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Lo-Tech ISA ROM Board trouble....

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First post, by Predator99

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I open a new thread as I think I need some assistance with my Lo-Tech ISA Rom-board...
ISA card with ROM at F000:0 ?

Summarized:
Unfortunately I didnt see I got an Rev.3
https://www.lo-tech.co.uk/wiki/Lo-tech_ISA_ROM_Board

while ordering parts for the older one.
https://www.lo-tech.co.uk/wiki/Lo-tech_8-bit_ROM_Board

This is my board after assembling:

IMG_9692.JPG
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Problem: Regardless of the DIP setting, the ROM appears in a 32 kb window at E800:0 or also C800:0.... 🙁

I have the following differences to the Rev. 3 bill of materials that can potentially cause this issue:

1) SN74F521N instead 74HCT688N: are exactly the same, e.g. according to
http://www.ti.com/lit/ds/sdas228a/sdas228a.pdf
"′ALS688 Are Identical to ′ALS521"

Therefore it should work?

2) 74LS33N missing:
Only needed for XT-slot-8

3) 7-pin instead of 8-pin resistor network:
The "empty" Pin 8 is connected to DIP-1 which switches the XT-slot-8 on and off. When the DIP is "off", there is no connection to anything else. Therefore this shoulnt be an issue?

4) Resistor network itself:
A "10k Bussed Resistor Network - Bournes 4608X-101-103LF" is required:
https://www.mouser.de/datasheet/2/54/600x-776645.pdf

I checked with a spare of my 7-Pin:
- There is 10k between Pin1 and each of the other 6 Pins
- There is 20k between the other Pins, e.g. Pin 5 vs. Pin 6
--> this should be the correct one

5) I use the board with a SST-PH29EE010 instead of SST39SF010A. Both have same capacity and pinout. I am programming it in my TL866 and it can be read as SST39SF010A in the TL866. It is also read in the Lo-Tech board, but at the wrong adress as mentioned above. Theerfore this is also no problem?

Also re-checked all solder-points, seem to be OK.

I also assumed the SN74F521N is damaged. Unfortunately the 521/688 are not supported by the TL866 for Logic-IC test. I have in total 5 of the SN74F521N and also tested another one, but its the same...

Anybody have an idea where to start..?

Thank you!

Reply 1 of 26, by bakemono

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Can't see any problems. I'd check the signals going from the switches to the comparator. If they look right then assume the comparator isn't working and try another one.

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Reply 2 of 26, by root42

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Show us the backside! That's where all the solder problems might be.

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Reply 3 of 26, by Predator99

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Thanks for your feedback!

Nothing to hide on the backside: But I have to admit, some points look "weaker" on the photo than with my eyes:

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I also came to the conclusion that the only possibility is the comparator. As written, I already tested another from the same lot. In the meantime ordered a new CD74HCT688E...I hope this will be the solution.

Reply 4 of 26, by matze79

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I also would solder the "weaker spots" again.

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Reply 5 of 26, by keropi

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yep use a little more solder on the weak spots just in case

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Reply 6 of 26, by root42

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Seconded and thirded. I see a lot of empty spots on the populated pads. I think you are a little low on the solder for those.

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Reply 7 of 26, by gdjacobs

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Many of the joints do look a little dry, although none look over filled or blobby. Just ensure everything has proper fillets, then clean that flux off.

All hail the Great Capacitor Brand Finder

Reply 8 of 26, by Predator99

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OK, I was rather surprised how bad this looked on the photo...maybe I need some glasses.

So I put some more solder on all points...:

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Indeed, something has changed: Now I see 64k at E000:0, but still no change when switching the DIPs. So this was only part of the problem...

Reply 10 of 26, by stamasd

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Cleaning flux is for chumps. 😀

(actually I do recommend cleaning the ISA connector to make sure there's no contact problem there)

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Reply 11 of 26, by Predator99

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This card drives me crazy... 😢

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CD74HCT688E arrived. Still the same. ROM is at E000 all the time.

Flux doesnt matter at the moment and contacts are clean. Also tried different slots...

Reply 12 of 26, by keropi

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ca you check with a multimeter the socket pins in case one next to the other makes contact?

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Reply 15 of 26, by Predator99

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Last hope was to assemble another board...did this and use another EEPROM - AT29C020 this time.

It exactly the same. DIP settings dont matter, ROM is at E000.

At the moment I conclude that these PCBs have an issue....

Reply 17 of 26, by Imperious

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According to the schematic Pin 8 of the resistor network goes nowhere at all, not even to dip 1.

I wonder if Your motherboard bios is the problem instead of this board.

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Reply 18 of 26, by Predator99

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I did most of the tests in a VLSI 80286. But I also tried a Suntac 286 and a Pentium Pro...

At least in 286s, there should be no BIOS fetaures that relcoate the ROM.

I also noticed (again) that the ROM is moving in 32KB mode, but in most cases not to the location where it should be.

Reply 19 of 26, by Predator99

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To make it more systematic I tested some jumper settings:

32KB Address Mapping

Base Sw3 Sw4 Sw5 Sw6 Sw7 Sw8 result (ST62)
8000h* On On On On Off On
8800h* Off On On On Off On
9000h* On Off On On Off On E800
9800h* Off Off On On Off On
A000h On On Off On Off On
A800h Off On Off On Off On
B000h On Off Off On Off On E800
B800h Off Off Off On Off On
C000h On On On Off Off On
C800h Off On On Off Off On C800 and E800
D000h On Off On Off Off On D000
D800h Off Off On Off Off On
E000h On On Off Off Off On E800
E800h Off On Off Off Off On E800
F000h On Off Off Off Off On
F800h Off Off Off Off Off On

64KB Address Mapping
8000h* On On On On On Off
9000h* On Off On On On Off E800
A000h On On Off On On Off E800
B000h On Off Off On On Off E800
C000h On On On Off On Off
D000h On Off On Off On Off E800
E000h On On Off Off On Off E800
F000h On Off Off Off On Off E800

As you can see, result is in the ST62 board in most cases E800. It shows only 32kb in all settings. I tried both card with the different EEPROMS.
I rember in the VLSI-board it was at E000 with 64kb when using the 64KB Address Mapping.

Thats strange and no system in it.

I have a VGA and an XT-IDE ROM installed, but no other drivers. I tested with debug.exe (D XX00:0) to see the where the window is.