Hello Robert,
I am currently using the scheme of the BUSY line in two systems I have developed. However I am using a single pin serial line. What I have done is created a system with a master board and 20 slaves. I have a busy line that is normally high. When a slave has a message for the master it first checks that the busy line is not low then makes the busy line low, sends its data to the master. The slave clears the low on the busy line. The master makes the busy line low just before it receives the data. It will keep the line low until the end user acknowledges the event received. The it clears the busy line allowing new data to be sent. To prevent crashes of data, each slave must wait 10ms before sending any data. The first board waits 10ms the next 20ms and so forth. The likely hood of two signals coming at the same time is virtually nill but why take the chance. Waiting 200ms isn't going to hurt anything.
I keep all of the slaves serial pins as inputs until the SEROUT2 command is used then as soon as they are finished sending I make the pin and input again.
All of the slaves are plugged into a card cage backplane so I'm not running cables all over.
In one system I am using a 16F877A for the master and 16F872 for slaves. In the other system, a 18F452 is the master (needed more memory) and the slaves are 16F74's. One system has more things to do than the other.
Hope this helps you out.
BobK
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