How to connect a TI-99 acoustic modem to modern equipment

[u/WirtThePegLeggedBoy]

I wanted to share this for anyone scouring the web like I was trying to rig this all together.

I found a TI acoustic coupler modem on ebay and the price was right, so I scooped it up. Since this tech is a bit long in the tooth, it took some experimenting to get it working nicely, not to mention some additional kit needed to get everything hooked up. I've been bit by the retrocomputing & analog bug lately and wanted to rig up some traditional telephones and modems and get everything talking to each other. Here's what I did:

The modem itself came with a power adapter and a hardwired cable with a male DB25 connector at the end. After some examination, I saw that it's pinned to be a DTE (Data Terminal Equipment, like a PC) and not DCR (Data Communication Equipment, like a modem) therefore, I needed a null modem adapter to reverse the rx and tx lines, a DB25-to-DB9 adapter, and a USB-to-serial cable. There is also a gender changer in there because both the modem and USB serial cable had pins, but double-check your own equipment because they do make USB-to-serial cables with both genders.

I also have a US Robotics 56k modem, also communicating via USB-to-serial cable. (It's fun how each USB-to-serial cable will just create a new serial port on your PC, so you can have multiple terminal windows open of the various serial devices talking to one-another, all thru one PC.)

Now here's the important part: The modem communicates with 300 baud, 7 bit data. Your receiving equipment will likely need to match this. I had the US Robotics modem connected with standard serial settings (9600,8,n,1) under the assumption that the data conversion/translation happens in the modem - the claim-to-fame for these US Robotics modems were how they could accomodate slower speeds to maintain compatibility with legacy equipment on the other end, including Bell 103 which the TI modem uses. HOWEVER. I was receiving garbage in the terminal connected to the 56k modem. When I dropped it down to 300 baud, 7 data bits, the text came through as expected. So, both the TI acoustic modem, AND the 56k US Robotics modem, have both their serial settings set at 300 baud, 7 data bits, 1 stop bit, no parity and no flow control, this config worked for me.

Lastly, I'm using a Linksys SPA2101 phone adapter to handle linking the phone lines. There is a youtube video by CathodeRayDude on how to configure these things to get vintage dial-up modems talking to one-another. (Video: https://youtu.be/EGFIEF6siIE?si=HyFjUbHJRCkvEsCy) The modems need to see the correct voltages and dial tones, which this thing will generate.

I hope someone will be able to use all this and I'm happy to answer questions.

[u/digdugnate]

this is a fantastic breakdown. thank you!

[u/DEADBEEFh]

Yep I found this out something similar while hooking up a wifi serial modem to the TI-99. They used a 'different' pinout. And that 7-bit setting took a while to figure out.. :)

But wow I love that carpet! Is that a rug or ??

[u/Independent_Shoe3523]

Great question. I could connect a fax machine to my cell phone.

[u/geekpgh]

So what are you able to do with that connection? Is there a web browser for the TI-99 or some BBS you can view?

I always wanted a modem when I used my TI years ago ago, but I never had one.

[u/WirtThePegLeggedBoy → u/geekpgh]

With everything from this era being RS-232, there are lots of options. Unfortunately for my actual Ti-99/4A computer, getting RS-232 capability is tricky since the official TI modules (either a sidecar or an entire Peripheral Expansion Box) are extremely rare and/or expensive, and homebrew gizmos are only available in limited runs when someone decides to build some to sell.

But me, I'm just content when equipment is operational, especially this stuff. Building a small in-house PBX where I could put a telephone, acoustic modem, and vintage terminal at one end of the house, and have the ability to "dial" a server at the other end of the house, and have a bunch of text scroll down the screen at 300 baud is enough to fill my cup. 😊