Question about modems

Frank McConnell fmc at
Wed Nov 13 12:03:25 CST 2019

On Nov 13, 2019, at 6:40, allison wrote:
> On 11/13/19 9:17 AM, geneb via cctalk wrote:
>> On Wed, 13 Nov 2019, Jim Brain via cctalk wrote:
>>> Did Hayes modem really do that?  I thought most later modems self
>>> detected parity and speed and thus would have switched both the comm
>>> on the serial port and the data sent to the other side in the same
>>> parity (if the terminal was 7E1, the modem would configure as 7E1 and
>>> send 7 bit data to the other side.
> No, well maybe a few of the winmodems did.
> Generally all the modems I have (ISA,S100 and external serial) you need
> to set the baud rate, parity and word length to match the modem (is it
> has at or similar protocal) and also to match the other end (usualy the
> same).

I remember old dumb modems that just turned 1s and 0s into different 
frequencies on the audio channel.  Think Bell 103 here, and acoustically
coupled modems where your fingers do the dialing.  No concept of parity
in the modem, it was just another bit passed through.

Anyway, maybe this is an authoritative reference:

Section 1.4, page 1-53 (PDF page 64 of 160):

(begin copy-pasta)

Modem commands begin with an AT prefix that gets the modem's attention.
The speed and character format at which the DTE sends this prefix tells the
modem the speed and format for responding to commands, and at which
speed to attempt the connection.

(end copy-pasta)

Similar language in the July 1991 version of the manual (also on bitsavers
in that directory).

-Frank McConnell

More information about the cctech mailing list