CoCo 1 Motherboard Issue

After getting the CoCo SDC into the hands of some users, it was discovered that Flash programming does not work correctly on certain CoCo 1 motherboards. The two earliest CoCo 1 boards known as the 'D' and 'E' boards are the culprits.

The Cartridge Select signal (CTS*) on these boards exhibits too slow a rise-time which causes a problem for write operations to the high-speed Flash chip. The issue does not affect normal operation of the CoCo SDC in terms of being able to read data or execute code from the Flash.

Identifying the Problem Boards

To determine if your CoCo 1 has one of the problematic motherboards you will need to open the case and look inside. The boards in question have a large metal shielded area that encloses all of the main logic chips including the RAM, CPU, SAM, VDG and PIAs. There is a number printed on the board just below the cartridge port which ends with "-D" or "-E" as seen in the photo below.

If your board has a smaller RF shield which only covers the SAM and RAM chips, or has a number printed on the board near the front-left corner that ends in '285' then this is what is often referred to as the 'F' board. The 'F' board CoCo 1 does not exhibit the problem and needs no modification.

Location of Motherboard Version for D and E boards 

Work Arounds

There are a few options to deal with this problem:
  1. Use only an 'F' board CoCo 1, a CoCo 2 or a CoCo 3 to perform Flash programming. This option is not ideal, especially if you don't have one of the suitable CoCos in your possession.

  2. Use a Multi-Pak Interface when programming the Flash. The signal buffering in the MPI acts as a kind of filter for the CTS* line and produces a fast signal transition for cartridges in the MPI slots. This is a good option if you do not wish to modify your CoCo and happen to own an MPI.

  3. Perform a simple modification to the CoCo 1 motherboard to fix the problem (see details below).

Board Modification

Please note that any modification to the CoCo is performed at your own risk. Although it is unlikely that this modification will cause problems with other hardware, I can't be held responsible for any damage or loss of function that may occur should you choose to go through with it.

The modification is rather simple and involves cutting just one leg of a capacitor. Be sure to disconnect power to the CoCo and discharge any static electricity from your body before touching any of the components inside the CoCo. The affected capacitor is located within the shielded area so you will need to remove the metal cover to gain access. Find the capacitor labeled C85 which is located next to the cartridge port (see photo). Using an appropriate tool, cut the front leg (the one nearer the keyboard) of the capacitor to sever the connection.

Board Modification : Cut Front Leg of Capacitor C85

That's it! Replace the metal cover, close up the case and you are good to go.