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Retro Challenge 2024/10 – Defeating The Riddler

The schematic for the ZX81 Module I designed in2016 had been designed in a very old version of KiCad, and hadn’t survived all the upgrades too well. This resulted in every component being replaced by a question mark in a box. Luckily I had some photos of the schematic from previous Retro Challenge posts, as well as an expectation as to what should be resistors or capacitors or whatever.

Things now look much better, and actually make some kind of sense.

2016 Me didn’t consider the importance of putting component values on the schematic. Or the PCB. My module was based on the original ZX81 schematic though, so it was pretty much a case of matching up components and updating their values.

The observant amongst you will notice that the ZX81 schematic has a lot more stuff on it than my module. These changes can be categorised in three ways;

  • Components which the RC2014 already have, such as the Z80 CPU, ROM or RAM
  • Components which need to be added, but are not part of the module, such as the keyboard or data/address bus resistors
  • Components which will not need to be included, such as the TV modulator including the USA/French options, or power regulation.

In addition, I noticed my module design had some extra components that were not part of the ZX81 schematic.

More specifically, this is a transistor and resistor. This actually matches the composite mod that the donor machine had. So, well done 2016 me for thinking ahead and including this!

Something which is a bit more concerning though is the way that the keyboard lines all seem to be shorted against the TV Out netlist. I think this might be an artifact of the schematic rescue, as I am sure that 2016 Me would have noticed that. However, I still need to test the PCB in case this fault has made its way through to the board.

So, with a donor ULA, and all the component values, I have everything I need to finish the module. There are other important things that I need to do to the RC2014 though. Firstly, I need to burn a ROM.

You may remember from a recent post that I did a partial ROM dump from the donor machine. This was a simple BASIC program

10 FOR F=1 TO 200
20 PRINT PEEK F;
30 NEXT F

I remember reading that the ROM chips used on old machines were almost, but not identical pin out to the 27C EPROM chips, and that it was a couple of data pins which were swapped over. Or something like that. So my worry was that, for example, D3 and D7 on every byte was swapped. So if I could compare the first 200 bytes or so then I would know if everything matched, or if the difference was consistent.

Luckily, everything matched with the ZX81.ROM file I have.

Although converting decimal to hex becomes less and less fun as you go. But at least I know that I should be able to just burn that image to a ROM and it *should* work.

The memory map on a ZX81 is a ZX81 is interesting. The ROM shows up in 4 locations, and the RAM is mirrored too. A13 is the only line used to decode if ROM or RAM is accessed. Therefor the memory map looks like this;

FFFF
      RAM (Shaddow)
E000
DFFF
      RAM (Shaddow)
C000
BFFF
      ROM (Shaddow)
A000
9FFF
      ROM (Shaddow)
8000
7FFF
      RAM
6000
5FFF
      RAM
4000
3FFF
      ROM (Shaddow)
2000
1FFF
      ROM
0000

As I understand it, Only the 8 ROM from 0x000 to 0x1FFF and the 16k RAM from 0x4000 to 0x7FFF actually matter. However, my plan is to burn two consecutive copies of the ROM and set the Pageable ROM Module to a 16k bank size. I will also use a 64k RAM Module with RAM starting at 0x4000. This will mean that the shaddow ROMs from 0x8000-0xBFFF will actually be RAM. I very much doubt that anything makes calls to that address range expecting it to be ROM, but as long as it isn’t critical to bootup, I can write a routing to fill that RAM with the ROM contents.

So, with all of that sorted out… I just need to solder some components, burn a ROM, and put it all together.

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Retro Challenge 2024/10 – Finding a donor

If you remember from my initial post at the start of the month, the first thing that I needed to do in order to make progress on the ZX81 Module was to find a donor for the ULA. Some things can’t be rushed though, so this has only just happened. Luckily I have several candidates to choose from.

I had some constraints though. Obviously the ULA had to come from a machine that worked. Ideally one that had undergone a composite mod. And preferably not the original one that my dads school friend bought back in the day and passed on to me a few years ago. (The ULA will only be borrowed to check that things work, but I would rather not risk damaging one that has sentimental value)

This one seemed to fit the bill. Apart from the fact that it is housed in a full size case with mechanical keyboard, there is nothing special about it. I think this may well have had various expansions or add-ons back in the day, but those are long gone now.

