DC-DC converter

I have a vague recollection that 2048 comes into the scaling of these things

153600/2048 = 75

Which may, or may not, be relevant to something

MichaelG.

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P.S. __ I have mentioned this before, but it may be worth linking here:

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

At 2 March 2012, 10:30:16 user Hans U. Meyer wrote:

Edited By Michael Gilligan on 10/04/2020 19:32:26

Bit higher that I expected, but almost certainly related to comms or LCD data rate. Incidentally:

3 x 512 = 1536

so definitely powers of 2 come into it.

SoD: Your task for tomorrow is to stick the 'scope on the comms and see what the bit rate looks like.

Andrew

Andrew's wish is my command!

Hard to see what the LCD feeds do when the slide is moved because there's no way of getting a probe on the LCD lines after the caliper has been reassembled.

However, the two of the 4 wires I soldered on to the caliper are data for an external display. On these the minimum pulse width is 80uS, and the caliper signals in 1.23kHz bursts.

I don't know if this output and the LCD update together at the same rate. These things aren't meant to come apart and in my clunky hands the LCD has stopped working! I've also lost one of the buttons.

Dave

Dave (SOD), I've been following your experiments with great interest. Regarding the LCD, from the PCB photo it looks like the LCD has a zebra strip that presses against the PCB. Having an appliance I have been trying to debug/repair for several months with one of the same style of connections, it only takes a slight misalignment for the contacts to be in the wrong place, so perhaps a bit of fiddling around with the alignment and pressure would help.

Are you going to try a decoupling cap on the position you have shown?

Paul

Hi Paul,

I think you're right - the contacts between LCD panel and board are via a sort of rubber strip thingy, which fell off! Very likely I put it back out of alignment. Also, one of the screws has gouged through the board and it's not applying pressure to the strip at one end. Fortunately, I'm experimenting with a cheap caliper I dropped on a concrete floor and then stood on, not a brand-new Mitutoyo!

I intended to try decoupling but I can't test it now I've broken the display. Why is nothing is ever easy?

Dave

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ferinstance : **LINK**

https://www.c-a-d.com.hk/design.htm

MichaelG.

'Zebra strip' (some are black and white) with alternating conductive and non-conductive stripes. The spacing should be cleverly designed so that you don't need the strip perfectly aligned, just the PCB and the display unit lined up with each other.

Usually the problems are either tiny bits of grit breaking a contact or finger grease. Degrease in alcohol (meths works OK).

Neil

It happens to us all. I knocked a cup of coffee over my keyboard this morning and I'm only just back up and running. Although to be fair in the meantime I've also had lunch, gone for an 8 mile cycle ride and had a long sulk sitting out in the garden.

SoD has it nailed! Some playing with the numbers gives:

153600/128 = 1200

or 1200 baud for the serial comms. The measured value of 1.23kHz is pretty close allowing for measurement error and tiddly crystals that aren't that accurate.

I hate zebra strips. Many years ago in the mad world of motor racing the company I worked for used them to connect the electronics to the graphics LCD display on a handheld unit. The build instructions consisted of assembling the unit, seeing if the LCD worked and if not dis-assembling and repeating until it did. Mind you the draughtsman/mechanical 'designer' was incompetent and got the heave ho shortly afterwards.

Andrew

Some of us are more accident prone than others! Intending to follow Neil's advice, I took her apart again and discovered someone who cannot be named (it was the perpendicular pronoun), had put the strip on the insulated side of the LCD.

1200 baud is convincing, I hadn't twigged that relationship!

Another noise relevant observation. Running the caliper directly off a 1.5V Zinc Chloride 'C' battery and short leads, the self-generated noise falls to about 40mV max. (Distinctly better than running it off a 9V battery and potential divider.) So far so good, but touching the metal slide with my hand caused the noise to double. Turns out the caliper's metalwork is connected to battery positive. Normally this wouldn't matter, but it's likely to introduce noise if the remote battery or power supply is on the end of a long lead.

There's a magic smoke implication too. If a mains DC power supply is used to run a positive frame caliper touching an earthed machine, make sure the mains PSU isn't DC negative earthed, as many metal boxed units are. (This is DC fuse blowing problem rather than a mains shock hazard.) Wall warts should be OK, unless someone knows they ever have a DC line connected to mains earth? I assume their DC outputs always float?

Dave

Edited By SillyOldDuffer on 11/04/2020 17:55:37

To be fair it's the only practical solution if the display has the contacts on the glass.

That's a magic number in comms. The first modem we received from the US in 1964 was 2400 baud.
Made by ITT and built like the proverbial, about 18ins x 12 x 18 with the individual plug-in modules enclosed in square alu frames and edge-connectors down.
Later came 4800/1200/300/150/75 baud modems.
The 1200 baud modems went to Paris and the PTT had to type-approve them and stuck engraved brass labels on each one. (he's rambling again...)
DaveD

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Just reviving this thread to add a note which may be useful

My Mitutoyo caliper was still working fine, but displaying B for battery warning

Installed a new Energiser 357 [SR44] and checked the voltage of the old one: 1.37V

MichaelG.