Lines! So many lines!

The PX-8 display is, you might say, not really up to modern standards. It's an eight line LCD with no backlight and only a contrast control to try and make it easier to read. Even back in the day, the display was criticised as being difficult to read, partly due to the dim display but also because the text does not have 'true descenders' i.e. the letters like 'y' are contained on the one line and don't dip into the line below. Apparently, this makes things difficult to read. The reviewer clearly had never used a ZX Spectrum... But I digress..

Not exactly 4K..

To get around this a company called Oval Automation came up with a clever box called the UD80 to extend the PX-8 display to a monitor that allowed for 40 or 80 columns (the LCD is actually 80 columns anyway) but with 24 lines of text. Ooooh! A whole 24 lines of text! Luxury.

UD80 on the bottom
DS8 on the top

The unit itself is in a very pleasingly 70's brown with a military grade metal case held together by four screws. There are three connectors on the front as well as a recessed button (the button may once have had a plastic insert that has since been lost but I don't know for sure). These are:

Serial I/O - 8-pin DIN connection to the back of the PX-8

RGB - 5-pin DIN RGB connector

Aerial type socket - this is actually composite (more on that later) and not RF

Opening the thing up shows a fairly chunky transformer which makes a period correct 'buzz' when switched on. It also means I need to be careful about touching anything that could make me very sad while it is on.

What's in the box??

Inside there is a Thompson EF9345 'semi-graphic microprocessor' that can generate 8 colours with 128 alphanumeric characters along with 128 'graphic' characters. By todays standards it's vey primitive, but for 1985(ish) it was a definite improvement over the PX-8 LCD display. Also inside are an EF6803 8-bit microprocessor, derived from the 6800 (the Dragon 32 used the 6809 of similar stock).

The question is, does it still work? After a couple of quick electrical safety checks I plugged it in and turned it on. No bangs or flashes. No magic smoke. So far, so good. Now to see if anything was actually coming out of it. So I took my oscilloscope and probed the 'Video' output. 

Is this a video signal I see before me?

With the scope showing what looked like a video output I went looking for a cable and found one in the bin (really) with the correct aerial plug on one end and phono on the other and plugged it into the back of my tiny Sony TV.

Yes! A video output! A very SMALL video 
output!

Success! But its a bit small. As it's composite I tried plugging it into my recovered laptop LCD panel. The composite on the control board isn't the best but it is much bigger than the Sony TV and it's clear to see that the unit is up and working.

That's bigger. 

Then I hit a snag. In the huge selection of cables I'd been given with the PX-8 I didn't have an 8-pin DIN to 8-pin mini-DIN that I would need to connect it to the UD80. 

After a lot of detective work i.e. I found a 5-pin DIN to 8-pin mini-DIN that is used to connect to DS8 to the PX-8 (did I mention the DS8 yet?), I think I have managed to establish that out of the 8 pins on the UD80 DIN connector, only five are actually needed.  These are:

• Gnd
• RxD
• TxD
• DTS
• DSR

I did think that I could make a 'straight through' cable with all the parts I have, but a quick investigation of the port on the front of the UD80 revealed that pin 2 is GND. This is NOT the same as pin 2 on the PX-8 which is actually TxD. Other 5-pin DIN applications seem to fairly consistently use pin 2 as ground so the UD80 is possibly still wired up a standard way. I just don't know which of the other pins are which. Dangnabit.

But there's another issue. After an initial panic that I did not have the relevant driver file available for the PX-8 (which is called UD80-DRV.COM) I realised, after a couple of people prodded me, that this file is already in the ROM on the UD80 and using the FILINK app on the PX-8 I should be able to get it to download. I even checked that it is by dumping the ROM and having a look!

ROM Dump

Sadly, even though I can run FILINK I have no idea what I need to do to prompt the UD80 to download the driver. I have tried probably a few hundred combinations of the cables into the UD80 with limited success (although it does display some garbage with some combinations) but without the actual instructions on what to do in FILINK it's all a bit moot. Sad face.

