Patrick J wrote:
>
> Is it possible to take a 'cheap' digital sliding caliper apart and use it as a
> digital linear sensor w a '877 ? How do they work?
> I have a 80mm long movement which takes min 0.10 sec which I'd like
> to measure with 0.3 mm resolution. I know there are expensive versions
> avail w RS232, I dont think they can handle that fast movement though

Machine tool supply firms sell milling machine quill DRO retrofits
which are functionally identical to digital calipers and have con-
venient form factors for many apps. These have a range of 150 mm.
Larger versions available.

Note that most cheap digital calipers DO NOT have outputs. All such
devices are quite slow - Reading a Mitutoyo "SPC" type device takes
about 34 ms. Resolution is .01 mm for the cheap stuff. Max speed is
1 meter/sec.

Mitutoyo SPC protocol is 3-wire, CLK, DATA, REQ.
CLK and DATA are open-collector outputs.
Pulling REQ low produces 52 clocks, DATA valid on
CLK low, each group of 4 bits represents a hex #,
lsb to msb:

d0-d3     0Fh

d4        sign digit, 0 is positive, 1 negative

d5-d10    measurement value, BCD, MSD to LSD,

d11       decimal point position in the 6-digit
          measurement value, counting right to left

d12       units, 0=mm, 1=inches

Its easy to use these things if you can live with the speed.
Faster devices are optical, referred to as "linear encoders",
"linear scales", or sometimes "glass scales". AFAIK all these
devices have quadrature outputs.

regards, Jack

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