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TRS-80 Acoustic Modem

J

JoJo_ReloadeD

Experienced Member
Joined
May 12, 2007
Messages
120
Hello all :)



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I have this one, and I'm trying to get it to life.

Got a 20-volt power supply, and it looks ok (altough back sign reads it works at 24v), can send bytes at 300/n/8/1 and I literally hear them...


... but modem does not answer to classic hayes commands, so I can't dial, answer...

Can it be broken?
 
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From your description, it sounds as if the modem is working perfectly :) It will NOT respond to Hayes commands. It can't dial. AFAIK, it's a modem, in the strictest sense: it modulates and demodulates, purely analog.

Get your nice old bakelite phone out. Dial the number (by hand). Wait for the whistle of the other modem (assuming it's old enough, too). Place the handset on the cups. Congrats, your connection is established. To hang up lift the handset off the cups and hang up.
 
I don't think a modern (<10 or even 20 yrs old) will connect to such an acoustic coupler. Modern modems negotiate to set rates between each other, and usually just give up of slower than 1200 baud. (You might be able to send Hayes commands to the modern modem to force it into "Bell 103" mode, but frankly I'm guessing only)

Ideally, you have another acoustic coupler (or modem of similar vintage) on the other end. Just make sure one is set to Originate and the other to Answer.

It is possible of course that the oscillator(s) have drifted off the standard frequencies. You would have to check that with an Oscilloscope.
 
mark66j said:
As I recall these acoustic couplers often did not work very well even back then. You probably need to make sure the phone handset fits well in the cups (which probably only old Bell phones will).
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Indeed, you will probably need one of these (or the wall-mount equivalent) to get an exact match with the handset. Even a modernized equivalent with the squared-off (rather than rounded) mouthpiece and earpiece won't fit into the coupler correctly.

Model_2500_Telephone.jpg
 
mark66j said:
As I recall these acoustic couplers often did not work very well even back then. You probably need to make sure the phone handset fits well in the cups (which probably only old Bell phones will). There are probably very few modems in service that can handle 300.
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Actually, you might be surprised by how many modern modems (and ISPs for that matter) do still handle 300.
 
Thank you for the answers :)

I'll try to get a modern modem in bell 103 mode by using the hayes commands.
 
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