Garbage - but it's displaying something!

UD80 end with 8-pin DIN to breakout

Breakout with one of hundreds of
combinations installed

PX-8 end

But, on the plus side, I have something that is so incredibly rare that if you search for it on Google, a lot of the top results are my posts on Twitter! And it does look like it would work, if I could find the step by step instructions..

One to re-visit.

More photos for anyone Googling:

DIP switches - changes things like 40-col or
80-col mode, background colour etc

Connections through DS8 switching box

EEPROM, video controller and 6803 CPU

Bottom of UD80 board

8-pin DIN lower left of board

Video RAM upper right of board

RGB Connector centre bottom of board

DS8 Switchbox with a bit of a design flaw!
Case screw screws into the cap!

Bottom of DS8 board. Pure art.

Screwed cap replaced.

Two units together.

PX-8 - It's not all bad..

With my escapades ongoing with the PX-8 RAMDisk you'd be forgiven for thinking that the thing is cursed. Of course it isn't because a) it was free, nothing that is free can be cursed, and b) curses aren't real.

The PX-8 has two slots for ROMS. The ROMs fit into little carriers that I shall call pods because it sounds cool to say 'ROM pod'. So there. These pods contain additional applications to run on the PX-8 which could be anything from a word processor such as WordStar to a diary or calculator. The pods can also accept EPROMs of different capacities and this unit came with a bunch of them.

Pod, pod, pod.

Pod, pod, pod, pod, you get the idea.

The first thing to do with these is to dump the ROMs so that I can re-use the chips for other things but easily restore them to their original state if I need to. This is particularly important for the DS8 video output boxes that came with the PX8 which are probably the rarest things I currently have in my collection. (I don't know if they work but if I don't plug them in and turn them on, they might..) Reach for the Dataman S4!

I'm not sure if I mentioned but I modified the S4 to run directly from a mains adaptor by hooking up the battery terminals directly to a modern power supply. This removes the need for an internal battery which also removes the risk of a damaging leak but has the drawback I have to re-load the 'library' of supported EEPROM devices every time I want to use it. A minor inconvenience.

To get going I just boot up the Dataman software from 2003, which works flawlessly with Windows 10 if you were wondering, and just select 'Download Library'. Then I select the library file, which is still available from the Dataman website, and then wait a couple of minutes. A message 'START LIBRARY?' appears on the display and by pressing 'ENTER' everything is ready to go.

Library 3.00 ready for action

To select the correct chip type it's a simple matter of pressing 'PROM' then using the up/down arrow keys to find the right manufacturer then the left/right arrows to select the right chip. Easy peasy.

This EEPROM is a National Semiconductor.

So, what to do. I take one of the dumped EEPROM chips and a PX-8 ROM file downloaded from the most excellent resources located here and here. How do I get it on to the chip? Well, first of all this is a UV erasable EPROM so I can't clear it with the S4, I need some other way. The datasheets for EEPROMs will normally give an indication of what's required and in most cases it's a blast of UV light at a wavelength of 257nm (nano metres) for about ten minutes.

Intel D27256 EEPROM
Note the window where UV enters

I have to tell you now that the EEPROM erasers that are available on various auction and other retail sites are complete garbage. I shall quote from one review of a unit that someone purchased from a well known site:

"This is basically a 'use at your own peril' device. It's really badly made with gaps all around the base and top cover. I'm sure if Her Majesty's Customs had anything to do with this, they'd incinerate them on sight. ... although it feels as if there's a reasonable chance they would do that on their own.

The warnings that come with this are crazy; maybe because they're in broken English. The gist of it is 'turn it on, run and don't look back'.
...
Having read the other reviews , the first thing I did was take it apart and put in a new power cable and a fused UK plug. The one you're replacing is terrible - there's around 5 threads per core.
...
After leaving on for seven minutes (with an extension lead going to the device, which I sat in a bucket with wood over the top. If I had an x-ray screen I would have probably gone behind that too) it did actually blank the AMD 27C400 inside.

