I recently bought a Fluke 45 on eBay and I want to replace the RS-232 connector on the back with a nice micro-USB connector so that I don't need to use specific RS-232 cables and an adapter, but can just take any regular micro-USB laying around.
So, as a beginner, I try to do this all slowly and carefully, because the last thing I want is to break my Fluke 45 or influence it's workings in any way.
So I started reading all the details of the RS-232 in the service manual, read the datasheet of the used MC155406DW RS-232 chip and designed a (extremely simple beginner) solution based on a cheap micro-USB CP2102 board in combination with a cheap MAX3232 board (powered by the CP2102 board)
Checked and double-checked all specs, signals and voltage-levels used.
Before testing anything on the Fluke, I test first with a DB9 cable to the MAX3232 which is connected to the CP2102 micro-USB and an old laptop with a RS-232 DB9 connector playing the 'role' of the Fluke.
Now the weird part:
When I first connect the micro-USB and then the RS-232, everything works fine. But of course, when I want to build all this nicely inside the Fluke, the RS-232 side will normally be powered first until I decide to hook it up to my laptop and connect the micro-USB side.
When I test that (first connect RS-232 to old laptop and then micro-USB to my laptop), the MAX3232 becomes very hot in a matter of seconds. So I disconnect it quickly again: Nothing broken.
Tried the 3V3 and 5V from the CP2102 board to power the MAX3232 board, but no difference.
Then I realized according to the service manual of the Fluke, the DTR pin of the RS-232 is always on +5V. The same is true for the old laptop I'm testing with. So I now power the MAX3232 board on the RS-232 side via the DTR signal from the RS-232 and disconnect the 3V3 or 5V from the CP2102 on the TTL side of the MAX3232.
This works! The only strange behaviour I see is that the first byte sent by the RS-232 when the connection is made, echos back a random byte to the RS-232 input. I don't know/think that is a problem?
But now my question:
Before actually hooking it up the Fluke (first via the DB9 test cable, but later by building it in and replacing the whole DB9), will I run any risk after all tests I've done with the old laptop?
Secondly, can this in some way influence the workings/precision of the Fluke? Maybe the MAX3232 can create interferences I know nothing about? I have no idea what to think of but want to play it safe :-)
On the other hand, if possible and safe, I really like to replace the RS-232 with a simple common micro USB UART connector...