Digital Caliper Interface?

[xrunner]

I picked up a nice little digital caliper the other day from Harbor Freight Tools. It seems to be very accurate.

There are two removable covers. One is for the battery. The other is for ... what?

There is no mention whatsoever of the other cover in the manual, yet, it's there - designed right into the tool and not hidden. When you remove it, you see 4 terminals/pads. Any guesses?

[c4757p]

If it's anything like my very similar one, that's where the gremlins enter at night to drain the battery.

Stick a scope on it. UART?

[Chet T16]

Is this one the same?

http://www.43oh.com/2010/12/interfacing-ti-launchpad-to-digital-caliper/

[DavidDLC]

http://www.nerdkits.com/videos/talking_calipers/

[DavidDLC]

Another one:

http://www.nerdkits.com/videos/digital_calipers_dro/

[notsob]

This may be of some use

http://www.pcbheaven.com/exppages/Digital_Caliper_Protocol/

[chickenHeadKnob]

These have been extensively hacked, there is a uart output. see hack a day search results:

http://hackaday.com/?s=digital+caliper

[xrunner]

Yea that's it thanks!

Funny it's not mentioned in the manual though.

[ben_r_]

URGH! Lookin at those cheap calipers makes me cringe! I literally have three of them on my shelf that all stopped working WAY too quickly. I finally got fed up and bought a Mitutoyo. Been WAY happier ever since.

That aside, what would one want the digital output of a digital caliper for anyway? To re-display it elsewhere? Are you really going to "log" your measurements?! Create the interface and then a program to drop the measurements into Excel or something? Do people really take THAT many measurements?

[ve7xen]

That aside, what would one want the digital output of a digital caliper for anyway? To re-display it elsewhere? Are you really going to "log" your measurements?! Create the interface and then a program to drop the measurements into Excel or something? Do people really take THAT many measurements?

They're cheap linear position indicators you can integrate into other mechanical devices. If you want to keep them as calipers though I have no idea. Maybe useful if you're trying to collect a lot of measurements quickly for CAD modelling or something?

[ben_r_]

That aside, what would one want the digital output of a digital caliper for anyway? To re-display it elsewhere? Are you really going to "log" your measurements?! Create the interface and then a program to drop the measurements into Excel or something? Do people really take THAT many measurements?

They're cheap linear position indicators you can integrate into other mechanical devices. If you want to keep them as calipers though I have no idea. Maybe useful if you're trying to collect a lot of measurements quickly for CAD modelling or something?

OH gotcha! Now THAT IS a good idea! Thank you.

[eKretz]

It's there for SPC.
Statistical Process Control. Basically, yes, people do take that many measurements and enter them into spreadsheets. It is sort of a requirement when manufacturing to ISO standards. (I know you've seen "ISO 9001" etc.). Certain high confidence manufactured parts (think aerospace, passenger jets, etc.) require serial numbering and records kept of every single size on critical dimensions. These dimensions are stored by the manufacturer in case of critical defect so the parts can be dimemsionally analyzed post-mortem even if they are destroyed or lost. That port allows for a measurement to be taken, then a button is pressed to send the final dimension to the computer/spreadsheet.

[GeoffS]

If you want details of the protocol used, have a look at Shumatech.com
There's also details of an ARM based DRO with open source software.

I have one of the DRO-550 on my mill, works well.

[superUnknown]

Arrg. No thumbwheel makes fine adjustments impossible! 
Spend the extra $5 and get the cheapo 6" calipers WITH thumbwheel.

[elektrinis]

Old topic, but could not find the info.
I have a project where I need to reset the caliper not to zero, but to other relative value, say, 10mm.
Is it possible to do via this digital interface? I see it supports 2 external buttons for zeroing and switching modes, but what about writing a relative value?

[tsman]

I have a project where I need to reset the caliper not to zero, but to other relative value, say, 10mm.
Is it possible to do via this digital interface? I see it supports 2 external buttons for zeroing and switching modes, but what about writing a relative value?

The serial data stream is output only. There is no command interface beyond the mode and zero buttons.

[elektrinis]

I see it has dual data output, a relative position (what you get azter zeroed out) and an absolute position. Couple sources say the absolute position changes after every power on. But does it? If it actually starts with the same number (given that caliper was not moved), this actually can solve my issue, as I would then adjust it mechanically.

[JVR]

A power on is defined by sticking a battery in. These cheap calipers never actually turn off, they just turn the LCD off.

[elektrinis]

Yes, by power I mean power, not a button press.
So, does it power on with absolute value based on sensor position, or just a random number?

[shteii01]

http://www.robotroom.com/Caliper-Digital-Data-Port.html

[KL27x]

That aside, what would one want the digital output of a digital caliper for anyway

https://youtu.be/XIAC1zyYGrc?t=624
If only.

[KL27x]

A power on is defined by sticking a battery in. These cheap calipers never actually turn off, they just turn the LCD off.

Most all digital calipers go to a low draw sleep mode when turned off. Many of the cheap calipers use the same microcontroller, even the same board in some cases. All of mine use the same board, and there are a pair of redundant buttons on the board so it will fit into many different models. I believe AvE, or maybe one of his viewers, measured the sleep current in a HF caliper and it is 4x the draw of a Mitt.

But the biggest difference is the battery. Mittutoyo come with a silver oxide battery. I put Energizer silver oxide cells in my cheap calipers, and they last over a year. But don't buy the cheapest batteries you can find. I tried some generics, and they lasted 3 months. I measured the cell voltage on them, and I got 1.588, so I'm pretty sure they're alkaline. A true silver oxide is 1.550 on the button, when new.

[Kilrah]

Couple sources say the absolute position changes after every power on. But does it?

A power on is defined by sticking a battery in. These cheap calipers never actually turn off, they just turn the LCD off.

Some do, some don't. Nearly all of those I've used keep the value, but the other day I came across one that zeroed at every power on. Very annoying when you're used to the other.

Mine must turn off since a battery lasts several years (but doesn't zero).

[KL27x]

Unless there's a hard switch on it, it most likely just goes to sleep. The sleep current is low enough on a modern micro that it close enough to being off when used with a silver oxide battery. It's not really worth the extra cost of putting in a FET to switch the power on. It's just a matter of user education that they ship these things with an inappropriate battery for this application.

I would like someone with a mit to put an alkaline in there and see how long it lasts. I bet it's pretty disappointing.

[kosine]

I repurposed one of these calipers as a linear encoder back in 2011 (time flies!), and digging through my notes here's a few bits of info that may be useful - assuming the module used is the same (looks like it):

Pinout is same as shown on the robotroom link above:

1 Battery -ve
2 Data out
3 Clock
4 Battery +ve

My steel caliper was +ve ground which means that the entire metal framework is connected to the +ve terminal of the internal 1.5V battery. This may cause problems if the caliper is to be attached to anything metal. (I.e., don't ground the metalwork.) Probably not an issue with a carbon-fibre model.

If nothing has changed, then a suitable PCB connector for the caliper is a 4-way 2.0mm pitch Hirose DF-3 SMT socket. (Requires dissammbly to fit.) You'll also need the mating plug.

I seem to recall that shorting some of the pins forced a reset, though can't recall which two. This is probably a bug not a feature, though my caliper is still kicking around and working all these years later.

Another bug is that these cheap calipers do sometimes jump to a random value, so not ideal for important applications, but still useful for casual projects.

Last time I checked, you could also by these as standalone encoders, i.e. same module on a slide, but without the caliper head. They came with mounting brackets for each end instead. I used a couple on a lathe project many moons ago.