Asymmetric serial port problems after reinstall

#1
I use a nullmodem cable to transfer data from an old PC controlling a scientific experiment (custom software for DOS). I have been using this method for the last 15 years on probably 8+ different PCs (always connecting to the same old PC). I have been using this method since Windows 7 came out also on my current PC without any problems.

Recently, I had to reinstall Windows 7 and since then, the connection is asymmetric: using terminal emulators, I can see any key typed on the old computer in my win7 terminal, but nothing I type in my win7 terminal appears on the old computer. The hardware is identical and untouched, only win7 has been reinstalled. To make sure no hardware was affected accidentally during the install, I have used two different USB-to-serial converters, a PCI serial card and two different nullmodem cables - all with no effect whatsoever. Bidirectional connectivity is required for Z-modem download of the data to the new PC. Does anyone here have the slightest idea why the serial port is now so asymmetric, after the reinstall, but worked just fine on the same machine with the same hard- and software before the reinstall?

I've been solving my computer problems myself for as long as I can think, but this problem stumps me and my colleagues here at our university.
 


patcooke

Microsoft MVP
Staff member
Premium Supporter
Microsoft MVP
#2
It's not clear whether the connection is showing as asymmetric because of different baud rate settings at each end or whether the connection is asymmetric and requiring different baud rates to be set. What baud rate settings were used at each end on the original setup and are they still the same?
 


#3
Both COM ports are set to 115200 as they have always been, no change there. I have tried other settings, but not exhaustively. I've also tried asymmetric settings, which only had the effect that the connection did not work at all.

A colleague here tried to connect a few of his computers with my (and his own) cable and found that apparently only those with comparatively old motherboards could be connected. I'm not quite sure what to make of that, especially since he uses a variety of Windows OSs, not only win7, but it may suggest that it's a specific Windows Update component that does something to the COM port of newer/compatible hardware. This made me realize that I re-installed Windows 7 with the USB-to-serial converter attached, while it wasn't attached during the original installation. Whatever that may mean.

Finally, I have since tested the connection on my 64-bit installation that I had completely forgotten to test (separate harddrive) and there, I get exactly the same symptoms.

This is totally freaking me out! Next on my list is to change the old ISA-serial card in the old PC (I think I have a spare one lying around...
 


patcooke

Microsoft MVP
Staff member
Premium Supporter
Microsoft MVP
#4
I'd be tempted to focus on the asymmetry and try each possible baud rate on the Win 7 pc leaving the old pc at 115200.
 


#5
Good idea. I tried every single setting while letting the old machine at 115200. No effect. I never saw anything on the old machine. Now I need to find that old ISA card somewhere...
 


#6
I used another old machine and it worked fine there. This means, coincidentally to my reinstall, one of the COM ports on my old machine broke. It was not a Windows 7 problem after all, just a very unfortunate and unlikely coincidence.
Thanks for the help!
 


patcooke

Microsoft MVP
Staff member
Premium Supporter
Microsoft MVP
#7
Thanks for the feedback - glad it's sorted. As far as your statement " just a very unfortunate and unlikely coincidence" goes - it's just complying with the universal law of computing - Sod's Law Rules!
 


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