In 1989, a gentleman named Marty Goodman posted the his discoveries “TPDD.DO” regarding his disassembly of his Tandy Portable Disk Drive “TPDD” cable.
He noted the following:
”There were three odd little surface mount components in the circuitry for the DTR, TXD, and RTS lines going FROM the TPDD TO the DSR, RD, and CTS pins of the DB25. All other lines were wired straight thru. The three odd devices looked like litte resistors or capacitors, and had the designation “14” on them. Curiously, they had THREE leads, two on one long side and one in the middle of the other long side of each of these tiny rectangular components.”
After probing and measuring he came to the conclusion that these devices behaved like a diode/resistor combo.
Since that time the internet has grown and there is much more information available. With some quick research and checking an original cable with a device checker, I have come to the conclusion that the devices may actually be PNP Transistors with built in bias resistors (DTA114EKA).
Note that these transistors are marked with the number 14 and the combined resistance base to emitter is 20K, close to the 22K Marty found and match his description.
I have designed a cable using these devices oriented the same as I found them in the the OEM cable I have. I am designing a PCB that will mount into a DB 25 connector shroud.
Now, I know I used a DB9 for my test cable, but the PCB is designed for the DB25. I will share the pcb once I have proven the design. Here’s the layout.
To be continued…
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