Programmed a network using RS485 Master/Slaves.  An Apple was the master and
the slaves were several single board computers logging data.  The master
would poll each slave and get current status and if it had a completed log
message to send.  There was no acknowledgment in the protocol.

If the slave had a log message ready, the master would ask the slave for
that particular log message (each log message had a sequential number id
(modulo something) that it had sent in the polled status message.  After the
slave sent the logged message, it would mark that message as sent, but not
destroy the message until it needed that logging space again.

The master would remember what log message it had asked for and keep asking
for that same message id until it got a good copy of it.  Again, no
acknowledgement of messages took place.

The master or any/all slaves could be powered down at any time.  The data
received as the master did it "status" poll of each slave was for
informational use and not critical.  The logs were important but for
statistical use only and not for safety reasons.

All messages were in ASCII characters with numbers being decimal.  The
fields were separated by a "tab" character delineator.  This served two
purposes.  It was easy to see what each message contained and could be
directly imported to a spread sheet.

Each message contained a "to", "from" and "message type" field.  Followed by
the data fields for that type of message, ending with a very simple check
sum of all digits.  All fields separated by a tab character.  There may have
been a special field(one character)used as "start of message", and a
different one before or after the checksum field used as "end of message".
There was also a special id for broadcasting to all slaves.

The slave would look for a "start of message" character/field and read the
"to" field.  I it was not that slaves id, then it would go back  looking for
the "start of message" character.  If it was for that slave then it would
read the whole message and comply if the checksum was ok.

Each slave had a seven segment led display a couple led's and a keypad.
With this, the activity on the line could provide some information for
diagnostics.

Might be a good ideal to use a led to blink when a slave received something
on the line and double blink when it responded.

Bill

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