An alternative way to the Carrier sense method is have the PC as the =
only device that sends, and have each slave pull on CTS or DTR if it is =
busy when it sees a message with its address. This can be achieved by =
the slave device using an open collector transistor to pull down the =
line which is pulled up by a resistor.

This sort of method could be used on rs232 or rs485 type lines by being =
careful at the receiving end.

The important thing is to make sure all slave devices release the "busy" =
line when no messages are being transferred.=20

If they are not busy when they see a message addressed to themselves =
then receive message and act on it.

If they are busy when see a message address to themselves then pull on =
busy line, and look for end of message character. Wait for a time (say 2 =
or 3 character times) and then release busy line.

The controlling PC can then sense the busy line at any convenient time =
during the message and determine if the addressed slave is busy, or the =
message has been acted on.

An improvement is probably to reverse the operation of the "busy" line =
to "available" so when a slave is available it pulls down on the line, =
and the PC then knows the slave is both present and available. If the =
"available" line is not pulled down the slave may be busy, dead, not =
present or any of several other failure conditions. The PC can then =
schedule the message to go to the next appropriate slave.

This also leaves the return data line available for slaves to return =
valid data to the PC without mixing the data with the busy signal if =
there is any advantage in this.

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