How Can I Update the coils on Quantum PLC...

[theta@vsnl.com]

Dear Sir,

I am in the process of configuring Lookout4.5 build9 with Quantum PLC.

I am trying to write two O/p coils of the PLC using two latchgate
objects. I am using two different set of push buttons for latching
and unlatching the two latchgates. After this I have directly
connected the two latchgates to the O/P coils of Modbus object
in modbus ethernet mode of communication.

Now I am using the PLC simulator s/w of PLC to watch the status.
When I start the first pump (coil1) and Stop the second(coil2),
after sometime it is seen that the position is reversed such
that the coil1 is off and coil2 is On. Though here in Lookout
I see that the latchgate 1 is ON and latchgate2 is OFF.

I am
unable to understand this change over in the status of the coil.
I have read one of the documents on NI website (Document ID:15QA2P9C
"Why can't I update the coils on Quantum PLC without reading them
back in?" Report dated:01/27/98). I think my problem is similar
to that.

So please tell me a proper method of updating the coils on Quantum
PLC or how can I update the coils by reading them back ?

OR is there any problem regarding the modbus.cbx file, if so please
send me the latest modbus.cbx for Lookout4.5

Waiting for your earliest response...

Yours Truly,
Girish Nalgirkar
Theta Controls
Pune, India.

[Greg Caesar]

The KnowledgeBase that you refered to is 3-1/2 years old, and the problem in the modbus driver is long-since fixed. Their are no other known issues with the Modbus driver.

The method you are using to update the coil sounds appropriate. What I would first verify is that Lookout is not sending a command to toggle the coil.

1) Write the simplest possible process that shows the problem (ie 2 pushbuttons, the modbus object, 1 latchgate).

2) Create a "modbus.ini" file in your Lookout directory. Place the following in the modbus.ini file:

[ethernet]
DiagnosticPath=c:\

3) Run your program until the problem occurs. If it does, find the "modbus1" file created in the C:\ directory. This file contains a log of all modbus communication that moved back and forth. Check the
log to see if Lookout sent a command to the PLC. If no command was sent, then the change occurred internal to the PLC software.

Regards,

Greg Caesar
National Instruments,
Applications Engineer