brown@nicmad.UUCP (Mike Brown) (06/27/87)
Well today I finally got a copy of the 50 & 60 Technical Reference Manual. We have a model 50 and were attempting to hook up a 1.44Mb, 720Kb and plain old 360Kb 5.25" drive. The jist of the story is that only the 5.25" drive will work worth a damn. The configuration only allows for 1.44Mb or a 360Kb drive in the B slot. Also, the 360Kb is listed as external. Makes sense, considering that it won't fit inside the PC. The 1.44Mb drive that we have is one of the new TEAC 55 series 3.5" drives. It is not IBM plug compatible. I couldn't get the drive to work until today, when I got the manual. I needed to see the pinouts. BTW, the IBM external adapter is used inside the PC. This adapter is basically nothing but a means to take the 3.5" drive B connector and make it available to the outside world. The tech ref manual doesn't contain the circuitry of the adapter board, but it does contain the pinouts of what goes on internally. After doing some looking around, I came to the conclusion that IBM doesn't want anything connected to the outside world except a 360Kb 5.25" drive. Why? Because two signals are completely missing and one of them is inverted. The two signals are: Pin 2 - High Density Pin 34 - Changed Disk The signal with the inverted logic is on pin 2. The TEAC drive wants to see Normal Density as the asserted low true signal. The changed disk signal is needed by the BIOS when a 1.44Mb drive is used. A read directory is kept in memory, unless the diskette is changed. If it isn't changed, the the directory in memory is displayed. Yes, the drive does appear to spin, but it is just the BIOS checking the Changed Disk signal. When a 360K drive is used, the signal isn't there, so the directory has to be read each time. So, if the Chnaged Disk signal isn't wired up and you read one diskette, change it and then do another read, you will get the previous diskette's directory. But, luck is kind-of there. When pin 34 isn't connected, it floats low, so the BIOS thinks that the diskette has been ejected, so it fails. When you pull up the signal, say with 10K ohm, then you read a directory, but as mentioned above, changing diskettes will not get you the newer directory. After a little bit of playing around, looking at signal levels, etc., I have made a little modification to the adapter board that will allow using the TEAC 3.5" drive externally. The first part of the mod requires taking out the adapter boards. You need to add two wires to the long board that goes into one of the expansion slots. The wires go between the 34 pin edge connector pin 2 and the 37 pin D connector pin 3. The other between (34) pin 34 and (37) pin 19. The other mod takes a little more work. You will need an IC in this case. I used the 74LS242 for a couple of reasons. One, it is 14 pins (explained later) and is tri-state. I suppose that an open collector part could have been used, but it looks like IBM wants to drive the line without having a resistor on the other end of Changed Disk. Without the Tech Ref manual on adapters and other goodies, I don't know what the IBM 1.44Mb drive uses for various output drivers. The tri-state is a safe chip. You will need to remove the adapter from the drive B slot. Disconnect the cable from the adapter. Get out your soldering iron, jumper wire, etc. Bend pins 7 and 14 so that the face in the opposite direction. Clip the remaining legs on the IC. Only clip the thin part, leaving the thicker portion of the leg. This will make sure that the pins don't hit the cover. Solder the two pins to pins 7 and 14 of one of the ICs on the adapter board. This is done on the circuit side of the board, so the mod IC will be sitting on the board with its legs up in the air. Now you know why I used a 14 pin IC. Add a jumper between pins 11 and 5. Since this is an inverting part, we need to double invert the Changed Disk signal. The High Density signal is only inverted once. Add a jumper wire from pin 13 to ground. This turns off the A to B- direction of the IC. Add a jumper from pin 2 of the 40 pin edge connector to pin 4 of the IC and a jumper from pin 10 of the IC to pin 2 of the connector for the ribbon cable. This is the High Density signal. Now add a jumper wire from pin 34 of the edge connector to pin 9 of the IC. Add a jumper from pin 3 of the IC to pin 34 of the ribbon connector. This is the Changed Disk signal. A 1K ohm resister needs to be added between pins 3 and 14 of the IC. This is the pullup resistor for the Changed Disk signal that is driven by an open collector part on the TEAC drive. Lastly the Drive 1 Select signal needs to be added to turn the IC on only when drive B is selected. Since I am doing this posting from at home, I am doing it from memory, but I believe the signal is on pin 12 of the 40 pin edge connector. This can be confirmed by looking at any diskette drive pinout. The famous IBM signal flipover is done for drive A. If it is wrong, I will make a corrective posting. In any event, wire it from the edge connector to pin 1 of the IC. When you solder wires to the edge connectors, do it at the very end, the end furthest from the edge of the board. You want to be able to get the connectors on. (I know it is obvious, but one must remind one when doing these things.) The modification is now complete. Put everything back together and attach your external drive, which can be any of the two types of 3.5" drives or a 360Kb 5.25" drive. The 1.2Mb floppy is not supported by the new PS series. Start up the computer with the Reference Diskette so that you can configure the system. If you get the 162 error, just press F1 to continue. You will be sent through the autoconfiguration. Say yes to the question. You are now the proud owner of a system that does a little more than what IBM really wanted. Also, a little cheaper than what IBM wants for their external drive. It doesn't look as nice, but it is very functional. I hope that this mod can be of some use to some of you. Hopefully it will keep you from re-inventing the external drive wheel. -- harvard-\ ihnp4--\ Mr. Video seismo!uwvax.......!nicmad!brown rutgers-/ decvax--/ terminus-/