I have played more with this little cutie.
"macboy" gave inspiration to try the mV-ranges.
https://www.eevblog.com/forum/testgear/canadian-tire-mastercraft-dmm-new-and-old-revision-teardown/msg927276/#msg927276They are available here too, and seems quite useful. They must be programmed to either the ohm, capacitor, diode or continuity position of the rotary switch. I don't use continuity, so it became DCmV. Continuity became a second function under ohm, so it is available. If you don't want to add the "select" button and have no need for one or more of the original four switch positions, you just program for instance DCmV as first on continuity as I did.
It's a lot of info floating around on the internet about the DTM0660, and I have tried to compile the most useful for VC921 into the modified HEX-dump attached.
As of today I have RS232, 8000 count scale length, temperature in °C and °F, warning level of 110V AC/DC and overload for 120V AC/DC (I don't trust VC921 for 230V) NCV, and toggle between AC and DC when in ACV or DCV.
I have found that the TX-LED for RS232 becomes lit when the instrument enters auto power down, so I have disabled APO. I have also found that the warning and overload does not function as expected if programmed to a value below 100V. In my instrument with 8000 counts, the warning did not work at all, and it entered overload at 80V regardless of what I programmed. From 100V upwards it seems to be OK. In the Chinese datasheet for DTM0660 they suggest a 500 ohm resistor in series with the TX-LED. I have not encountered any problems using that, and the benefit is less current consumption.
I am awaiting some small micro push-button switches from China so I can get the "select" on front of the instrument. While waiting I made an X-ray like image with a 5mm raster to help finding some free space for the switch. I attach it here.