Modbus communication

[Bruce Meissner]

Has anyone used PB Dos to talk to Modbus devices?
I'm in a crunch to get data from a building power meter.
I need to know what the data strings would look like to talk to this device.
I need to get power consumption readings and date stamp them then put them in a file.
Problem is, getting data to the power meter and gatting back some useful information.
I appreciate any help available.

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[Lance Edmonds]

Are they serial devices, USB devices, or something else?

Does the manufacturer offer any documentation on interfacing and communicating with them?

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Lance
PowerBASIC Support
mailto:[email protected][email protected]</A>

[Bruce Meissner]

The power meter has a Modbus two wire RS-485 output feeding a RS-232
converter on the IBM compatible PC. The power meter has an address
of 20. Serial rate is 9600,N,8,1. The meter manual has some info
but not a lot. Sample inquiry given: 0A 04 00 00 00 0A 71 76.
0A=periheral number
04=reading function
00 00=initial address (first register)
00 0A=number of registers to be read
71 76= CRC character

I use pbasic 3.5, If I can a data string back from the power meter
then I have no problem dealing with the data. Sending the data
string to the meter I can't seem to get any response back. I was
hoping someone may have done this type of communication before.



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[Mel Bishop]

If you are going through an RS-232 converter and you have your
comm parameters set up correctly, then there should be no
problem.

One possibility: If you are using software flow control, send
the meter an XON character, either chr$(17) or chr$(19) (can't
remember which).

Also, check to see that your cable matches the flow control you
are using. Sometimes, a modem cable is of the 3-wire variety
which mandates the use of software flow control. If you are
using hardware flow control, you have to have the RTS/CTS wires
present in the cable.

Just a couple of thoughts.
Cheers


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[This message has been edited by Mel Bishop (edited March 21, 2003).]

[Tom Hanlin]

You might start with the TERMINAL.BAS example code, which ought to
provide a good basic framework for the comm part. You should also
be aware that the Modbus data needs to be appropriately encoded and
decoded. For example, that 0A 04 00 00 00 0A 71 76 string goes out
something like this:

PRINT #1, CHR$(&H0A); CHR$(&H04); CHR$(&H00); CHR$(&H00);
PRINT #1, CHR$(&H00); CHR$(&H0A); CHR$(&H71); CHR$(&H76);

...assuming your comm port was opened as file #1.

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Tom Hanlin
PowerBASIC Staff