wiretap wrote on 2021-08-29, 03:04:

Here's the first rev of the first board soldered up. I'll setup a system to test it on and flash a BASIC ROM for testing. This i […]
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Here's the first rev of the first board soldered up. I'll setup a system to test it on and flash a BASIC ROM for testing. This is really the first day in about a week I've been able to have 20 minutes free.. haha. I've been covering for my manager at work while he's on vacation and it's been some lovely 18hr work days. I hope to have Monday free to get this tested, but no guarantees. My wife starts back to college Monday, so I'll have to keep my 4-year old occupied as well. Busy times 🤣.

Thought I had some 20-pin sockets for the 688 chip, but apparently not. I'll have to order some.

Hi! Would you be so kind and share the BASIC ROM you've used? I cannot find working one... Thanks!

I built this cute little card, but ran into a problem, the address selection jumper doesn't work, the address is always D800h.

Thank you, this card is very useful. I built one for my 386 which doesn't support LBA, to use the XTIDE BIOS instead. Works great!

I didn't have the binary comparator chip in a DIP package in my parts bin, so I dead-bugged a SOIC. Also the resistor footprint seems a bit too small for the common 1/4W resistors.

What is the largest rom that can be used this way? Is it possible to have one that boots a OS? Like freedos or dare I say. Windows?

A rom/cf card would probably be more practical

But as a thought experiment.

Having dual rom, that had xt-ide and memtest would be cool also.

This is a very simple card with no mapper chips.

As such, the largest contiguous block allowable would be 1 full segment, or 64k.

The IBM PC specification only allotted for 384k of "adapter rom region" between 640k base memory and the top of the first megabyte.

ROM-DOS cards use a method called "bank switching", which use a small "window" in this adapter region (usually 32k or 64k in size), with some means of remapping which part of the ROM-DOS card's disk image is mapped to this window, usually by writing to an IO address with an instruction word.

Again, this card is very simple, and has no mapper. 64k is the theoretical maximum.

Reminds me I still need to build this..

Sphere478 wrote on 2023-11-25, 04:41:

Having dual rom, that had xt-ide and memtest would be cool also.

Dual rom cards are availabe, as open source DIY or can be bought from Monotech:
* https://github.com/monotech/ISA-DoubleROM
* https://monotech.fwscart.com/product/doublero … le-rom-isa-card

Depending on the size of the option roms you wish to have installed, burning two or more into a 64k rom is a possibility.

I made this card and had a strange problem, when using 28C256, the address is always D800, after changing to 28C64 or 27C64, the address is always DE00, the jumper does not work, can anyone tell me why this is? Maybe I should replace the 74LS688 with the 74HCT688?

PS:
Something magical happened, I got a few pieces of MM74HCT688N, replaced the SN74LS688N with it, and then everything worked fine, the address jumper worked correctly, 0xC800, 0xD000, etc. were available.
I'm using an EEPROM PCB, 28C64B.

appiah4 wrote on 2023-11-25, 07:47:

Reminds me I still need to build this..

OK, so I got around to building it:

A few questions:

1. I see there are polarity markings for the capacitors. I used 100nf ceramics, and I see other builders also have done the same. I am thinking this should not be an issue in a DC circuit, right?
2. I'm not sure I get how the memory location jumpers ought to work. I need to look at my PC's memory map and locate 64K of free memory at a specific location, then jumper the card accordingly, right?

Cheers,

Ideally, to avoid having multiple devices trying to drive the bus, you want a region with nothing mapped in it. (Even RAM).

There are various tools to visualize the 1st megabyte of memory, but msd is stock in dos6.22

Run it with no memory managers.