Luckily, it also has also been composite modded. In particular, this has been done via a transistor, rather than just sending the video signal out of the socket. This means that I can test that this works with my monitor before taking the ULA. I know that some modern monitors are fussy with ZX81 signals as there is no back-porch generated. Adding the circuitry to do that isn’t overly complex, but if I can keep things simple then all the better.

The TV image is actually really sharp, and certainly works with my monitor. So it looks like I have a donor chip!

Before removing it, however, there were a couple of checks that I wanted to make. Firstly, I have a file on my computer called ZX81.ROM which I think might be exactly what it sounds like. When opening it up in a hex editor, though, there is nothing readable in there at all. I would have expected to see something like the copyright message or the keywords listed. So I will take this opportunity to dump the first couple hundred bytes of the ROM to the screen so that I can compare it to the file I have.

I also want to take a quick look at the output signal on a scope.

Everything looks good and just what I would expect from a video signal (without a back porch)

So, with no further ado, out comes the ULA, ready for its new mission in an RC2014

Next up, I need to finish the schematic so that I know what value components I need to populate the rest of this board…

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Retro Challenge 2024/10 Talking The talk… at 9600

Following on from the failure to get the Portfolio talking directly to the RC2014, I decided to turn my attention to the Cambridge Z88. I already have a custom RS232 to TTL serial adapter for that, and the RC2014 is all set up ready for the Portfolio at 9600 baud, so this should be straightforward…

Well, would you believe it, it was a success! The Z88 is a fantastic machine, with some good built in programs. One of those is a terminal emulator, so it really was just a case of plugging the RS232 adapter in to the RC2014, with Port B set as the primary port set at 9600 baud.

To test it properly, I typed in the test program for the LCD module https://rc2014.co.uk/modules/lcd-driver-module/ (The initial test failed, but after I plugged the LCD in correctly, everything worked fine).

Buoyed on by the success there, I decided to turn my attention back to the Portfolio. Knowing that the Z88 RS232 to TTL serial adapter worked meant I didn’t have to burn my fingers again! The only issue was that the Z88 had an almost but not quite standard pinout on the DB9

I blogged about the Z88 RS232 to TTL Serial adapter back in 2013 https://sowen.com/466/z88-to-ftdi-lead-via-max3232/ The important takeaway from this is that Gnd is on Pin7 on the Z88, although it is Pin5 on the Portfolio. It is also worth noting that Pin1 and Pin9 both carry 5v, and whilst these aren’t used in my adapter, they could break other things they are plugged in to. Pin2 and Pin3 are Tx adn Rx respectively on both the Z88 and Portfolio.

Time for a Frankenconnector to adapt them!

The other bit of unfinished business on the Portfolio was putting some terminal software on there. A copy of xterm2, as supplied on the Portfolio utility card was easy enough to find. In the past I had transferred programs to the Portfolio via the parallel adapter, however, I had recently bought an APF-01 memory card from https://apfram.com/ Whilst I had to use some propriety software on a Windows machine to copy the files on to it, it worked really well.

With everything connected up and Xterm2 started, the Portfolio and RC2014 were talking!

Again, I typed the LCD test program and sent a message to be displayed from the Portfolio to the RC2014

Although, of course, it was only as I type this blog post that I notice the Off-By-Ten error in that message! Ooops!

So, that is the Z88 and the Portfolio both ticked off of the list of computers to talk to the RC2014. Whilst the Portfolio wasn’t a complete failure before, I am really glad I revisited this and got a more satisfying result. And it has reminded me just how good both of those two machines are. I really should use them more.

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Retro Challenge 2024/10 – Getting my fingers burned

If you read the intro post to this years challenge, I set myself a secondary goal of getting many of my old computers talking to my RC2014. So I decided to kick things off with the Atari Portfolio. This was going to be an easy intro in to things as this little DOS compatible palmtop has a serial port expansion and can run DOS based terminal software. So this would be pretty straightforward, right?

