neals@midas.TEK.COM (Neal Sedell) (03/07/87)
-------- My COCO3 PAL arrived several days ago (cost was $7.50) and last night I finally got around to checking to see what had changed. I had heard that the memory contention with the 3's new registers was due to R-S "not decoding the address space enough". Since I don't get Rainbow (yet) I decided to find out for myself. After some serious ohming out of the Pak I had a schematic. Luckily the security fuse of the old PAL14L4 was still intact, and it revealed that the three outputs used are the slot select read and write strobes at $FF7F and the 74LS245 data buffer enable at $FF40-FF9F. The data buffer enable uses two terms so the space from $FF80-FF9F was quite intentional as opposed to oversight. It does not depend on Q or E which explains why my logic probe indicates active when nothing is accessing that area. The buffer enable also goes true if the CTS cartridge ROM select output or the SLENB internal address decoder disable are true. Having the old PAL figured out, I turned my attention to the new one. Funny, it has a weird part number, pLS153..... The programmer wouldn't read it as a PAL14L4 so I looked up the Signetics parts list and sure enough the '153 number was listed. Crossed my fingers and it seemed to load. I don't have a Signetics book that shows what's in that particular part, but it appears to be a PLA (Signetics Integrated Fuse Logic) as opposed to a PAL. Educated (hopefully ;-) gussing about which fuses represent which input terms it appears to be identical to the original PAL except for the ommision of the buffer enable product term for $FF80-FF9F. Nothing suprising. Now if someone would just offer a 512K upgrade board for around $20... Neal Sedell P.S. It probably wouldn't be a good idea to reply to me directly because this is my last day on this system. I should be on the air again next week at tek_somewhere_or_other.