Peter,

If you email me, I can send you a specification for the protocol.

Kind Regards

David Huisman

-----Original Message-----
From: Peter L. Peres [mailto:plp@ACTCOM.CO.IL]
Sent: Friday, 27 June 2003 6:24 PM
To: PICLIST@MITVMA.MIT.EDU
Subject: [EE]: MODBUS protocol programming example


If you use ASCII modbus you can use expect and tcl/tk to build the
interface directly on Linux. expect does serial line scripting. Both
systems also run under Windows but I have no experience with expect
under
Windows, i.e. ymmv.

Modbus consists simply of packets which are strings with ASCII modbus.
The
host queries the terminals (each in turn) and they answer within a given
time or are assumed to be faulty. There is no special magic on packets
or
responses. Collisions only occur when a slave module is faulty or when
you
do not wait long enough for an answer. It depends on what you are trying
to do ? Real time control or just programming ?

Peter

> I have been asked to install and commission a cryogenic temperature
> controller. The system uses a mixture of various commercial components
> to provide closed loop temperature ramping from -200C to +10. Several
of
> the components provide control via a MODBUS interface. We wish to make
> use of this feature but we have no experience in programming the
MODBUS
> protocol. The physical interface is half duplex RS485, 9600 baud, 8E1.
>
> Can any one guide us to some sample code, in either VB, VC++ or Delphi
> which we could use as a starting point. I have scoured the Web but
have
> found no really good examples. The control system will operate,
> ultimately, from a Linux system, so ANSI C examples would be
preferred.

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