Going through Modems [telecom]

From my BBS days I vaguely remember an optional command that could be used in the modem command string. It was used to force modems to terminate a call on lines that would not drop loop current properly at a call termination. Symptom is just what you describe, the modem ends a connection but the phone line is not released for use by others. The modem is not detecting the loop current drop when the modem call ends and so stays "off hook" regardless of what the other end does.

The actual command eludes my memory but it would force the modem to release the line regardless of the loop current condition at call termination. I remember not all modems supported the "extended" command but those that did it would stop (and mask) the problem.

Hopefully someone here will have the actual command sequence at hand for you but I seem to remember it may have been an S register

True it does not fix the possible phone line condition causing the problem but it will force the modem to stop being a part of it.

The modem is not going off-hook and is pulling down the line so the line appears off-hook. If this is a quality modem, it will have MOVs and protection diodes on the incoming line. Check them to find out which one is shorted or leaky.

--scott

I just realized the modems I was using on the old BBS were industrial grade modems from Motorola which may have had many more options than the routine store bought models. It helped that I was working for them at the time and had free access to use them on the BBS provided I would give them feedback on how they worked at my place.

Now on with the show...

I can think of one test that would help eliminate or identify an internal house wiring problem, if your line has a modern telephone demark. The kind with the little rj11 jack you remove and can plug in a telephone to get a direct connection to the phone line.

What I would do is wire a rj11 jack into one of those surface mount phone jacks, the kind that accept two male rj11's. Wire both female jacks in parallel so that the common male rj11 can be used for both connections. Yes, just your basic phone line splitter. A store bought splitter can be used provided it is able to be plugged into the telephone line interface box provided it is not one of those Line 1 and Line 2 kind that look for two actual phone lines (numbers using the same common rj11.

Then plug a phone into one side and your modem into the other (you may need a long rj11 to rj11 phone cable) and plug the male rj11 of the splitter directly into the phone interface. Try using the modem to call the internet and then terminate the call and then see if the phone on the splitter can draw a dial tone. If the problem goes away while using this rig you most likely have an internal wiring problem. If the problem persists then it's either your modem/phone still causing the problem or most likely a phone line problem for telco to fix.

Last remove the setup and test again with your normal house phone jacks to verify the problem is still there. Would not be the first time just reseating a jack or moving wires has "cleared" the problem.

For the modem to turn off, so to speak, the computer's connection to the phone line, wouldn't the modem need to go "on hook"?

(if I'm wrong, please be gentle).

Wouldn't a +++ escape followed a second later by an ATH[Enter] (or an ATH0[Enter]) take the modem on-hook and thus release the line?

Cheers, -- tlvp

Right. That process is not working correctly. The modem is not "turning off" and disconnecting from the line. Most probably one of the protection devices in the modem is damaged and shorted across the line.

--scott

Not if the modem is broken in such a way that it is stuck off-hook. Which is a very common failure; high voltage trash on the line causes the protection diodes to fail.

I'd suggest the original poster get the telco to replace the protection network in the network interface, and possibly add additional line protection at the network interface. Also check the network interface ground.

--scott