VOGONS


The XL-7 TURBO 8088 XT build

Topic actions

First post, by computerguy08

User metadata
Rank Member
Rank
Member

I'm new to the Vogons forum, forgive me for any mistakes 😀

So I've acquired this gem for free a while ago from my local high school (yes, for FREE). It's a XT clone motherboard (XL-7 Turbo), likely pulled out of a bulgarian Pravetz-16 8088 PC.

GC7KMTK.jpg

It was in a really awful state before i got it, somebody aggressively desoldered all of the RAM sockets along with a few other chips. Thankfully, with some patience and some new parts I successfully put it back together... sort of. I am still missing the two PROM chips (see photo, PROM1 and PROM2), it appears to be a PAL16L8ANC from other photos online. AFAIK, these are programmable array logic chips, which unfortunately I didn't get with the motherboard (I have everything else, including the original BIOS EEPROM)

I have tried to test it with spare PAL16L8ANCs (not knowing what they did at that time) but it appeared to be completely dead (no beeps). A debug card didn't help, the POST codes may be sent on 60h instead of 80h ? (just like the IBM 5160). Later, I have attempted to "reverse engineer" the 5160 motherboard in the hope that i could replace the PROMs with TTL logic chips, but that got way over my head.

I've also found a page about this motherboard, but it is in russian, which doesn't really help me, since I don't speak russian: http://www.phantom.sannata.ru/forum/index.php … um_view&o=&st=0

The only other things i know so far is that the reset circuitry works (tested with debug card) and the CPU runs at 4.77MHz and 8MHz (with turbo switch), which I tested using my oscilloscope. Below there is a attachment with a circuit diagram of a very similar XT board from Russia if it helps.

I'm a newbie in the vintage PC world, especially in the XT era, any advice is appreciated.

If anyone can give me help or knows someone who can help me to put this thing back on its wheels, I would be truly grateful.

Attachments

Last edited by computerguy08 on 2019-02-13, 13:50. Edited 1 time in total.

Reply 3 of 26, by retardware

User metadata
Rank Oldbie
Rank
Oldbie

U31 creates the bus signals from the processor pins.
U42 does the address decoding.
Every one of both is critical.
Without them, no ROM will ever get a CS signal.

The logic is not complicated.
it was described en detail in the Intel and AMD 8086/88 handbooks.
Each one more than 1000 pages.

Reply 4 of 26, by computerguy08

User metadata
Rank Member
Rank
Member

Here is a little pinout I made last evening with my multimeter on the U31 chip (which I am missing) :

YFk8rIE.png

@retardware Not sure how to safely do that without destroying something else on the motherboard, that's why I'm asking for help.

Reply 5 of 26, by retardware

User metadata
Rank Oldbie
Rank
Oldbie

@computerguy
I don't understand the purpose of the table you did with the multimeter. The schematic tells it all.
What you need, respective have to create is a set of truth tables, which needs intense consultation of the according hardware manuals and data sheets.
If you don't know what a truth table is, I'd suggest you to put away the mobo until you found somebody else with a working board, try to read the PLA using a PAL programmer, hoping the readout fuse hasn't been blown. If the latter is the case, there is no way around creating truth tables.
More than 30 years ago I had a similar challenge, finding out the logic in the C64 82S100 PLA that liked to get ultra hot and to die, and that took some time.
So I again say, this is a thing that is a manageable challenge if you have skill and experience with logic.

Edit: Just proactively I'd like to say you that I didn't look intensely enough at the schematic to say for sure whether powering up the board without the correct PALs is safe. In particular there is the risk of multiple tristate outputs mutually shorting out. Officially the buffers usually can sustain such, but the reality shows that this can cause damage.

Reply 6 of 26, by Nprod

User metadata
Rank Newbie
Rank
Newbie
computerguy08 wrote:

likely pulled out of a bulgarian Pravetz-16 8088 PC.

