Building an Acorn System 1 replica - Part 7 (29/05/2020)
This project runs in frantic time blocks of activity, followed by periods of doing nothing, and these time blocks are controlled by how long it takes for a box of PCBs to arrive from JLCPCB. To be fair, they are pretty quick, it's the postage that's the delay and largest cost.
A couple of days ago a new box of PCBs arrived meaning that work can resume.
The first thing I wanted to test was the final version of the piggyback PCB that allows for a 6522 to be used in place of the legacy INS8514 IC that's used for the keypad and LED display board.
A couple of days ago a new box of PCBs arrived meaning that work can resume.
The first thing I wanted to test was the final version of the piggyback PCB that allows for a 6522 to be used in place of the legacy INS8514 IC that's used for the keypad and LED display board.
Assembly of the board is pretty straight forward, you just need a fine tipped soldering iron and make sure that the pin header strips are soldered as vertical as possible. I found the simplest way was to just lay the PCB down, drop in a header strip, solder/tack one pin, adjust it's angle to make sure it was vertical and then solder the remaining pins. Laying the board down on a surface prevents the pins from peeking out from the flip side of the pads.
Don't be tempted to use the square header pins instead of the turned header pins. The square pins won't fit properly into the IC socket on the main PCB.
However, and there's always a however, in this version of the PCB there were two little mistakes with the silk screen.
The transistor is shown back to front (for a ZTX313 anyway, and I had to remove the transistor and turn it around), and the text for the two link wires (O2 and R/W) are reversed.
This is what happens when you rush.
The only odd thing about this board is the ZTX313 transistor. I use these as general purpose jellybean transistors as I've a box full of them. Interestingly, on a previous prototype I used a BC548; another general purpose NPN transistor, and the module refused to work. It was only when I switched back to the ZTX313 that it worked correctly. I did test the BC548 and it was fine. Since it's just being used as an inverter for the Address 0 signal, so only running at 5v, maybe 1 mA and at best a couple of MHz, it should've been fine. It's probably something to do with the resistor values. Just be warned.
I've corrected the PCB foil layout anyway and the foils for Version 1.2/01 are correct.
Another board that I received in my PCB box was a new, updated LED display for the KeyPad/Display board.
Don't be tempted to use the square header pins instead of the turned header pins. The square pins won't fit properly into the IC socket on the main PCB.
However, and there's always a however, in this version of the PCB there were two little mistakes with the silk screen.
The transistor is shown back to front (for a ZTX313 anyway, and I had to remove the transistor and turn it around), and the text for the two link wires (O2 and R/W) are reversed.
This is what happens when you rush.
The only odd thing about this board is the ZTX313 transistor. I use these as general purpose jellybean transistors as I've a box full of them. Interestingly, on a previous prototype I used a BC548; another general purpose NPN transistor, and the module refused to work. It was only when I switched back to the ZTX313 that it worked correctly. I did test the BC548 and it was fine. Since it's just being used as an inverter for the Address 0 signal, so only running at 5v, maybe 1 mA and at best a couple of MHz, it should've been fine. It's probably something to do with the resistor values. Just be warned.
I've corrected the PCB foil layout anyway and the foils for Version 1.2/01 are correct.
Another board that I received in my PCB box was a new, updated LED display for the KeyPad/Display board.
The new display adapter board contains MOSFET drivers to enable larger LED digits to be driven and can supply larger drive currents. This board is pretty much mandatory if the CPU board is running a 6522 VIA chip as this chip cannot supply sufficient drive current.
I did experiance two weird problems. Whilst the prototype display I constructed on a breadboard worked fine, the PCB version that makes exclusive use of SMT MOSFETs and resistors didn't work correctly; the segments on the displays didn't switch off correctly.
The fix was pretty simple and requires the addition of 16 x 10K resistors to either pull up, or pull down the MOSFET gate pins. I've updated the PCB layout to support this.
The other niggle is something I will need to look into. With the CPU running at the default 1MHz clock speed, the display flickers... and it's annoying. Running the CPU at 2MHz fixes the problem. This could be either an optical effect; in which case there's nothing that can be done, or it could be something to do with the MOSFETs not switching on or off fast enough.
One other piece of good news is I assembled the SAD board (Segment Address Decoder) and initial checks are excellent.
This will make addressing peripheral expansion boards a lot simpler and reduce the amount of electronics needed on each board.
I did experiance two weird problems. Whilst the prototype display I constructed on a breadboard worked fine, the PCB version that makes exclusive use of SMT MOSFETs and resistors didn't work correctly; the segments on the displays didn't switch off correctly.
The fix was pretty simple and requires the addition of 16 x 10K resistors to either pull up, or pull down the MOSFET gate pins. I've updated the PCB layout to support this.
The other niggle is something I will need to look into. With the CPU running at the default 1MHz clock speed, the display flickers... and it's annoying. Running the CPU at 2MHz fixes the problem. This could be either an optical effect; in which case there's nothing that can be done, or it could be something to do with the MOSFETs not switching on or off fast enough.
One other piece of good news is I assembled the SAD board (Segment Address Decoder) and initial checks are excellent.
This will make addressing peripheral expansion boards a lot simpler and reduce the amount of electronics needed on each board.
In the next thrilling instalment, the final (yea, right) CPU board.