Tandy Portable Disk Drive Cable Revealed (c) Marty Goodman MD, Sept. 1989 I recently picked up a Tandy Portable Disk Drive (TPDD) at a Ham Swap Meet for $10. It came WITHOUT the usual black 8 pin dual-in-line to DB25 cable needed to connect the drive to a Model 100 / 102 / 200 / NEC 8201. I figured... what the heck... it's worth a gamble... I own such a drive already... if I am lucky and this works, I will have a spare I went home and, using the cable that came with my original TPDD I ascertained that the drive I had bought was in perfect working order. Fine. Now... all I needed was a CABLE for the damn thing. I called Tandy National Parts, and after considerable difficulty, ascertained that the catalog number for the needed cable was AW-1028 and they would gladly sell it to me for $25.09 plus shipping. TWENTY FIVE DOLLARS for a durn CABLE!!?? And here I had no more than ten dollars invested in that spare drive, anyway! I dedcided to make up a cable for myself. The first difficulty was that the 8 pin dual in line connector used to hook the cable to the drive is very non-standard, and I had not a prayer of finding a source of those. But, 15 minutes spent with a hacksaw, a 10 pin dual in line, and a soldering iron (to melt and weld the plastic of my hacked up former 10 pin dual in line connector and its strain relief) sufficed to adequately clone that connector and attach it to a length of 8 conductor ribbon cable. The real problem came when I tried to "buzz out" the connections for the wires of the cable using an ohmmeter. I kept getting weird results... strange resistances, and some lines behaved as if they had DIODES in them! DRAT! The buggers must have put some CIRCUITRY into that cable! I then looked in the manual for my Tandy Portable Disk Drive. Like so many other Tandy product manuals, they had very conscientiously and thoughtfully included a schematic diagram for the drive. BUT, there was NO diagram for the required (and normally included) conneting cable! Drat! Foiled again! So... I teased apart the hood and the DB 25 connector the hood to (which seemed like the only logical place they'd try to hide any circuitry. and, sure enough, there was a blob of what appeared to be hot glue all around a little CIRCUIT BOARD inside that hood! About an hour spent destroying the hood to free up the whole mess and carefully, tediously removing the hot glue using soldering irons and awls and screwdrivers and exacto knives finally resulted in my having the circuit board totally revealed. 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. Some probing with an ohmmeter revealed that these things behaved like diodes across some leads, and like a resistor across others. I never DID exactly identify WHAT the components were, but using a diode and a resistor I was able to get a circuit to replace each of them that resulted in a WORKING cable. After a total of about five hours on this project (meaing I was getting virttually minimum wage, even if you assumed all components were free) I had my spare cable. But I ALSO had the knowledge of HOW the cable was made, and so the ability to make more or repair the ones I had. And here is that information: Note that while this is a serial cable, all signal levels are TTL level, NOT normal RS232 level! This has to do with the way the signals are generated inside the Tandy Portable Disk Drive. (By the way, to deal with these TTL signal levels, the folks at Lap Dos had to provide a special cable that has inside it a CMOS level converter to convert in both directions the signal levels to RS232 level, so it can be hooked to a normal PC type RS232 port. The folks at Lapdos cleverly used some diodes and capacitors, by the way, to POWER that level converter off the voltages found ON the pins of the RS232 port. Very smart! Very sneaky!) TPDD 8 pin Dual In Line DB 25 RS232 connector wires connector signal direction ------------------- ---------- ---------------- gnd 1 ------------------------------------- 7 gnd CTS 2 ------------------------------------- 4 RTS to TPDD DTR 3 --------o--->l----------------------- 6 DSR from TPDD l 22K resistor l gnd RTS 4 ---------o--->l----------------------- 5 CTS from TPDD l 22K resistor l gnd DSR 5 ------------------------------------- 20 DTR to TPDD *TXD 6 ---------o--->l----------------------- 3 RD from TPDD l 22K resistor l gnd *RXD 7 -------------------------------------- 2 TD to TPDD 8 not connected 1, 8-19, 21-25 all (inclusive) not connected Note that for the DTR, RTS, and *TXD lines from the TPDD, there is a 22K resistor going to ground (pin 1 of the TPDD and pin 7 of the DB25), then a DIODE pointing its anode TOWARD the DB25 side of the circuit. (The little "o" is used to signify a 3 way connection point in this ASCII graphic). I used Radio Shack Schotty diodes (cat no. 276-1165) because Schotty's have ultra low voltage drop across the diode in the forward direction. Tho I SUSPECT any durn diode would do in this circuit. This cable / circuit worked perfectly between the TPDD and my IBM PC clone using LAP DOS, and between the TPDD and my NEC 8021 lap computer (a "sister machine" to the Model 100 / 102). Happy Hacking! ---marty