If you read the datasheet for those ICs – the ES519xx series –
you’ll make the interesting observation that they all do RS232
compliant output. So a vast number of multimeters – quite possibly
yours - are capable of serial communication, but most manufacturers
don’t break it out to the front panel for you.

Fair enough perhaps – it adds to the cost and it might not be a
popular feature that everybody wants. On high end bench instruments
(voltmeters, lock-in amplifiers, electrometers…) it’s a completely
standard feature. Typically with those instruments you’re recording a
lot of data, and you don’t want to be writing it down on a piece of
paper like some chump from the 1960?s. For hobby applications, let’s
imagine you want to monitor the resistance of a sensor… once a second.
For 2 days. Maybe you want to measure the temperature of a heatsink as
a function of the current going into your circuit to determine some
operational limits. Maybe you built your own signal generator, and you
want to calibrate it by measuring with a multimeter in 1Hz steps. And
so forth and so such; I’m basically telling you that this can be a
pretty handy tool to have kicking around.

Let me highlight that this is not an RS232 meter,
there is no provision for it anywhere. That said, to enable serial
output you just need to touch the ‘RS232? pin to the ‘V-’ pin. You’ll
hear it beep, and you might even see an RS232 segment light up if the
LCD has one. Note that this mode is not permanent – you can change
measurement settings without losing it but every time you turn off the
meter it will leave this mode. You’ll want to rig up a momentary
contact, normally open switch across these two pins and make it
available from the outside of the case. I’m going to leave the build
details up to your engineering imagination, since this is not actually
my personal meter and I’m therefore not at liberty to cut holes in it.

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