Modem connections

Dear Friends
This is my first question so go easy on me…
I would like to use you experience regarding modem connections.
I’m using a Unitronics PLC and in this controller one can have a very good remote access capability as long as he has an external modem or a modem that gives a CDC signal while connected.
I get more and more questions regarding problems with internal modems and also people complain about the fact that after working with our tools they have to restart the modem (its easy when it’s external, when internal it’s the PC) in order to go on the web (using the same modem).
I was wondering if you could tell me how it is done with other PLCs.

Thank you in advance

Avner from Israel

Dear Avner,

Modem connections is sometime a touchy subject, even for the experience PLC programmer. Its so easy when it works but darn, its time consuming when its not.

Dealing with internal or external PC/modem to PLC is one of those "thing". Why do you thinck that the manufacturers sell there "compatible" modems at such a high price.

-Mitsubishi will not even market there modem in North America, they would go for over 2K (canadian dolloards of course).

First, you have to imagine that the modem internal buffers, any deviation from a set rules of operations could leed to the failure of the comunication link.

You have to make certain that what "they" do is the same as what "you" do.

If they use there modem for Fax and other purposes, they are just asking for trouble.

Personaly, I use both type and PCMCIA card also. Most problems comes from the memory buffers being set to parameters that will be used by programs not related to my PLC link. WinCE links, Fax, PDAs, etc.

I'm asking for trouble but, I need my FAX!!! My PDA, my embeded PC need to be synch, at least my mouse is on USB

My advice is to tell the End-users to use there modem only with this app. If they want to use it with other apps AND want to find what goes wrong, they will have to look into interrupts. Windows has a bizarre way of reserving a port when it uses it and sometime it does not release it. It could be many things...

ON thing you could try is a "Cold-Reboot".

First, get out of any app using your com port.

ALT-CTRL-DEL, then click Explorer, then OK... then wait for the Windows shutdown window. Click CANCEL. then wait for Explorer stopping message Windows... click OK. Wait a few second, you'll see your screen flickers. You've just reboooted your memory. It could do the job...