internally generated trigger is starting the device. This signal could be useful for other devices, too. To export this trigger signal, you must enable the tristate buffer associated with the trigger terminal, so the device can drive the pin with the trigger signal. It is important that there is no external signal hooked up to the trigger terminal. If it is inconvenient to unhook the external signal, you must make sure the external signal is at least tristated.
The trigger terminal is being driven by both the internal device trigger AND an external trigger signal. Driving the trigger terminal both internally and externally is called double driving. If the internal and external sources drive the signal in opposite directions, it signals problems. Usually the driving hardware is damaged, but more extreme consequences can occur as well. Remember to be very careful to avoid double driving any terminals on your I/O connectors.

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