Mega 4 two chip TOS with no 74LS11

[satjanand]

Hi guys, this is my first post. I've been coming here for a while now, and this forum has helped me with a lot of wisdom But now there is something I can't find the solution to. I have tried to find the answer on this forum and elsewhere, but I can't crack the case. So now I'm counting on your help.

I have a Mega ST 4 that runs a set of two chip TOS 1.02, with no W4 and W2 and W3 set to 2-3. My board is a C100167 Rev. 5

Mega 4.jpg (366.02 KiB) Viewed 3317 times

Now, according to the schematic for C100166 rev 8 (which actually shows my rev on the silkscreen) the configuration for different ROM types looks like this:

schematic.JPG (25.12 KiB) Viewed 3317 times

And according to this very table, my two chip TOS configuration should not work, since I have no 74LS11 installed in U12. That is puzzling on its own.

Since I don't mind why it works, as long as it works, I went on to upgrade the TOS. I wrote TOS 1.04 on six M27C256B ROMs, but when I configured W2, W3, W4 as in the table (1-2, 1-2, and short) and leave U12 unpopulated, I get a white screen with nothing initializing. When I remove the jumper from W4 (and leave W2 and W3 on 1-2) then the floppydrive initializes but I get two or four bombs.

So although it is interesting to know why my two chip configuration works with U12 unpopulated (because the solution could be in there) I am more interested in how to get my system working with a six chip TOS 1.04. Could it be that the W2/W3/W4/U12 configuration is different in my rev. 5 board? Or is there something else than W2/W3/W4/U12 that could cause this?

I hope someone can help me get this puzzle solved?

[exxos]

You're right, it does not make sense..


2 CHIP TOS

2.JPG (140.47 KiB) Viewed 3310 times

6 CHIP TOS

6.JPG (104.82 KiB) Viewed 3310 times

Chip selects from the GLUE go via the LS11 to generate the chipselect signals for the ROM.

However, either this board has some other mods under it somewhere ?

Or The chipselect is just dropping low enabling the ROMs all the time. Workaround for this behaviour is because the address lines are connected for output enable for the chips. So I guess it could possibly "work" but is relying upon a rather odd fault condition for it to happen and I would assume it would not be reliable operating that way.

[satjanand]

So I guess it could possibly "work" but is relying upon a rather odd fault condition for it to happen and I would assume it would not be reliable operating that way.

So I have ordered a new 74LS11 in your webshop yesterday, and I am wondering what's going to happen when I populate U12 in this two chip configuration.

Here's the rest of the board. No mods that I am aware of. Anything I could do to rule out any odd fault conditions at play here?

IMG_20211105_201423.jpg (413.78 KiB) Viewed 3299 times

Thanks for your help (and webshow ).

[exxos]

So I have ordered a new 74LS11 in your webshop yesterday, and I am wondering what's going to happen when I populate U12 in this two chip configuration.

If you have the jumper set correctly for 2chips and the LS11 in place, it should all work fine.

[satjanand]

So I have ordered a new 74LS11 in your webshop yesterday, and I am wondering what's going to happen when I populate U12 in this two chip configuration.

If you have the jumper set correctly for 2chips and the LS11 in place, it should all work fine.

Alright, will post back once your shipment comes in.

[satjanand]

Update:

The 74LS11 did not affect behaviour: jumpers set for two chip TOS works, but set for six chip TOS: system won't boot, white screen, no floppy action.

I did, however, find this configuration in the bottom side of the motherboard. Any Idea what it's for? And if this could be why my two chip TOS configuration was operating with no 74LS11 installed?

All was connected to a switch on the back: flipping it off made the Mega ST hang at boot with white screen and no floppy action. I did try to see if this switch could be the answer to my six chip failure, but no success.

IMG_20211123_171257.jpg (396.58 KiB) Viewed 3145 times

[exxos]

I have no idea what that's supposed to be doing. Someone did a right hack job on it for some reason. I can only assume the chip is a gal and someone tried to bodge a decoder for TOS206 ?

[satjanand]

What would you think that would happen if I'd take that "old 2" wire and connect it to "old 6" and try again?

Screenshot_20211125_004522.jpg (175.83 KiB) Viewed 3139 times

[exxos]

What would you think that would happen if I'd take that "old 2" wire and connect it to "old 6" and try again?

Screenshot_20211125_004522.jpg

IMHO I would just remove it as it could be causing problems .

[rubber_jonnie]

Ok, so looking at your pictures above, none of the jumpers are correctly set for 6 chip TOS.

W2 & W3 need to have pins 1 & 2 jumpered and W4 needs to be installed.

All of your pictures show W2 & W3 pins 2 & 3 jumpered and W4 as open, as below taken from the most recent picture in this thread:

Screenshot 2021-11-25 at 08.57.18.png (996.84 KiB) Viewed 3114 times

Do you have a current picture of the jumpers?

Here's a link to my upgrade guide for reference: https://www.exxosforum.co.uk/forum/viewt ... lit=6+chip