Project Logs

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• Construction

  Keith • 06/19/2022 at 16:21 • 0 comments

  2022-06-08

  PCB arrives. I have noticed that the legend on the PCB indicates '74LS245' while the parts list specifies 74HC parts (or 74HCU for the crystal oscillator circuit). The original COMX used mix of 4000 and 74LS  series parts. I'm going use CMOS parts with LS switching thresholds, because I have a large stock of 74HCT14, 74HCT86 and 74ACT574 chips. 

  2022-06-10

  Parts arrive. They supplied a 74HC04 instead of the recommended 74HCU04.

  2022-06-12

  Fitted many parts. Soldered the logic and RAM chips directly, because they are too cheap to warrant sockets these days and sockets introduce unreliability. I have fitted sockets for the DIP40 chips.

  2022-06-18

  Board currently looks like this:

  Lacking most of the passive components. I had many 100n decoupling capacitors available. I have the chips for the sockets, but won't fit them until all other components are fitted, to avoid exposure to static discharge.

  I plan to modify the project to input characters from a USB module instead of the keyboard, so I can enter programs by pasting text into a terminal emulator.

• Component sourcing

  Keith • 05/19/2022 at 18:30 • 0 comments

  COMIX-35 Assembly Resources are already well documented.
  

  CDP1802 still widely available. Other chips less so:
  

  CDP1869
  

  CDP1870

  CDP1871

  2022-06-01 

  I found two sources for the PCB.

  On eBay, https://www.ebay.co.uk/itm/115300569917 for £22.79 + £7.20 postage = £29.99.

  If was a third cheaper from a UK supplier, https://www.dcselectronics.co.uk/comix-35.html for £18.50 and chipset for $31.20 (£24.95), so I order it from there.

  I found a kit of ICs on eBay.com https://www.ebay.com/itm/114536103330 for $24. It was cheaper and less fuss than sourcing them separately. I paid with my card, as this has a slightly better currency exchange rate than eBay.

  ETA 14-17 June for the chips, 11th June for the PCB.

  2022-06-08

  PCB arrives. I have noticed that the legend on the PCB indicates '74LS245' while the parts list specifies 74HC parts (or 74HCU for the crystal oscillator circuit). The original COMX used mix of 4000 and 74LS  series parts. I'm going use CMOS parts with LS switching thresholds, because I have a large stock of 74HCT14, 74HCT86 and 74ACT574 chips. 
  

  2022-06-10

  Parts arrive. They supplied a 74HC04 instead of the recommended 74HCU04.

  2022-06-12
  

  Fitted many parts. Soldered the logic and RAM chips directly, because they are too cheap to warrant sockets these days and sockets introduce unreliability. I have fitted sockets for the DIP40 chips.

  2022-08-19

  Received low-height crystals from https://www.ebay.co.uk/itm/265237934281 which will make the board look a bit nicer and less vulnerable to knocks.

  2022-08-25

  Received crystal insulating washers.

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