bill@hp-pcd.UUCP (bill) (01/28/86)
You can't just replace the crystal in an XT and expect it to work for one main reason: some of the video timing is derived from the same crystal, and *must* run at 14.318 MHz. If you really want to speed up the XT, I suggest you get a copy of the IBM PC-XT Technical Reference Manual (it contains logic diagrams). Also, don't expect miraculous speed improvements: a lot of the system timing becomes pretty marginal at faster speeds (like talking to plug-in RAM). The fastest we could push an XT and get fairly good reliability was with a 16 MHz crystal (yielding a 5.33 MHz system clock as opposed to the normal 4.77 MHz). At 17 MHz things were still pretty reliable, but occasional flakies showed up in some applications. At 20 MHz the thing will only talk to system board RAM; it seemed to work OK for things like Flight Simulator, though. We never really tried a faster processor; I think we were constrained by buffer delays talking to plug-in RAM. If you really really really want to experiment, I'll give you some "suggestions". However, I don't take responsibility for anything you do to your machine, and it'll undoubtedly void your warrenty. In order to speed up the XT, you must add a second oscillator to the system board. One way to do this is by replacing the 8284A clock generator (located near the edge of the board, next to the 8088) with an 18-pin DIP socket, and then building up a small card containing *two* oscillators (the original 8284, and a new one with another crystal) that can fit into the socket. If you do this, you'll have to make the 18 "pins" extending from the new card down to the socket fairly long (there isn't much area) so as to raise the new board up somewhat above the 8088; this way the board can be a little larger and lie parallel to the system board and about 1/4" above the 8088. Warning: the original 8284 is probably soldered in place, and can be *very* difficult to remove in order to replace it with a socket (don't be upset if it gets destroyed during removal). One of the 8284A's on the new card must be wired directly to all of the socket's pins except for pins 2, 12, 16, and 17 (PCLK, OSC, X2, and X1). Wire the X1 and X2 pins (17 and 16) to the faster crystal (and also, if it's an NMOS 8284, through 510 ohm resisters to ground; if it's a CMOS 8284, through 20 pF capacitors to ground). Leave OSC and PCLK unconnected. This chip becomes the new system clock generator. The other 8284 must be an NMOS part (assuming the 14.318 MHz crystal that's still on the system board has 510 ohm resisters attached to it). Connect it to the socket's pins 2, 12, 16, and 17 (to generator PCLK and OSC using the system board's crystal). Connect pins 1, 4, 6, 9, 13, and 15 to socket pin 9 (ground). Connect pin 18 to socket pin 18 (+5V). This chip becomes the 14.318 MHz oscillator for the video card. Like I said before, don't expect miracles. One of the big reasons the PC-AT is so much faster has nothing to do with the clock. Apart from talking over a 16-bit bus, the processor in the AT simply executes many instructions in far fewer clock cycles than the corresponding instructions for an 8088. If you can find meaning in Norton's SI utility performance index, our XT got a rating of 1.1 with the 17 MHz crystal; with the 20 MHz crystal, it went to 1.2. We tried some games at 20 MHz (Silent Service and Flight Simulator), and the speed improvement was noticeable; at 16 MHz, you really can't tell much difference from an unmodified machine. >>> You take full responsibility for any changes you make to <<< >>> your machine. Please don't send me mail. If you mess up <<< >>> your machine, I don't want to know about it. :-) <<< bill frolik hplabs!hp-pcd!bill