Hi Longpole 001,
Back in 2005 I built 2 160-Input annunciator panels using 20 - 16F74 chips "talking" to a 18F452 on the master board and 20 - 16F872 chips talking to another 18F452 master. I used 1 line for the "talking" and 1 line for a busy indicator. The 18F452 was doing various tasks while also checking the busy line to see if any of the 20 boards had data for it. As soon as it sensed a low on the busy line it stopped whatever it was doing also made the busy line low and kept it low until it was done handling the data before resuming its tasks. The boards would wait for 10ms after sending its data before resuming its tasks. The systems have been in service since the fall of 2005 and have been working fine since. The master board also sends a report to a printer for a hard copy record of events. I had planned a series of upgrades to the systems including an periodic message from each of the 20 boards indicating the boards were there and working. Any board not responding to the interrogation after a couple misses would then cause the master board to send an alert to the people working in the office. I have also started building a smaller system using RS485 as the means of communication with several more features added to the system.

Does this sound like something you are looking for? Let me know.

BobK