Hello,
I got this Turbo XT clone board from ebay. It came with no CPU or BIOS. I have put a NEC V20 on. The first sign of life I got was that using an AMI XT BIOS, partway after powering it on the turbo LED comes on, and this didn't happen with no BIOS at all.
I got Ruud's diagnostic ROM booting in it. On a (modern) ISA POST card, I get nonsensical codes like 0xbe and 0xbf that don't appear anywhere in the source. On the parallel port, I am getting the correct codes in the source with three error codes--0x86 (8253 channel 1 failed), 0x88 (8253 channel 2 failed), and 0x8a (8237 failed). Accordingly, none of the BIOSes I have tried nor this diagnostic ROM can make the PC speaker beep.
With the 8253 pulled, I get 0x84 (channel 0) failed plus 0x86 and 0x88 as expected. With an 8253 borrowed from another board, I also get all three codes. Huh? It's like the 8253 this board came with is specially blessed so that channel 0 works enough to pass the test. The Ruud ROM writes 0000h to channel 0 and waits for it to decrement to 0ffffh; can't 0ffh appear on an I/O port coincidentally if nothing is there?
I also tried swapping the 8237, the CPU, the 8288 to no avail. I tried testing with the 8237 pulled (no codes at all) and with the 8255 and 8259 pulled (no difference).
I also pulled all but 256k of the ram and have also tried with no ram at all.
The 8253 clk pins show a 1.19 MHz signal, the 8237 clk pin shows a 4.77 MHz signal.
Ideas? Might it be something with the 74 series logic?
Excuse the overhanging EEPROM which is adapted to work with the 28-pin pinout and known to work in other boards.
I got this Turbo XT clone board from ebay. It came with no CPU or BIOS. I have put a NEC V20 on. The first sign of life I got was that using an AMI XT BIOS, partway after powering it on the turbo LED comes on, and this didn't happen with no BIOS at all.
I got Ruud's diagnostic ROM booting in it. On a (modern) ISA POST card, I get nonsensical codes like 0xbe and 0xbf that don't appear anywhere in the source. On the parallel port, I am getting the correct codes in the source with three error codes--0x86 (8253 channel 1 failed), 0x88 (8253 channel 2 failed), and 0x8a (8237 failed). Accordingly, none of the BIOSes I have tried nor this diagnostic ROM can make the PC speaker beep.
With the 8253 pulled, I get 0x84 (channel 0) failed plus 0x86 and 0x88 as expected. With an 8253 borrowed from another board, I also get all three codes. Huh? It's like the 8253 this board came with is specially blessed so that channel 0 works enough to pass the test. The Ruud ROM writes 0000h to channel 0 and waits for it to decrement to 0ffffh; can't 0ffh appear on an I/O port coincidentally if nothing is there?
I also tried swapping the 8237, the CPU, the 8288 to no avail. I tried testing with the 8237 pulled (no codes at all) and with the 8255 and 8259 pulled (no difference).
I also pulled all but 256k of the ram and have also tried with no ram at all.
The 8253 clk pins show a 1.19 MHz signal, the 8237 clk pin shows a 4.77 MHz signal.
Ideas? Might it be something with the 74 series logic?
Excuse the overhanging EEPROM which is adapted to work with the 28-pin pinout and known to work in other boards.