Model T to OSX Serial Communications
Mac Serial Ports
On the Mac, your only option is likely to be a USB->Serial adapter for a serial port. There's nothing special about USB -> RS232 serial ports vs. the usual PC com ports. Don't worry about it any more than you would regular com port.
One issue you do need to care about is cabling. To connect a Mac to a M100 you need a "null modem" cable, not a straight-through wired cable. I suggest a Belkin Serial Laplink cable. They are usually available on the web for <$15. You will need a DB-25 M - M gender changer adapter to go with it.
Terminal Program Configuration
Next, you need to get the line parameters to match.
That means your Mac terminal program should be set to:
Appropiate communications device name 600 baud 8 data bits No parity 1 stop bit Flow control DISABLED
I suggest starting at 600 baud and working your way up.
Model 100 Configuration
To match, in TELCOM use the STAT command:
STAT 48N1D<ENTER> TERM<ENTER>
Try It Out
At this point, you should be able to be able to type characters on one side and see them on the other. If garbage shows up, you may have a mismatched parameter.
Tuning For Speed and Reliability
The parameters given above are fairly conservative to make communication reliable with the least number of moving parts. But it's not terribly fast. If you can get flow control working, you should be able to achieve higher baud rates.
Once you have some success with the setup recommended above, you can experiment with increasing the baud rate and turning on software flow control (XON/XOFF) on both sides.
Straight ASCII transfers work but more feature-filled systems exist. If you have a REX, dedicated TS-DOS ROM chip, or wish to bootstrap TEENY, you can directly transfer text and binary files and perform those transfers reliably at very high speeds.
To do so you need to use a TPDD (Tandy Portable Disk Drive) emulator. There are two which are compatible with OSX:
These applications let serve files to your M100 from a directory of your choosing on the Mac. The transfer operations are under direct control from your Model T keyboard. That means once DLPlus or LaddieAlpha are started, you "drive" completely from the M100 side