CompactFlash Reliability

Connecting a relatively modern high-speed device, such as a CompactFlash card, to a slow-speed retro computer is not without its challenges.

Most CompactFlash systems described on this site essentially connect a CompactFlash card directly to the processor bus. This makes for a simple and low-cost interface that offers optimal performance.

However, there can be reliability problems using this method. There are a variety of technical reasons for this, including ground bounce caused by the card switching multiple high-speed data signals at the same time.

These problems can show up as random errors but are particularly visible when copying files from the serial port to the CompactFlash card using programs like DOWNLOAD.COM.

It is worth testing the reliability of your system before spending time installing lots of CP/M software. There are a number of recommended tests to perform.

  • Use the SCM App CF_Test. This can be left to run for a while as it writes and verifies patterns of data to an entire card.
  • When you first install CP/M, add DOWNLOAD.COM and install a few programs whilst watching for error reports. The process is described here and here.
  • When you have a few files on the CompactFlash use the PIP command to copy them to another logical drive with verify enabled. eg.
    PIP B:=A:*.*[v]

If all of the above tests work without error then you can be confident you have a reliable system. If not, consider the following.

CompactFlash cards vary. Some are more problematic than others. As a general guide, it is best to use older, lower-capacity cards. These tend to be slower and cause less electrical noise. 128MB cards are optimal for plain CP/M 2.2 as CP/M 2.2 can only access 128MB of storage. Some brands also work better than others. Cisco cards are generally regarded as a good choice.

Another factor is the proximity of cards on the backplane. Generally, the CompactFlash card should be as close as possible to the processor. The position of other components can also be a factor. For example, I’ve seen the position of the serial interface cause success or failure when using DOWNLOAD.COM. It may be necessary to experiment with these to obtain the optimal layout.

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