We have two devices connected via a cable, an ARM based computer on one side and a sensor on the other. This cable carries an RS232 TX, RX, and ground. In addition there is 28V provided from the sensor to the ARM computer. When the sensor is powered on, it provides the power to the ARM platform so both systems power up at the same time.
It was observed that in this configuration the sensor refuses to communicate with the processor. If the processor is powered on a seconds after the sensor, then communication works as normal.
An o-scope trace showed that when the processor was powered on, the RS232 TX goes from undefined to 0 and then immediately pulses to 1 and then back to 0. My guess is that if this pulse occurs during the sensors boot, this will cause the sensor to ignore any future IO.
The simple solution would be to disable the TX line for a few seconds while both systems boot. What is the simplest circuit that could be used to perform this? This circuit would need to be built into the cable.
I have seen a lot of delay on and off circuits on this site but most of them look to be switching power, I am not sure if it would be different if it was an IO line.