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I used to work at an ISO in 1997 and we tested Mototola Sufr 28.8 and USRobotics Sportster 33.6 modems this way, just a short pair between them back-to-back. ATX1D on one side, ATA on the other, connection established.

However, recently a friend of mine tried to use the same trick with two USB modems from Trendnet and it failed. It seems they either don't have oomph enough or they rely on 9vdc off-hook carrier voltage to work.

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Cool! I was an intern with the modem folks at Motorola Codex back in the 80s; I probably worked with some of the folks who did the data path on that one.

(hmm, actually by that time it's likely that all the Codex tech had been killed, and your modem came from the Motorola UDS folks in Huntsville. Makes sense - Motorola Codex couldn't build a $500 modem to save their lives, never mind a $100 one, and a lot of the folks interested in building consumer modems left to found Zoom Telephonics)

> 9vdc off-hook carrier voltage to work

In the US it's nominally 48v, although it drops to a small fraction of that with any significant (or even fairly insignificant) load.

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