> Modbus is a really half-assed protocol riddled with
> fundamental flaws that people have to 'cheat' around.
> It was probably cobbled together by people with no
> appreciation of networking protocol problems.

Your criticism is easy to make with 20 years of hindsight.  Modbus
was invented by Modicon when they were the leader in PLC's.  It
probably predates RS-485.  Profibus didn't come until 20 years
later and is more complex.  Modbus is a standard that both simple
and adequate.

> For example there is no unique character or other way of
> identifying a packet start, so if a slave wakes up
> halfway through a message it cannot tell if a byte is
> part of the packet frame or the payload.

Huh?  Isn't the Modbus packet start character ':' a unique
character? :-)  No protocol, including PPP or ethernet will allow a
slave device to wake up in the middle of a packet and properly
handle what it didn't receive.  i.e. special state-change characters
such as start of packet, end of packet, or next character is special


Thomas J Macauley, KD7BDW
thomas@advancedcontrol.com
(208) 362-5858