It's possible, that's the board they used for the 16/ES when they outsourced the production to Taiwan in the late 80s. On older models you're more likely to see a few soviet and bulgarian chips here and there with К and СМ prefixes in their names.

The folks at the russian forum are discussing whether or not standard PC/XT roms will work, apparently that very same board was used in the ukranian Poisk-2 PC clones. From all the pics i can see those two ROM sockets are unpopulated so they may have been like this from the factory. The other unpopulated socket is for an 8087 coprocessor, as the silkscreen says. Since you already have the BIOS rom (that the person in the russian forum is missing) it shouldn't be a concern.

About the PAL16L8ANC - programmable logic arrays aren't the same thing as ROMs, they function as "glue logic" and are programmed only once at the factory.

For the PROMs you're going to have a hard time unless you have a file with what's supposed to be on it and a programmer that can handle one of these. These old PROMs often use very high programming voltages that more modern programmers can't handle.

Reply 7 of 26, by retardware

User metadata
Rank Oldbie
Rank
Oldbie
Nprod wrote:

or the PROMs you're going to have a hard time unless you have a file with what's supposed to be on it and a programmer that can handle one of these. These old PROMs often use very high programming voltages that more modern programmers can't handle.

That is, if you manage to obtain old stock... these haven't been manufactured since decades and are probably very expensive now at specialized NOS components dealers.

Reply 8 of 26, by Nprod

User metadata
Rank Newbie
Rank
Newbie
retardware wrote:
Nprod wrote:

or the PROMs you're going to have a hard time unless you have a file with what's supposed to be on it and a programmer that can handle one of these. These old PROMs often use very high programming voltages that more modern programmers can't handle.

That is, if you manage to obtain old stock... these haven't been manufactured since decades and are probably very expensive now at specialized NOS components dealers.

It's true the stock is very old (they were considered obsolete even in the 80s), but finding the IC isn't actually that hard - obtaining the program and uploading it is. Like EPROMs, a lot of different companies made them with the same pinouts so you don't necessarily need the exact same part. An N82S129N is 5$ on ebay for example. Trouble is, they are one-time programmable so you need to be extra sure you have the right file. Best way to go about it is to borrow one from a fellow nerd that has the same board and dump the contents.

Reply 9 of 26, by retardware

User metadata
Rank Oldbie
Rank
Oldbie
Nprod wrote:

Trouble is, they are one-time programmable so you need to be extra sure you have the right file. Best way to go about it is to borrow one from a fellow nerd that has the same board and dump the contents.

I think the OP could be lucky using a translator and ask on Russian vintage boards forums.
I could well imagine that he even could get the source files there, not only the binary images.
These details were communicated very openly in the Soviet bloc, as in lack of competition, there was no need for manufacturers to hide this information from customers.

Reply 10 of 26, by computerguy08

User metadata
Rank Member
Rank
Member

This is the biggest problem I have to solve, getting these binary images for the PAL16L8ANCs.

Unless there is another way around this (building an external circuit, etc..), I'm gonna have to ask on that russian forum for help getting the binary files for the PROMs, because I'm 99.9% sure nobody else that I know so far still owns such a board. (hoping google translate won't mess up my message)

Also, I do have a few spare PALs from the same school I got the motherboard. There is a small chance I have the original PALs form my motherboard, I just don't know which one is which without a reader.

Reply 11 of 26, by computerguy08

User metadata
Rank Member
Rank
Member

FIRST SIGNS OF LIFE :

155000855291078152 (1).jpg
Filename
155000855291078152 (1).jpg
File size
227.73 KiB
Views
409 views
File license
Fair use/fair dealing exception

I could never thank enough to the russian people at phantom.sannata.ru for showing me the nail in the coffin, one of the ICs had to be a PAL16R6ANC (register array) and the other one a PAL16L8ANC(logic array).

