Fax handshake protocol troubleshooting

I am trying to set up an old XT computer with some MS-DOS fax software so it can send and receive faxes. I have used the same software in another machine some years ago and it worked fine. The modem is a 33KB Conexant and seems to work fine otherwise. When trying to send a fax the process is interrupted and I am trying to troubleshoot. Following the fax handshaking protocol we have:

1)- Answering fax answers call and sends initial ID message, using V.21 300 b/s modem signal, containing info about receiver’s capabilities.

2)- After successfully receiving the above the calling machine sends a message, also via a V.21 300 b/s signal, which directs the called machine to configure itself appropriately to receive the FAX image.

3)- After the set-up commands, the calling FAX sends a training signal intended to allow the receiving demodulator to adapt its channel equalizer and to otherwise initiate itself for the image transmission.

4)- If the receiver can successfully initiate itself using the training sequence, it sends a signal to the transmitter, confirming the receiver’s ability to receive. If equalizer training was not properly attained, then a message to that effect is sent. In this case the transmitter will usually attempt to reconfigure the receiver (via another set-up message), this time at a lower image transmission baud rate.

5)- After initiation and confirmation, the transmitting FAX machine sends the image.

In my case, it seems like the sending fax (XT computer) it does not recognise the confirmation signal (4) sent by the receiver or cannot agree on some protocol or something. But the receiving fax does seem to send this signal because it moves on and says it is ready to receive the image.

I have been tinkering with a few configuration parameters (speed etc) but nothing seems to work and the process always does exactly the same. I have tried sending to different receivers so I know it is not the receiving machine.

Any ideas?

Try turning the speaker off by adding M0 to the AT init string, and don’t listen in - I’ve had cases where with the speaker on, the signal wasn’t “loud” enough to establish a connection.

Set port to 38400 baud, hardware handshaking on.

That’s all I got right now…

I have tried that and still nothing. The problem seems to be with the software or configuration because I tried another modem with the same result.

What IRQ and COM port setting? It’s got an IRQ all to itself, right?

Also, try 9600 baud instead (XT might be too slow for 38.4 or even 19.2).

Another thought - is the XT too slow to build a fax page in a timely manner for transmission?

AmbushBug, I do not think it would be related with the IRQ and port settings as the computer otherwise communicates just fine with the modem. I did try tinkering with the speed an other parameters but nothing. The program does not build the fax file on the fly, rather it first creates the fax file and once it has completed that it starts the transmission. The fax file is successfully created and can be viewed an manipulated.

At times like this I wish I could monitor the serial port to see what the communication is and what is going wrong.

Just wondering; one of those FAQs mentioned IRQ conflicts as one cause of fax problems, due to the differing expectations of fax software versus terminal software.

I’m out of ideas. Do you have more info on the specific modem other than “Conexant”, since “Conexant” isn’t a modem brand but is the neo-Rockwell chipset offering?

ATI3 or the FCC number on this page?

AmbushBug, I have tinkered with pretty much everything. I do not think the modem is the problem because I have tried another modem and it won’t work either. I am inclined to think it is a software problem. but I don’t know where to go because there’s not much to tinker with there. When I used this software in the past it just worked. IThe only thing I can think of is installing the software in this computer, start in in DOS mode and see if it goes. I hate to install stuff just for tests but I think I might do that. Although, two things can happen: either it works or it doesn’t. And then what? I’m scratching my head.