SatPC32 works similar as Instant Tune (see Anthony Monteiro's detailed answer):
Thank you Anthony and Erich for your replies. I'm always impressed by the generosity of this list's members.
During the past week I've spent hours debugging my code because it was originally supposed to work the way that you have both described. Actually it did, or at least I thought it did, but that was before I bought a laptop computer and a USB to serial converter. The code has been updated many times during the past eight years and during one of those updates I changed the user interface so that the user had to stop the tracking during manual tuning; effective but clumsy.
Reading the FT-847's receiver frequency originally caused me some frustration. I had included a delay after each read and write command because the FT-847 doesn't return a command acknowledgement. While writing this reply I realised that instead of a delay I shroud be waiting for the serial port to receive five bytes from the radio.
This has turned out to be more reliable but I still get occasional read errors and I feel that it's due to the USB to serial converter. When an incorrect frequency value is read, at one second intervals, the same value is reread from the serial port buffer each second until the tuning knob is moved again. Maybe the read command is working correctly and instead it's the write command that instructs the radio to output it's frequency that's not working reliably.
I remember reading about some sort of delay that the FT-847 requires but I cannot remember what it was all about. It could be important.
A lecturer once told a class that I attended that explaining a problem to anyone, even your dog if no one else will listen, often leads to an answer. It's helping in this instance, but not enough to lead me to the final answer.
To add to my frustration, my ISP has made changes to their mail server which now prevents me replying to any e-mails that are outside of their domain. I have a workaround but it's a real pain.
Thank you Wayne for your thoughts on frequency drift. I spent many hours listening to AO40's beacon and did manage, eventually, to get the drift under control.
-- Regards, Phil.