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Success! Following the schematics above, I was able to build a cable which is working. My fix to the source code linked above also works.. Basically anywhere there is Serial.begin(xxx); was replaced with Serial.begin(xxx, SERIAL_7E1); - 7 data bit support is built into the Arduino Serial library.

Ok so I've verified the adapter that I have does break out the pins from DB9 to DB25 the way the TI is expecting for Port 1. I've updated the source code for my wifi modem to use 7 data bits (source is based on https://github.com/jsalin/esp8266_modem) and verified that worked, yet I still can't get this thing to talk to the TI. One thing that may be a problem is that the RS232 adapter in use only brings out VCC, GND, Rx and Tx to the ESP8266. Are the other lines controlled typically by the MAX232 chip? EDIT: Well maybe I spoke too soon. I just found this diagram which differs from the one I was using. I'll give this a shot.

Thanks, I'll take a look. What if I wanted to connect up an actual 300 baud modem (with a db25 connector)? I would still need this special cable?

Thanks everyone.. where would I acquire such a cable, or will I have to splice/build one? I see a breakout diagram that was posted here Would that work?

Hello I have a PEB with an RS232 card, and a WiFi serial adapter (configured for 300 baud) plugged into it with (with a db25/db9 adapter). I start up the Terminal Emulator II cartridge.. accept the default settings basically (even parity, com 1, 300 baud). When I type on the TI keyboard I see the PEB slot flash, but I don't receive anything back from the modem. The screen is completely blank even as I type AT commands which should return some data. Any pointers?

Ah shoulda checked there first, thanks

I do have a TI with a working PEB/Disk Drive. How would I get the data from the disk transferred to my modern PC?

I have a bunch of disks for a TI-99/4A system that I got when I purchased the system from its original owner. Some disks include "GREAT TI GAMES", "TI SALES DEMO", "WIZARD'S LAIR" and "SUPER FROGGER". I'd like to back up these disks and get them into a repository somewhere. First of all, is there an exhaustive repository somewhere that I could contribute to? If not I could at least get them up on archive.org. I am surprised to see there wasn't really a large category of games under https://archive.org/details/softwarelibrary. I did find this https://archive.org/details/basic-games-ti99 but obviously that's not everything.. Second, how would I properly back up these disks? Would ImageDisk on a DOS computer with a 1.2MB drive work? If so, what settings should be used?