Fluke 45 Serial Communication

[WaveyDipole]

I am trying to get my Fluke 45 working with serial communication. I have one of these cheap USB-to-serial adapters. When I plug it in to the Fluke, the display shows all segments and freezes. I did a search on this forum and found that someone solved the problem by using a null-modem cable, although using a hardware serial port to connect to the PC. I have made up a null-mode cable but have so far been unsuccessful in making this work. Baud rate and parity are set correctly on the meter to match my terminal client. Using a logic analyzer I can see data being transmitted, but nothing being received back from the meter.

Unfortunately, Fluke does not document the pinout but does refer to RS232, so standard 9 pin RS232 is assumed at the moment. Only pins 2,3,4 and 5 are present which narrows down the possible permutations. Has Fluke used a non standard pinout on these?

[bw2341]

The first Google Image Search result for "fluke 45 back panel" shows the pinout diagram right above the DB-9. It shows:
2 RX
3 TX
4 DTR
5 GND

This is the standard pinout for DB-9 serial introduced with the IBM PC-AT. A standard female-female DB-9 null modem cable should work.

Does your USB to serial adapter have a DB-9 or DB-25 male? If it doesn't, it probably has a pin header. The cheapo USB-serial boards with pin header outputs are using 0-5V or 0-3.3V for "TTL" level serial. They're used to connect to serial consoles on modern embedded CPUs and ASICs. They are not compatible with traditional serial ports that use -15V levels.

You can check the serial level of both the Fluke and your USB-serial adapter with a voltmeter. Just measure between GND and TX. Since serial ports idle on logic high, you should see a positive voltage on a TTL level serial device. A traditional serial port will have a negative voltage.

[Zermalmer]

In this older video from Dave on youtube ( https://youtu.be/mK-Sc57SA0I?t=10m10s ) he connects  terminal emulation to the serial port.

At the end of the sequence you also see that there are some cases out there which have printed pinout on it.

If I understand Daves comments right the serial port is ONLY a sending port (for printing) and doesn't allow communication in both directions.
For communication I think you will need the IEEE port.
I was wrong.... sorry for that

You will find the full manual on the Fluke webside and there is also a description of the RS232 in chapter 5
You can find there also the Service manual which contains procedures for communication over RS232

I wait for my own 45... still somewhere in the mail  ... so I can only talk in theory.

[WaveyDipole]

Thank you for the helpful replies. I have both the user manual and service manual from the Fluke website. The Fluke service manual does mention that there should be a 5v on the DTR line at idle time, which I checked and indeed there is.

I couldn't locate the image you mention, bw2341, but thanks for confirming the pinout which in turn confirms that the RS232 standard is being adhered to by Fluke. This obviously points to the adapter as the culprit.

I actually have two usb-to-serial adapters - an older PL2303 with a male plug which pretty much works with anything, and the new one that I purchased recently with the female plug and which is identified as 'QinHeng Electronics HL-340 USB-Serial adapter'. Both are 9 pin, but it is the QinHeng that I have been trying to use with the fluke and that is giving me problems. I have in the meatime been able to find a 9pin female D plug which I was able to use to adapt the null-modem cable into a female to female version which allowed me to test with the original PL2302 adapter.  The good news is that the genuine PL2303 adapter works and I got a response from the *IDN? command. It even works without the DTR line connected. It seems sufficient just to cross over the TX and RX lines.

I guess the QinHeng will be going back as not fit for purpose.

[Zermalmer]

The Fluke service manual does mention that there should be a 5v on the DTR line at idle time, which I checked and indeed there is.

Have a look to page 2-19 chapter 2-35 of the service manual.

Data Terminal Ready (DTR) is a modem control signal controlled by the
Microprocessor. This signal is an RS-232 output generated by driver A1U7-7; it is at
+5.0 V dc when the meter is powered up.

For the normal communication it isn't necessary, but if you have further plans you can maybe use this as indicator that the DMM is "ready for use"