Since I had cracked open a black serial TI-Graph Link cable to salvage its 2.5mm 3-conductor jack, I thought it would be interesting to see how its circuit worked. The black serial Graph-Link cable was the later revision with a female DB-9 connector and is only intended to work with Windows PCs. There is an earlier version of the TI-Graph Link cable which is grey and uses DB-25 and works with Windows and Macintosh computers. Some basic info on both of these from Merthsoft's Link Cables page

Black TI-Graph Link Cable (or "Black Link")
The black TI Graph-Link cable uses the control lines of the serial port to both power the circuitry and communicate with the TI graphing calculators link port. DTR (pin 4) and RTS (pin 7) are used to supply power and as outputs from the computer to the graph link. DSR (pin 6) and CTS (pin Cool are used as inputs from the graph link port to the computer.

Each of the graph-link pins use a pair of serial port I/O which is used on the inputs of LM339 (PDF) quad comparator:

  • tip: RTS/CTS
  • ring: DTR/DSR

RTS and DTR are passed through diodes CR3 and CR4 to create the supply voltage for the comparators and pull-up circuitry. Since the computer controls these and separate pins are used to send data from the graph link to the computer, at least one of these will remain high. RS-232 logic levels are +3V to +15V for asserted and -3V to -15V for deasserted. Diodes CR1 and CR2 keep the inputs at most ground level to the comparator. The voltage levels used will vary a lot between computer serial ports. USB-serial adapters often use lower +5/-5V or lower.

The KiCad schematics are included in a repository for preservation.

This is very interesting and a great way to preserve this cable's specs.
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