SEL_*: There would be one select line for each of the 3 slaves. A slave would be selected when its corresponding select line is high. A slave would drive this line low if it is busy and must ask the central 8052 to wait.

I would decode the sel (i.e. 8 selectables) and have a busy line that the selected unit should pull low if busy.

I have learned that if you do not make room for expansion you WILL find that you need it.

Murphy (of Murphy's Law) was an optimist.

Erik