chrism@iddic.UUCP (Chris Ebaugh) (12/03/85)
I have my SCSI board all built, but when I plug it in to the ROM slots, and plug the ROMs into my board, the MAC will not boot. It just comes up with a sad macs. I have spent 5 hours Ohming the board and know that my board is wired acording to the schematics. Using an Osciloscope I have determined that the NCR5380 chip select is being pulsed active constantly and therefore the Mac cannot read the ROMs without interference. The schematics in the Dr. Dobbs showed a 3 input NOR with one input tied to ground. I replaced this with a 74LS02. This 2 input NOR has one input tied to A16 and another to the ROM Chip select, which is asserted when low. I also replaces the 3 input NAND gates, which had one input each tied to logical high. I used a 74LS00, a quad 2 input NAND gate. The output of the 74LS02 is used as input to one of the NAND gates along with A20. The output of this gate is used as the chip select of the NCR chip. Therefore, when ROM chip select is low(selected) and A16 is low the output of the 74LS02 is high. If A20 is then asserted the output of the 74LS00 should be low and therefore the NCR chip should be selected. The 21st address line, A20, controls the odd megabytes of address space. 1 meg, 3 meg,...,etc, (in HEX 100000, 300000, 500000, etc). Since the current ROMS sit at 400000-47ffff, A20 should never go high when booting the MAC. Does anyone know what is going on, or where I have gone wrong!!!!????!!! Any help would be appreciated. Thanx in advance, Chris Ebaugh Tektronix, Wilsonville, Oregon tektronix!iddic!chrism .