Question about modems

Jim Brain brain at
Fri Nov 15 21:03:13 CST 2019

On 11/15/2019 8:09 PM, Fred Cisin via cctalk wrote:
> What do the various Hayes modemS do when you type
> "aT" ?

I can speak to all modems, as I gave away most of mine.

But the one I have right now has the following behavior:

AT -> OK

at -> OK

aT -> CR, no LF, essentially echoing back the CR that was sent

At -> same


aaaaaaat -> OK

Aaaaaaat -> At behavior

aAAAAAT -> aT behavior

AaaaaaT -> OK (AT found)

aAAAAt -> OK (at found)

abcdefgAT -> OK (AT found)

abcdefgat -> OK (at found)

AbcdefaT -> aT behavior

aBCDEFAt -> At behavior

The rule seems to be:

once you find an a, save it.  If you find more as, just ignore, but if 
you see a non a non T, reset.  Once a t is found, check case with saved 
char.  If same case, detect parity and continue with command.

> What do they do when you used mixed case?
> And, are they all the same?  (as mentioned before, "Hayes Compatible" 
> was never completely defined.)
I have no idea.  Guess I'll have to create a small text file and have 
people pipe it into modems and see what comes out.

It's been a cool exersize, and sorry I annoyed so many people with the 
trivial details of modems :-)


Jim Brain
brain at

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