Hi shakalnokturn,
No problem to make some meausements, just ask. If desired, you could also borrow my one for a couple of months to perform your reverse engineering, I do not use it so often. Just contact me via pm.
Thanks for the offer, I will consider it if really stuck but would like to avoid that as much as I can.
Here are the photos with a little reverse engineering, I may get round to cleaning the red numbering into actual names later.
The 4040 counter addresses RAM A0 to A11 the remaining A12 to A16 are straight from the µ controller.
Of the 11 empty footprints under the LCD, one is for a ceramic decoupling cap, for the others I expect resistors if these are needed at all.
Resistance measurements on the points labelled 1 to 9 WRT Vcc are mostly in the 2.2M
or 4.8M
depending on ohmmeter's polarity.
_Wim_ could you measure those points (1-10 on photo) WRT
Vcc on your MetraHit 22M when you have a little time for that?
A few side notes:
Current current consumptions (before adding anything to the 24S) and on Lithium batteries (Vbat=3.4V) are: 9 to 12mA depending on function selected, 184µA in standby.
Funnily I was expecting at least the SRAM would be powered straight from the batteries in standby, as lowering the supply voltage also reduces the current draw. This is not the case the MAX856 supplies the SRAM with 5V even when the meter is in standby.
OTOH the 93LC66 is not powered straight from 5V Vcc but gets power through the µ controller (@4.9V) and is not supplied in standby, this could be either to reduce standby power or as a measure to not corrupt memory on power transients (?)
Another thought on the "M" option for these meters is that it may be software enabled in the 93LC66. I'll dump the 93LC66 soon that will at least get me a backup of calibration data "just in case".
@ MichalZ: Thanks for the photos, they further confirm that different FW revisions have been used throughout the range and likely indifferently on "S" and "M" models. V7.2 is the highest revision I have spotted so far.