We have two NI6509 PCI boards - one in each of two linux machines (CPCCs). These two boards each have an input line attached to a single monitor point that is held to a TTL High by a pullup resistor. So both machines are monitoring the same signal. This is for redundancy. This monitor circuit goes to a smoke alarm. If an alarm occurs, a relay is closed and pulls the monitor point to ground which then causes the CPCCs to take action.
We noticed that when one CPCC is powered off, the monitor point is no longer held to a TTL High. It is like the NI Board input has a low impedance causing a pull down. Measuring the line directly on the powered off board does not show a low impedance. So I am confused.
I think I attached a schematic of the setup but don't know if I did it correctly. If you don't see it, please use this URL: http://www.aoc.nrao.edu/~kryan/smoke-alarm-monitor.pdf
Does anyone know why the input line of a powered off board would pull the other input line that it is connected to low?