US Robotics PCI Modem Not Working
Posted by TheMiningTeamYT26@reddit | vintagecomputing | View on Reddit | 16 comments
I picked up a US Robotics PCI Voice modem (model 0727 -- pictured below) from eBay to build a PC-DC Server (direct connection between a PC and a Dreamcast modem), but I can't get it to connect. It shows up in Linux, and I can send it commands, but when I try to answer the call, I only hear tones from the PC side through the speaker, nothing from the Dreamcast; not it dialing, not any data it sent, nothing. It's not the cable: I tested it by connecting my Dreamcast to my PowerMac G4 and confirmed it's fine. Interestingly, I checked with my oscilloscope, and the USR modem is able to send signals over the line *until* the device on the other end goes picks up the line, after which it stops sending completely. Any advice?
(BTW If you're wondering why I don't just use the PowerMac, it's because it's an MDD, meaning it's very loud!)
ProperEye8285@reddit
Forgive me if I messup some details its been 25yrs since I was seriously into telephony. You have a voltage drop/current draw issue. Back in the day some modems pulled current to run the "phone" circuitry from the telephone line, some pulled it from the computer power. PowerMacs powered their "phone" circuitry from the PC. I suspect the DreamCast and the USR modem are both trying to draw power from your line inducer and there's not enough to run both at once. That's why the Dreamcast's dial is very faint connected to the USR but works fine with the Mac. Doubling the current output of the inducer or boosting the voltage may fix you.
0EFF@reddit
The external speaker on the USR turns off by default when a connection is made. There is a AT command to leave it on all the time.
TheMiningTeamYT26@reddit (OP)
That’s not the problem. The problem is that it doesn’t make a connection in the first place.
orion3311@reddit
You say cable...are you running a line emulator? You cant just connect two modems together.
TheMiningTeamYT26@reddit (OP)
I built a DIY cable with a line voltage inducer. I know it works because I was able to get my Dreamcast to connect to my G4 modem.
orion3311@reddit
Some modems need a dialtone, or you have to disable carrier detection first.
TheMiningTeamYT26@reddit (OP)
how do you disable carrier detection for answering a call? (Not dialing from the USR modem, but something else dialing into the USR modem?)
seismicpdx@reddit
Review a list of AT commands for that modem. You might be able to toggle the command set into the modem and save it, or your PC server software that manages receiving the call.
I used to tune these things for performance, starting with 14.4K, but it's been a minute.
"USRobotics Courier 56K Business Modem: User Guide"
https://support.usr.com/support/3453c-ref/3453c-ref-ug/at_commands.html
TheMiningTeamYT26@reddit (OP)
I can try it, but I highly doubt it’ll do anything (in this situation) because the modem is outputting a signal, it just drops to almost nothing once the Dreamcast goes off-hook. Good to know, tho. Maybe I can squeeze some more performance out of it if/when I get it working 🤔.
TheMiningTeamYT26@reddit (OP)
I wonder if this is a case of bad capacitors. The only problem is I’ve no idea what replacements I need because electrolytic SMD cap markings are super inconsistent
seismicpdx@reddit
This was produced many years before the great capacitor plague.
TheMiningTeamYT26@reddit (OP)
Good point
SnooCheesecakes399@reddit
I would not think that modem would work. The USR modems that are supported are the soft-modems. I think the one you have is not a soft modem. (I double checked my thought process with compatible modems here: https://www.dreamcast-talk.com/forum/viewtopic.php?t=2478 )
TheMiningTeamYT26@reddit (OP)
I mean that seems like a list of modems that are known to work, not the only modems that can work. This modem should (theoretically) work for this. Still good to know, tho.
BTW, I was reading the thread you mentioned and this caught my eye
"They have their own line voltage inducer." (in reference to hard modems)
I am using an external line voltage inducer. I wonder if that could be a problem.
TheMiningTeamYT26@reddit (OP)
Well the modem doesn’t put out any voltage and the Dreamcast needs line voltage to output anything so if the external line inducer is the issue, I’m SOL
OldGeekWeirdo@reddit
I wouldn't expect a modem to output any DC voltage. That was always supplied by the phone company. You need a line emulator that will supply voltage.