View Full Version : Pc XT clone rom extension on expansion slot

July 23rd, 2017, 04:41 AM
I have a generic XT clone, CGA,640k with 8 roms sockets.
I set the roms to use 32k. I have only the bios rom (32k) installed. The other 7 sockets are empty.
Somehow this setting prevents any expansion rom (eg the 8k xtide rom) on the any expansion slot to be recognised.
If I use a 16k/8k bios in 1 socket the expansion roms (eg. xtide 8k) on the slots will be recognised.
I understand that this is good to prevent conflicting rom addresses. I cannot see this preclusion on any the pcxt schematics.
Any insights?

July 23rd, 2017, 10:35 PM
what is the address space of the 8k XT IDE rom?

July 24th, 2017, 12:10 AM
D0000h to D1fffh

The on-board sockets have no chips except the bios

According to the schematic, the buffered signals MRW,MWE, A0..A19, D0..7, *AEN* are directly connected to the expansion slots.
The empty on-board socket (at D0000h data bus D0..D7 shld be floating?) but expansion xtide rom at D0000h should respond accordingly.

Somehow if the on board rom sockets (C0000h..F0000h) ,even if empty, are all set to use 32K eproms, any expansion rom on any slots will not be recognized.

July 24th, 2017, 02:44 AM
I am guessing that your motherboard looks like the one at [here (http://minuszerodegrees.net/xt_clone_bios/X'Golden%20Board%20of%20revision%20G.jpg)].

Presumably, the ROM addressing will be per the diagram at [here (http://minuszerodegrees.net/temp/3/temp_kl8hffr209236.jpg)].

The empty on-board socket (at D0000h data bus D0..D7 shld be floating?)
Not if the motherboard has a driver chip between the data pins of the ROM sockets and the data bus. In such a case, it will not matter whether a ROM is fitted or not; the motherboard will still be driving the data bus when a ROM socket is addressed.

For example, a circuit diagram for the aforementioned motherboard appears to be in the rear of the manual at [here (http://minuszerodegrees.net/manuals/unknown/Unknown%20maker%20-%2016-Bit%20Motherboard%20-%20User's%20Manual.pdf)]. In that circuit diagram, such a driver chip exists. Sheet 4 shows the ROM data pins connected to bus XD0-XD7. On the same sheet, that bus translated to bus D0-D7 via driver chip U14.

August 1st, 2017, 07:46 PM
Thanks for the info.
And I thought I have understood the PC schematics from 30 years ago...