Modem won't stay OffHook=true

[Pete H]

I am connecting to a Modbus slave station using a telephone
modem. This works fine on the development laptop with it's
built-in V.90 modem. The OffHook datamember of the Modbus
driver has been set to =true so that the modem doesn't try
to drop the call. My poll rate is 0:01 (1 second). This
works reliably with few short or garbled responses.

The problem is that using other modems, I can't get the
phone connection to persist. Judging from the comm logs
generated by adding the DiagnosticFile= entry to the .ini
file, there are usually either zero or four messages passed
between the host and the remote before the call drops.

This is what it looks like:

ATX4VEM1DT555-5555[0D] DL205p2_Modbus ->
ATES0=0[0D]
ATX4VEM1DT555-5555[0D] DL2
05p2_Modbus ->
ATES0=0[0D]
ATX4VEM1DT555-5555[0D] DL205p2_Modbus ->
ATES0=0[0D]

etc....

It may have to do with an AT command or a register setting
of the modem. However, I've already tried resetting to
factory defaults on two different external USRs and also
setting up on the target desktop PC with an internal modem.
The internal modem has done a few polls before the call
drops.

I've also tried disabling the DTR line in the INI file or
enabled with a serial cable which includes the DTR line.

These modems all work fine using other applications. I need
this connection to work reliably no matter which phone modem
I use. Any ideas? Is there anything about the diagnostic file
that should mean something. How about dip settings or other AT
commands?

[Martin_S.]

Judging from the data this was posted it is too late for the answer, however it might be beneficial for others in case they run into the same issue.
One of the scenarious is following: If the diagnostic file really looks this way than you are not getting any replies from the device. Your DL205p2_Modbus object sends message out (->), but never receives anything back, otherwise you would see a line with a arrow pointing to the same object (DL205p2_Modbus <-) - you can find more on National Instruments site - this KNB: 288F1DJ6.
Because the object does not get any reply, it retries number of specified times - default is 4 - this is why you see a few entries before modem hangs. If the communication was successful it would not hang, but it isn't.
There is numerous reasons why t
here is no response, for any troubleshooting I strongly recommned loop-back test (PC->modem <--> modem->loopback) - connect pins 2 and 3, modem in autoanswer mode.
One more info you don't want to miss - from my pesonal experience with DL205: this PLC and some modems don't work together. I was not able to use Robotics 56k for example. I even got in touch with Automation Direct support - they were awere of the problem and had spent a lot of time on it, but were never able to find out the reason. They recommended swiching to a different modem. Slower modems - 14.4k (Robotics) seemed to work. The symptom is that the modem does not pick up if connected to the PLC.
Martin