155000855291078152.jpg
Filename
155000855291078152.jpg
File size
170.8 KiB
Views
409 views
File license
Fair use/fair dealing exception

Next step is to see if I can get some sort of video output out of any of my VGA ISA cards.

Reply 12 of 26, by computerguy08

User metadata
Rank Member
Rank
Member

A year has passed since my attempt at ressurecting this board, and now that I have a lot of free time, I made some progress.

Firstly, I learned that I wasted my time with that chinese POST card (even if the ROM would output at 80h, some critical connections are missing, mainly the AEN pin). Therefore, I flashed Ruud's diagnostic ROM and ordered a LPT laptop debug card. I also unsoldered all the crappy RAM sockets and installed one row of new sockets. Once that card arrived, I found out that the motherboard does POST up to codes 32 80. This basically means that it has problems initializing the first bank of memory. At this point, diagnostics would show some useful messages on the screen, but unfortunately, it is only compatible with MDA and CGA cards. I do have such cards (two MDA), but I do not have an old CRT capable to use them.
More on this topic can be found on the russian forum here.

So my questions now are the following:

* Is there any possible way to use a conventional VGA CRT, LCD or television, without paying for one of those special boxes that do the conversion (or buying an old monochrome CRT)?
Any solution is viable (homemade circuit), and the image quality does not matter.
Since the diagnostic rom code is open source, it could be possible to add basic VGA support (TVGA8900 and alike), but I don't have knowledge for such task.

* Once I get a video adapter working, I need some advice on how to get DRAM banks functional on XT class machines. Any ideas ? memory is now fixed, replaced two dead 74S157 ICs

Last edited by computerguy08 on 2020-03-26, 09:10. Edited 1 time in total.

Reply 13 of 26, by Vynix

User metadata
Rank Member
Rank
Member

Some CGA cards (such as the stock IBM CGA card) had a CVBS (composite video) output, if you have a TV that is old enough to have a composite input (practically any random TV could work), it's not going to be pretty but at least it should work in a pinch.

Additionally, I think I know why EGA/VGA cards are both out of the window, these cards have their own ROM that needs to be initialized by the motherboard's BIOS beforehand, and the diagnostic ROMs only seem to have the "routines" (not sure if this is the exact term) to initialize a MDA or CGA card.

A few topics ago I remember reading about someone making a makeshift MDA to CVBS converter, but it was rather hacky and still a very rough prototype at this point.

Proud owner of a Shuttle HOT-555A 430VX motherboard and two wonderful retro laptops, namely a Compaq Armada 1700 [nonfunctional] and a HP Omnibook XE3-GC [fully working :p]

Reply 15 of 26, by Cyrix200+

User metadata
Rank Oldbie
Rank
Oldbie

There is this, but it has some issues, including random lockups in text mode (never used it myself): http://aknamunka.uw.hu/mdah/1st.php

1982 - 2001

Reply 16 of 26, by computerguy08

User metadata
Rank Member
Rank
Member

Guys, I think I finally did it, the motherboard does POST. Here is the video.

I managed to get it running with a Realtek RT3105 VGA card somehow.

My question is: Is this board (or any XT) compatible with the common UMC I/O floppy controller (excluding the HDD part cause that is 16 bit) ?

Reply 17 of 26, by Vynix

User metadata
Rank Member
Rank
Member

Yes should work, though you may run into some problems with the BIOS not handling well the 1.44M FDC 😀

The IDE port won't work, obviously, but that doesn't really matter.

Proud owner of a Shuttle HOT-555A 430VX motherboard and two wonderful retro laptops, namely a Compaq Armada 1700 [nonfunctional] and a HP Omnibook XE3-GC [fully working :p]

Reply 18 of 26, by computerguy08

User metadata
Rank Member
Rank
Member

Do I really need 360KB floppies ? Or is the board capable of handling DD 1.2MB floppies (and drive) ?

The BIOS is one of the generic ones found on most XT clones.