This download package contains:

  • The circuit diagram
  • The memory dump for the EPROM
  • The source files of the SB-6668 V1.1 software
  • The source file and memory dump of original software for the PM-6668
  • A copy of the service manual of the PM-6668

Come to think of it, that's quite a treat, don't you agree.
The software must be programmed in a 27C256 EPROM and is supplied in the Intel Hex format.

A Little Note On Software Version V1.0

It is 2018 now, when I release software version V1.1. That's some 22 years after I have created this project, with software version V1.0. A few years ago my counter showed all sorts of strange frequency readings. It turned out that one of the two HEF4737 quad decade counters had died. Fortunately I was able to buy a replacement for it and I was very happy to be able to fix the counter again.
However, from then on, I started noticing something strange. Only some frequencies above let's say 1MHz appeared to be affected. For example 19.23074MHz was shown correctly, while 19.23075MHz was shown as 3.205125MHz. I was not sure, but I couldn't remember that this had ever happened before.
I had spent a little time investigating this issue before, but couldn't find the time or inspiration to trouble shout it completely. One thing was for sure, the counter worked flawlessly with its original processor, so it wasn't the hardware.

Thanks to Jerry, my new EPROM Emulator minion, I have now been able to pinpoint the problem. It appeared that the 8051 processor was a tiny bit too fast for the decade counter I had replaced a few years ago. Adding one NOP after selecting this chip fixed the problem. I've had plenty of NOP instructions lying around doing nothing, so I have generously added 4 NOPs, just to be on the safe side.

Therefore I highly recommend updating your software to version V1.1 if you have built this project too. The measurement error was not always obvious or consistent, making it hard to tell whether your device is affected by this issue.
As a bonus you'll get two extra SB-Bus commands if you do upgrade to version V1.1.