Tag: Circuit

RC2024 – Part 4 – A Quick Diversion To Look At The RC2014 ROM Board

I have been thinking about what else I want to achieve with this year’s Retro Challenge.

Apart from designing a PCB to interface rotary encoders to my RC2014 computer, I wanted to be able to use them from software. I have used BASIC for my testing so far, but I also want to look at using Z80 assembly language.

To use Z80 assembly language on my RC2014 Classic 2, I have a few options.

  1. Use BASIC to load and execute a hex dump of the my assembled Z80 code.
  2. Use SCM to load and execute a hex dump of my assembled Z80 code.
  3. Burn my assembled code into a ROM and run that directly on the RC2014.

The ROM board for the RC2014 Classic 2 has the ROM chip socketed. If I wanted to use my own ROM image I can lift this out and replace it. However, the board is low down and I would need to also remove the board from the RC2014 backplane. This would add mechanical wear and tear.

I have a similar problem if I want to switch between ROM images. For example, from BASIC to SCM, or to my own custom ROM image. There are jumpers on the ROM board, but I would need to lift the board from the backplane to be able to swap them.

So, what is the solution?

Well, Spencer does have an RC2014 riser card available on the Z80 Kits website to lift the board above the other cards. This makes it possible to reach the jumpers, but still difficult to remove and replace the ROM.

After writing about designing the PCB for the rotary encoder, I thought I could just build my own ROM card. What I want my new ROM board to achieve is…

  • Use a ZIF (zero insertion force) socket for the ROM so I can easily remove and replace it.
  • Use switches instead of jumpers for the ROM select so I can easily change which part of the ROM the RC2014 is seeing.
  • Have the ZIF and switches above the other boards so I can easily access them.

The circuit design of the ROM board is essentially the same as the original ROM board, just with some pull down resistors near the address switches. The board height will be the same as a standard RC2014 board.

I couldn’t find a DIP package with 3 switches in the EasyEDA library, so I have used a 6 pin socket in the design which should have the same footprint.

Here is my PCB layout.

I have sent this off to JLCPCB to be manufactured. The cost including delivery was just £2.53 using their 10 to 14 day delivery option. I’m not in a rush for this board, so I think that is a fantastic price. There is also a $2 discount at the moment, so that also reduced the cost.

I am expecting the board to arrive towards the end of the month but this won’t stop work elsewhere in the project.

RC2024 – Part 3 – Designing A PCB For The Rotary Encoder

One thing I really wanted to do as part of this year’s Retro Challenge was to build my first custom PCB for the RC2014 computer.

I have chosen to use EasyEDA to design my PCB. I was heavily influenced by watching James Sharman’s excellent YouTube videos in which he designs his own CPU using EasyEDA. The alternative was KiCad, which seems more complex for a beginner.

Spencer has kindly put the specifications for a RC2014 module template up on the RC2014 website. I will try to follow this when it comes to laying out my PCB.

The Circuit

The RC2014 Peripheral Addressing didn’t originally target exact addresses, instead it used 74LS138 chips to target ranges of addresses.

The SC219 takes a different approach, and uses a 74HCT688 8-Bit Magnitude Comparator to target a specific address. I think this is the best approach for my circuit. Using switches for the comparison will mean I can easily change the matched address if there are conflicts with any other cards on my RC2014. I will use the IORQ line from the RC2014 as the OE (Output Enable) line. I can take the output from the 74HCT688 and use a 74HCT32 OR Gate with the RD line to know when the peripheral address I want to respond to is being used. I will use a 74HCT245 Octal Bus Transceiver to present data on the data pins when active.

In my initial post I mentioned I had to use a 74HCT14 Schmitt Trigger to invert the input from the rotary encoder and help produce a cleaner digital signal. This will be connected to the 74HCT245’s D0 to D5 pins. This will allow me to support two rotary encoders, and leave D6 and D7 spare. I will provide two blocks of input pins, one block for each rotary encoder. Each of these will support the CLK, DT, and SW pins, as well as GND and +5V.

I will add some debugging support to the circuit. I like the input LEDs on the SC219 so I will also add these to my circuit so I can see the incoming data from the rotary encoders. I think a breakout of the incoming data lines before the 74HCT245 will be useful to see what is going on, and also allow the board to be used as an input device when the rotary encoders aren’t being used. Finally, I saw a discussion on one of the Sinclair Spectrum groups about adding a hook to GND to attach an oscilliscope to when debugging. This could be very useful if the circuit doesn’t work, so I will add in provision for this.

This is my initial design.

Rotary Encoder circuit

The PCB

As I mentioned earlier, there is a specification for the shape and size of PCBs for the RC2014.

Laying out the board I have tried to use EasyEDA’s measuring tool to ensure this is correct.

Adding the curved corner I found tricky, but eventually I was able to do this by targetting the BoardOutLine layer in EasyEDA.

I read on various forums that adding a ground plane to a PCB was a good idea, so I have done this on both the top and bottom layer of my PCB.

As my circuit is reasonably simple, I used EasyEDA’s auto routing functionality. This worked quickly and I didn’t need to do any manual routing.

For manufacturing I decided to use JLCPCB as EasyEDA has built in support for this manufacturer.

The board itself cost $3.20 for 5. I paid for express delivery because if I have made a mistake and need to create a new revision, it could take several weeks to arrive. The total for the 5 PCBs, delivery, and tax was £20.21. These are due for delivery on October 8th 2024.

JLCPCB adds a manufacturing code to each PCB. You can specify where this appears by adding the text “JLCJLCJLCJLC” to one of the silk layers. I put mine on the BottomSilkLayer so it’s not forward facing.

This is the final layout for my board. I can’t wait for it to arrive so I can solder it up and see if it works.

Rotary Encoder PCB