I didn't really doubt that it would, I just expected it to explode or at the very least start smoking as soon as I flicked the power on.

So? If you need to blank an eprom or two, then go for it, but, take enormous care with this piece of junk."

This was not what I would call a 'glowing' review.

But there is an alternative. In a bizarrely 'Hitch Hikers Guide' turn of events, I purchased a 'Phone Sanitiser'. This is basically a white box that you put your mobile phone into and, for ten minutes, it bathes your phone with UV light at 257nm, to kill bacteria and leave you with that smug feeling that you won't catch any virulent diseases while posting cat pictures on Instagram (or whatever the kids are doing these days).

You won't be wiped out by a virulent disease contracted
from a dirty telephone.

But did you see what it does? It emits 257nm light for ten minutes. And it only cost me £13. And it's a surprisingly high quality unit. There is mention of 'Samsung' on the box and in the small manual it comes with but it's not directly made by Samsung. No matter. It does what I need it to do.

Opening the lid reveals two small tubes, one at each end. By resting the EEPROM with the window directly facing the tube, it is completely erased in ten minutes. Blank EEPROMs are full of 'FF's which indicates they're ready for writing. Nice. I shall probably look at designing and 3D printing something to hold the chips more securely in sanitiser in the future. 

Open sesame..

FFs sake.

So now I have a blank EEPROM and I want to write my downloaded ROM to it. In the S4 software I just select 'Transfer > Send file (PC>S4)' and select the ROM file. The format of the file can vary, mine is a 'HEX' format, but the software supports at least four different formats.

After a minute or two the transfer is complete and on the S4 software I select 'Memory>Checksum RAM' to check what the value should be, then 'Device Pretest' which should tell me the EEPROM is blank. Then I can 'Burn' which copies the contents of the S4 RAM onto the EEPROM. Once complete I always do a 'Device>Checksum ROM' to make sure it matches the RAM checksum from earlier. 

In theory, the EEPROM should be ready for installation into the bottom of the PX-8. The little carriers aren't essential and the EEPROM can be fairly easily pressed into the socket without any problems.

Does my new ROM work?

Yes. (Sub Zone)

Yes. (Tower)

Yes. (Tank Attack)

Yes. (Snake Pit)

These are about the only games for this system (I do have a version of Othello I want to try though) and they all work perfectly and are surprisingly fun, even if my aged hands start to hurt on the cursor keys after a few minutes. 

Now, if only I could get that RAMDisk to work...

Where was I? Ah, yes, the PX-8 RAM Disk

Before the shenanigans with my oscilloscope I was busy trying to repair the faulty PX-8 RAM Disk.

I was in the process of testing all the chips. And I have actually finished testing them all now. All chips are now working, either as their original chip or with a replacement. I managed to also check the EEPROM by using the S4 Dataman EEPROM programmer to dump the ROM and compare it with a version obtained online from the most excellent PX-8 resource located here.

Here's my spreadsheet (everyone loves a spreadsheet):

Chips.

Sadly, however, even with all of the chips installed, the behaviour is unchanged and the PX-8 screen sits with a sad cursor, flashing on an otherwise blank screen.

But now I have a working oscilloscope so I can check some waveforms! Yay!

Very quickly, I noticed something a bit strange. If I probed pin 2 or pin 14 of chips 17A or 17B (although only 'A' is populated for 60kb mode), the resulting wave was very dim and unstable and would suddenly revert to no signal, as if there was a 'crash' occurring on the board.

Interesting. Very interesting.

The pins 2 and 14 are the DI (Data In) and DO (Data Out) pins on the RAM chips. For each chip (actually pair of chips e.g. 17A and 17B) the DI/DO pins are tied together. This does sort of make sense since the data on that line and its direction is also dependent on other signals so if data is heading into the chip it is irrelevant if that data appears on the DO pin, and conversely, if data is being read from DO it cannot be read into the chip at DI unless the 'Write Enable' pin is also active.

