Re: [CAD_CAM_EDM_DRO] Re: Hacking Cheap Digital calipers...

Lee:
have you figure out the mitutoyo as well?
mariano


Lee Studley wrote:

> oops had the 'section' descriptions reversed. below is corrected:
>
> Well, I figured out the serial format of cheap digital caliper I got
> on Ebay. Hopefully this will be a start to people wanting to make
> DRO's from this type of caliper and maybe the format is a common one
> (ie like Digimatic)
>
> It has and SPI interface with clock and data. The data "burst" has 2
> sections of 24bits data equaling 48 bits per packet. The first 24bits
> are shifted out on the rising edge and qualified on the falling edge
> by the reading device. Bit timing was approx. 13uS clock cycle
> yielding about 6.5us to detect the falling
> edge and sample the data.
>
> Section1:
> The first section of 24 bits is also shifted out lsb first. This
> 24bit record is an 'absolute' binary count with a constant error
> offset, for example, when the caliper is fully closed (0.000) the
> count was 007FD3h(32723 decimal). By subtracting this value
> from the record then dividing by 5000h(20480decimal) you get
> the 'inch' readout.
>
> Section2:
> Bits are lsb first of a 24bit record. The 2nd section of 24bits is a
> 'relative' 24bit count subtracted from 0xFFFFFF with scaling relation
> of 5000h(20480decimal) counts per inch.
>
> The mode buttons had no apparent affect on the data records sent out,
> other than freezing the 'relative' section, so the micro in the
> caliper must do a conversion by scaling for displaying the metric
> value.
>
> I wrote some quick and dirty Pic code to log data in hex format using
> Hyperterm. I cheated and used an emulator.
> I targeted a 16C558 running at 10Mhz, but may convert it to a 8pin
> part. I used 1/2 of an LM339 quad comaparator(cheap) to level shift
> the 1.5v logic to 5v for processing.
>
> Let me know what you think...
>
> -Lee
>
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