JR2DRV.TXT -- UPLoaded 2/22/87 (rev.4/1/87) **** Adding a second drive to PCjr thru modification of controller **** (Excerpted from now defunct Home Computer Magazine, Vol. 4, #4, 9/84) ### !!!WARNING!!! **\ This modification will void *any* warranty that may be left on your /** ***\ machine--the WHOLE machine, I'd guess--not just the controller card! /*** ****> But since the warranty was for one year, and since only about <**** ***/ a dozen or so new jr's have been sold in the last year or so, it's \*** **/ probably not even worth mentioning--but thought I would, "just in case!"\** ### Modification consists of: -"Piggybacking" of two additional chips, on top of existing ICs; -Addition of seven jumpers to locations on controller; -"Lifting" or cutting of two pins on existing ICs; -Changing position of selection jumper on internal drive; -Making up a cable; -Addition of a simple assembly language program, and installing a special AUTOEXEC.BAT file on your boot disk. 1. Prepare and install the two chips as follows: a. 74LS175--[a quad "D" flip flop with complementary outputs]: bend up 90 degrees ALL pins *except* 1,4,8,9 & 16; push on top of another 74LS175 (right end, 1st row, below large controller chip & 555 timer), making sure pin 1 goes over pin 1! Solder the 5 pins not bent up to the same pins of the original chip. CAUTION! DO NOT OVERHEAT!! b. 7410 (NOT 74LS10 as article says)--[a triple 3-input NAND gate]: bend up ALL pins *except* 7 and 14. Push down over the 74LS08 (the first chip to the left of the FDC chip, a uPD765, in the top row), taking care to match the LS08 pins. Solder the two pins to the bottom chip, as with the 1st IC. (NOBODY who is not fairly experienced at this type of work should attempt this conversion; but I'll share my experience with you experienced folks anyway, just in case it can make things a tiny bit easier!) 2. a. Using a pair of flush-cut diagonal cutters, cut pin 1 of the MC3487--first chip, left end, second row of ICs (same row as the '175)--as close to the board as possible. Then bend the just-cut pin up--we'll solder a wire to it later. Just in case you have a problem when finished and want to check the board with a digital probe or something--this is a quad RS422 line driver with 3-state outputs. Pin 1 is an input, which is output on pins 2 (non-inverted) and 3 (inverted). Only pin 3 is used-- when it goes low, it accesses the drive (B:) ("drive enable"). b. The other cut pin isn't so easy--it's pin 13 of the 74LS04, just to the left of the '175. There's a capacitor in the way. Heat the left lead on TOP of the board (it'll loosen easier that way), and lift the capacitor up out of the way. Clip pin 13 as close to the board as possible --not so easy, be careful of pins 12 & 14!-- bend the pin up for another wire connection later, then replace the capacitor. This chip is a Schmitt-triggered hex inverter. Pin 13 is an input; the output is on pin 12. This is the "motor enable" line. Both this and the "drive enable" line were disconnected from the original circuitry, and will soon be fed by the new circuitry you're about to install. 3. The JUMPERS-- For these tiny things I've found that heating the end of the wire rather than trying to strip the insulation usually gives you enough bare wire to make the connection. Then, just touch the iron to the pin first, to get a little solder on it. Now all you have to hold is the iron and the wire! Lay the wire on the pin, and a second or less of contact with the iron on the two should make a strong union. The jumpers go... [1] FROM pin 2 of the '175... ...TO pins 9,10 & 11 of the 7410 (here's one place where you may HAVE to *strip*!) [2] FROM pin 3 of the '175... ...TO pin 3 of the 7410 [3] FROM pin 5 of the '175... ...TO pin 7 of the FCD chip (the big one) [4] FROM pin 6 of the '175... ...TO pins 4 & 5 of the 7410 (maybe another strip) [5] FROM pin 7 of the '175... ...TO pin 1 of the MC3487 (left end-most chip, second row--same row as the '175) [6] FROM pin 13 of the 74LS04 (immediately to the left of the '175, same row)... ...TO pin 6 of the 7410 [7] FROM pin 8 of the 7410... ...TO pin 10 of 34-pin header where the cable connects) (these pins are numbered, top row: 1,3,5,etc., from the RIGHT to LEFT, looking at component side; and bottom row: 2,4,6,etc. R-to-L). NOTE: This wire must be wrapped around the pin at least once (one more strip job!), positioned as low as possible to the plastic bottom and soldered with the absolute MINIMUM amount of solder (or the plug won't sit right). 4. Now, check the little (blue?) jumper on the internal floppy drive's board, just behind the edge connector finger contacts. It should be on DS1. Move it to DS0. On your soon-to-be-connected second drive, the jumper should go on DS1, of course! This *may* not be the same type of jumper--it may be part of a DIP header or maybe even a DIP switch! In this case, you'll have to trace out the #12 edge connector finger (use a continuity meter) to see which pin of the socket it goes to. Pull the header, DIP switch, or whatever's in the socket, first. Then, jumper the pin that you find connected to #12 to the pin exactly opposite on the socket. (In my case, it was #3-to-#12.) ALSO--this should be(*) the ONLY jumper on the header--all others should be "broken," or disconnected. (*) In special cases, it may be necessary to leave the other jumpers connected--so before breaking the contacts, make a note of which pins were connected. If you have to put the jumpers back, just a little daub of solder should do it; OR, you can discard the header and just put in short stripped wires across the socket. 5. All that's left (of hardware work) is making the cable. You can probably buy one to do the job, but it won't be as precise as you can make it! Take a strip of about four feet of 34-conductor ribbon cable, and duplicate the IBM cable bends and folds on one end. DO NOT CUT THE CABLE SHORT. You'll need a special edge card connector that does NOT have a *higher* lip on the opposite side of where the cable "enters" the connection slot, because the cable will go "through" the connector on the internal drive. (NOTE: I've found the way to get these connectors onto ribbon cable is to use a vice.) After you get that part of the cable to fit right, put another fold- UP-and-over-itself crease in the cable, TO THE RIGHT (toward where a sidecar would attach)--you'll notice there's a notch in the case that'll let it slip under the cover when you replace it (with a insistence the first time). Then put another edge connector (any ol' kind) on the free end. All you need now is the second drive and a little program, which follows. Now we're going to have to tell DOS its got another drive to address. First, the following files MUST be on your boot disk: COMMAND.COM DEBUG.COM DISKCOPY.COM and a special AUTOEXEC.BAT file along with 3 other tiny files we're gonna make: BOOT.BAT MODBOOT.BAT SWITCH.BAT You can make these files with a word processor, or with "copy con:". MODBOOT.BAT is a short ml routine that is NOT CORRECT in the original article. It should be: A 0:9080 ;assemble following code at 0000:9080 (arbitrary) XOR AX,AX ;zero the AX register MOV DS,AX ;shift the zero into the DS address register OR BY [410],40 ;load 40h into 0000:0410, changing the bits NOP ;"no operation"; haven't the foggiest WHY ;(this blank line stops assembly) G=0:9080 9089 ;run the [above] program L 0:7C00 0 0 1 ;go back to the boot disk & load the boot record G=0:7C00 ;"run" [DO] the boot (boot starts over again) BOOT.BAT is a clever little deal that runs the MODBOOT ml routine by loading it with DEBUG: RENAME switch.bat config.sys DEBUG