And since there are eight RAM chips (or eight pairs with bank switching) this means that each chip/chip pair is looking after one bit of RAM. In this case, chips 17A and 17B look like they are looking after data bit '7'.

So, where does this combined line go to?  Well, it goes to chip 9B (a TC40H374P flip flop) at pin 19 and is a 'Q' input. It also goes to chip 8B (the other TC40H374P flip flop) at pin 18 which is a 'D' input. But it goes further. This line also goes straight to pin 13 on the Z80A which means that this line is confirmed as being specifically for data line D7. And it also heads over to pin 16 of chip 11C which is a bi-directional driver. Finally, it also runs to pin 17 of the EEPROM.

To check whether this line was fully connected was a simple case of doing a continuity check across the various points and pins on the right chips:

• Pin 2/14 to pin 19 on chip 9B - CHECK
• Pin 2/14 to pin 18 on chip 8B - CHECK
• Pin 2/14 to pin 16 on chip 11C - CHECK
• Pin 2/14 to pin 17 on the EEPROM - no continuity
• Pin 2/14 to pin 13 on the Z80A - no continuity

At this point I thought I had found the problem, a broken trace that prevents reading/writing of data bit 7! Yes!

But. No.

Before I started soldering wires anywhere I just so happened to run the multimeter probe up and down the legs on the EEPROM. And guess what I found? Data bit 7 doesn't go to pin 17 on the EEPROM. It goes to pin 19. And pin 19 on the EEPROM connects to pin 13 on the Z80A. I appear to have found an error in the technical manual of this thing. Or possibly a different board revision. Or it could be that the pin allocations are just different depending on the EEPROM version (but would still need a different board layout).

Arse.

Back to square one. 

Today I decided that I would re-flow the solder joints on the 'Gate Array' chip (I shall call him Gary) since this is the only chip that is not in a socket and is surface mounted. This escalated into a complete removal and re-attachment. Definitely not a fun 45 minutes. Although this chip is quite chunky with fairly big legs for an SMD chip, it's at the absolute limit of my soldering ability and I just about managed to do it without lifting any pads. The re-attachment is not pretty but the legs are all connected as they should be.

Gary removed..

Sadly, this activity appeared to confirm my worst fears. After removing Gary I plugged the RAM board in and the results were exactly the same as before i.e. a sad, lonely cursor flashing on a blank screen. After re-attaching it, the results were again the same.

This says to me that it's 99% certain that Gary is dead. Poor Gary.

These units are pretty rare so this means that Gary is the purest form of unobtainium. 

But I refuse to give up!

So, let's do some more continuity checking. This time, I shall focus on the pins of Gary and the destination of all 80 of those tracks. I feel another spreadsheet coming on!

Gary re-attached - not pretty..

Two spreadsheets in one post. I'm spoiling you.

After a couple of hours I had discovered a couple more things. Firstly, two pins on Gary had not been soldered on properly and were still loose, most importantly pin 45. Secondly, there is no continuity between pin 45 of Gary and pin 8 of chip 11C. This remained the case after I had carefully re-soldered pin 45 back to the board.

Attaching a jumper wire from pin 45 of Gary to 11C was straightforward enough, but, again, this made no difference at all.

Bodgit and scarper..

On the plus side, with my newly repaired oscilloscope I began to poke around the Gary pins and found sensible looking outputs for a gate array. So perhaps Gary isn't as dead as I thought. 

I am getting desperate with this now. My next step is to de-solder all of the original RAM sockets and check for unseen damage under the board. Then, I shall replace the few through hole resistors that are on the board (one looks to have caught some battery leakage quite badly) and consider doing the same to the four or so transistors. 

Or I might just put it away for a while and think about other things...