The Portfolio serial port is limited to 9600 baud, so this gives me a couple of options on the RC2014. I could either slow main clock right down, which will slow the CPU, but I will get 9600 baud on serial Port A. Or I could slow down Clock 2 and use Port B as the primary port. Rather than dive in headfirst and make a mess of things, I came up with a simple step by step plan;

  • Power up the RC2014 and check that it is all working
  • Plug in to PC and verify 115200 baud on Port A is working.
  • Connect to Port B and verify this works at115200 baud
  • Change Clock 2 and check the PC can communicate at 9600 baud on Port B
  • Install terminal software on the Portfolio
  • Plug the Portfolio in to a serial to USB adapter and check it can communicate with the PC at 9600
  • Build a simple MAX232 adapter to convert the RS232 on the Portfolio to TTL serial
  • Check this can communicate with PC FTDI
  • And, finally, connect the RC2014 and the Portfolio, via the MAX232 adapter, using Port B at 9600

The first 3 steps went without a hitch. Using Port B as the primary port was as simple as flipping a switch when using the Front Panel. However, when I dropped the RC2014 down to divide Clock 2 by 12, I was unable to communicate at 9600 baud. This didn’t bode well. In case I didn’t get the jumper right, I tried setting the terminal to 4800 and 19200, being the speed just above and below 9600, but to no avail. Anyway, long story short, I wasted an hour before discovering that indeed the jumper was of by one, but it gave 14400 baud, which my terminal software didn’t support! So ok, progress, but slow progress.

On to the Portfolio. I plugged in the Serial Port expansion, connected a RS232 to USB adapter, and plugged it in to the PC. Using the DOS command COPY AUX CON meant that whatever I typed on the PC came out on the Portfolio screen, and COPY CON AUX meant that whatever was typed on the Portfolio appeared on the serial terminal.

Now the easy part. I had a few MAX3232 modules which do the RS232 level shifting/inverting. On the Portfolio side I just need Ground, Tx and Rx. On the RC2014 side, Ground, 5v, Tx and Rx. Soldering was about as easy as things get, and it all looked good. Just to be safe, I decided to add an extra step and connect it to my FTDI adapter plugged in to the PC. A second later I dropped it as I felt burn my fingers!

Quickly unplugged everything and checked the soldering, the pin labels, and made sure I was 100% confident that everything looked fine. Plug it in again, touch the chip… Ouch! This was not good.

Time to bust out the FLIR thermal imaging camera. Wow! This thing gets up to 150’C (or 300’F) before I chicken out and pull the power.

Checking online and social media, people had reported problems with the cheap Chinese MAX232 modules. I had my doubts that the chips were genuine. However, some reports stated that the internal pull-up resistors are too weak, so without an input signal it is bouncing around all over the place and causing it to heat up. Others said that a resistor inline with the 5v power might fix it. Those ideas were certainly worth a try.

I can confirm, for my module at least, pull-up resistors made no difference. A resistor in series with the 5v meant that it didn’t get hot… but it didn’t work either. (although quite possibly the vital parts were already burned out?)

It was also pointed out that the chip is actually a MAX3232, which is a 3v3 chip. This would explain a lot! However, the datasheet says that it can work with Vcc from 3v – 5.5v. The TTL input can also be 5v even with a 3v3 Vcc. So, if it is a genuine chip, it _should_ still work

So I had to accept only a partial success, but mostly a failure on this one. The RC2014 worked at 9600 baud on Port B, and the Portfolio could talk at 9600 baud. It was just that they hadn’t talked to each other.

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Retro Challenge 2024/10 Update 1 – The Riddler

When I failed to complete the Retro Challenge in 2016 I finished by saying that the dog ate my homework. That sounded like a reasonable enough excuse.

This year any failure to complete is because The Riddler ate my schematic!

One of the first things I wanted to do was familiarise myself with what 2016-me did regarding the ZX81 module I designed for the RC2014. I have 3 PCBs here, and whilst they look great, they lack some info, such as the component values. So I opened up the schematic I created and discovered that all of the components have been replaced by ?? boxes!

To be fair, this isn’t the first time I have seen this situation. This was probably created in Kicad v4 or possibly earlier, and one thing which Kicad was terrible with back then was keeping track of component libraries whenever there was an upgrade. I am now using Kicad v8, so there have been a lot of upgrades.

I might be able to install a fresh copy of an old version and hope that has the right libraries and I can map them in to work with the schematic. I think that most of the components are resistors and capacitors though, so even if that worked, I wouldn’t have the values anyway.

