Fluke 724 calibration cable

[MatteoGaria]

Hello everyone,
This is my first post here, so let me introduce myself a little... I'm a software developer profesionally speaking, father and diy enthusiast and in the real life. Also electronic hobbyist when some time is left from all the previous.
Please forgive me for english language errors,  I'm almost totally self-taught about that.

Coming to the real topic, a couple of years ago I found a fluke 724 temperature calibrator in the dumpster at my previous workplace. It was discarded just because... battery contact oxidation, a broken input plug and a severely dirty case.
Well I gave it the love it needed and now is mostly ok, I just need to finish the job by 3d printing a bit of the plastic cover that's missing.

When used in DC voltage generation mode, a modality really useful for me, it shows an annoying 40mV offset on the output. It remains constant through all the scale, so, for istance, if I set the output to 1.000V it sources 1.040V. Now I can't remember but an offset probably is present also in other modes.
Obviously it's usable like that, just by setting it against a reference multimeter, but I was looking how to fix it and I came across it's calibration manual. It seems that it's requested to connect the instrument to the computer using a specific cable and Hyperterminal (or similar) is all you need on the software side of things. Plus some references but I can arrange for that.

The issue is the serial cable, the original is discontinued by Fluke (PN 1556747) and some knockoff are available online for about 150$. I sincerely don't want to spend this money (also because this instrument may be broken and a calibration may not solve the issue) and I was wondering what kind of magic is this cable to cost so much.

It's a serial cable, DB9F on the computer side, 10 pin 2.54mm connector on the other side, with a black box on one connector side.
Maybe it contains a level shifter of some kind, maybe it short-circuits some pins to tell the instrument that the cable is connected, maybe it does nothing of that and it's just pricey.
The calibration manual specify no hardware flow control for the serial communication so I think it's just GND-RX-TX.
Before starting by trial-and-error on reverse engineering the connector, by following traces and so on, I was wondering if someone knows this cable/instruments and has some information to share about that, helping me in building this cable.

Thanks in advance to all
Best Regards

[MatteoGaria]

I sorted it out!

Having a little time I decided to go ahead and take some measurements,
4 pins are grounded directly to the negative of the batteries... so it eliminates a few pins.
Some pins are showing a very stable 5V value, a couple are around 0V but not really grounded... by the way I found a pin that shows a not very stable 5V value and BINGO! it's the serial TX.
By following the instruction in the calibration manual I got the prompt asking me the calibration password.
Finding the RX took a little bit of trial and error, I placed a 100ohm resistor in series to my usb-ttl converter (an arduino uno adapted for the task) for a safety measure, but after a few tries I entered the password on a serial terminal and I was pleased by the full calibration menu appearing.

Maybe it can save some time to someone else, here a rude drawing of the connector pinout
Other pins purpose remains a mistery, but this device shares the internal electronics with the 725 and 726 models that have more functions.

Code: [Select]

^TOP OF THE INSTRUMENT ^

GND *  * SERIAL TX
GND *  * around 0V
GND *  * 5V
GND *  * around 0V
 5V  *  * SERIAL RX

A quick calibration of 0 and full scale V output solved the offset issue.

The 150$ cable is really an RS232 / TTL converter or, as in my case, an USB-TTL converter.

Best regards to all

[salayhan]

please
Fluke 724 calibration cable schematic pin number.