See related threads here:
https://www.eevblog.com/forum/microcontrollers/stymied-why-are-my-data-sometimes-doubled/https://www.eevblog.com/forum/mechanical-engineering/my-new-dro/Mine is an older unit from the early 1990's. There are 2, DB-9 connectors. The pinout I determined is: Pin 1, Mitutoyo SPC (Digimatic) data; Pin 2, REQ line (data request, active low); Pin 3, clock; Pin 4, DCt voltage of about 1.6V; Pin 5, circuit common (previously identified as GND); Pin 6, 5 VDC; and Pin 7 & 8 , chassis GND. There are only 6 pins on the connector. REQ comes from the display; data and clock come from the scales (open collector). Data format is shown in Attachment 1. In brief, thirteen 4-bit bytes are transmitted, MSB (first) to LSB. Each byte is BCD. Bit order in the bytes is reversed. That is, 0001 = d.8.
Attachment 2 shows a typical trace. Yellow = data, Blue = REQ, and Magenta = clock. I marked off the 4-bit bytes. The decimal value on the display was -0.0200 inches. The earliest peak (0001) = d.8, is the negative flag. The positional data read (decimal), 0,5,1, and 2. The "2" is the decimal. So, the whole value is 0.51 decimal. The display read 0.0200", and of course, the linear scales are only in mm. 0.51 mm = 0.02008".
Hope that helps someone.
Regards, John
EDIT: See update here for opening case and a picture:
https://www.eevblog.com/forum/mechanical-engineering/my-new-dro/EDIT2 (12/10/2023): Something didn't seem quite right. I opened the case again and probed around. Circuit Common and chassis ground appear to be isolated. Mitutoyo supplies a very brief design recommendation with RC LPF filters (22k & 220 pF). The Large display uses copious LC LPF's (10 uH & 330 pF). Tentatively, it appears the linear sensors are powered by about 1.5VDC from the display. No 5VDC to them; although, it is brought out to the DB-9 socket. The 5VDC regulator is for the display and pull-ups on the sensors open collector outputs. The same series of devices for the quill is powered by a single lithium coin cell. The longer sensors for the mill XY axes do not have batteries that I know of. Will update as I find out more.