I think it will actually be easier to convert the schematic to the v8 standard and manually change each of the parts to the resistor, capacitor or diode as approprite. I still won’t have the values, but the thing I do have is the original ZX81 schematic that this module was based on, so it should be a case of matching things up.

Mid-blog update

I looked back at the intro post that I made and noticed that the photo I reused from 2016 was a picture of the partially assembled module that was sitting on a schematic printout. The values can be seen too!

So that makes life easier!

Not only that, but another blog post from 2016 also has the full schematic (less component values) so that helps too!

The RC2014 has come along way since this module was conceived 8 years ago. I think some of the stuff that I had learned about the ZX81 back then had guided some of the decisions that I made later, which actually helps this a lot.

Firstly, the Backplane 8 and Backplane Pro both have the ability to add resistors on the data and address bus, as the ZX81 did. There is now an RC2014 keyboard which mimics the ZX81 or Spectrum layout and has the diodes onboard, so that makes things easier too. The Pageable ROM module supports 16k blocks, which will help if anything on the ZX81 needs to access the shadow ROM in 0x2000 – 0x3FFF. And, lastly, the 64k RAM module can be set to start at any address. With the ZX81 having RAM mapped from 0x4000 – 0x8000 this is perfect.

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Retro Challenge 2024/10 – Intro Post

It has been a while since I entered Retro Challenge, but I think I remember what to do. It goes something along the lines of stating at the start of the month by saying that you are going to design a ZX81 module for the RC2014, and then at the end of the month report back in with minimal progress. Looking back, I seem pretty good at that. Retro Challenge 2016/1 started off like this and ended like this. Then later for Retro Challenge 2016/10 it looked like this.

I hope you will excuse me for using the same image I finished on 8 years ago, but, honestly nothing has changed. The ZX81 module is still in this state.

So, before we get to far ahead of ourselves, lets take a look at how we got to this point. My original goal was to recreate either a ZX Spectrum, ZX81, ZX80 or Jupiter Ace module for the RC2014. They are all fairly similar architecture, although the Jupiter Ace has weird dual port memory RAM. They all have a Z80 CPU, some ROM, some RAM and a clock, the same(ish) as theRC2014, so those parts of the schematic can all be ticked off. The ZX80 does everything else with conventional 74 series logic. The ZX81 is almost identical, except that 74 series logic is all smooshed in to a ULA. The Spectrum is similar except the ULA does more stuff (which can also be done with 74 series logic).

I ended up choosing the ZX81 as a good place to start because it is pretty much just a ULA chip with a little bit of support circuitry. The ULA is no longer manufactured, which will mean taking one from a working ZX81 to get this up and running. There are, however, modern FPGA replacements available, so I might well end up getting one of those to swap in at some point. So in January 2016 I translated the relevant parts of the ZX81 schematic in to KiCad and designed a module around that. I got some boards made, but that was as far as things got. Then later that year I soldered in 9 components.

To get started this year I need to do two things. Firstly, familiarise myself with the ZX81 circuit and the one I designed 8 years ago. I seem to remember being fairly confident it would work, but at this stage I have Schrodingers PCB. It both works and does not work simultaneously, needing a test to clarify the actual state. The second thing is that I need to check my stock of ZX81 and select a donor one that the ULA can be borrowed from. I probably want to make note of some of the pin voltages or signal traces before taking it out so I can check those when it is in the module.

There is a chance that I will end up waiting for parts, so the secondary part of my Retro Challenge challenge is to talk to my RC2014 with as many of my retro computers as possible. This may include but not be limited to;

  • Atari Portfolio
  • Cambridge Z88
  • Psion Organiser II
  • Psion Organiser 3a
  • Sinclair Spectrum (with Interface I)
  • Sinclair Spectrum 128
  • Sinclair Spectrum +2 or +3
  • Sinclair QL
  • Amstrad PPC640
  • Toshiba T1200

I think this will be limited to serial communication, and luckily I have a few MAX232 chips and boards here. I expect it will mainly be using the device as a serial terminal for the RC2014, but it would be nice to maybe send a Spectrum loading screen from the RC2014 to the Spectrum. It might sound trivial, but understanding how the serial ports work on those devices, and finding/writing software to use them could be the challenging part.

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