Replacing Fluke 45 RS-232 interface with micro-USB UART - Page 1
Forum section: electronics / beginners

author: mdijkens
author_rank: Regular Contributor
timestamp: January 17, 2018, 10:54:04 pm
content: 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...

author: mdijkens
author_rank: Regular Contributor
timestamp: January 18, 2018, 05:13:30 pm
content: Anyone any thoughts on this?

author: james_s
author_rank: Super Contributor
timestamp: January 19, 2018, 12:39:59 am
content: I just keep a USB to serial dongle attached to mine, I've never seen any reason to modify the meter itself.

author: mdijkens
author_rank: Regular Contributor
timestamp: January 19, 2018, 08:58:21 am
content: I don't have any 'lab' space so I will have to store it away every time after use.If some things are sticking out on the back, there's a risk they will get a 'hit' sooner or later when taking or storing the meterActually that's also the reason it is way more convenient to be able to use a cable I have laying around everywhere instead of finding/searching that old RS-232/DB9 cable in my storage

author: SL4P
author_rank: Super Contributor
timestamp: January 19, 2018, 11:13:34 am
content: In the above posts, you allude to putting both the RS232 and USB chips in your mod.In most likelihood, you can remove the existing RS chip, and connect the logic level pins directly to your new Usb Serial chip.the UART functions are farther inboard the meter. the RS/USB chips are simply level converters (and port negotiation in the USB chip.)

author: mdijkens
author_rank: Regular Contributor
timestamp: January 19, 2018, 12:41:24 pm
content: Yes, I have also been looking into directly connecting the UART to the TTL side of the MC145406DW chip in the Fluke, but I don't dare soldering on the Fluke PCB, especially since it is very very tiny (at least for me)Cutting of the 4 wires of the DB9 and connecting my 'own' circuit there is very easy...But I am aware that I am then having 2 level-shifters in a row

author: james_s
author_rank: Super Contributor
timestamp: January 19, 2018, 06:56:08 pm
content: There's no reason you couldn't put the USB to serial dongle inside the case of the DMM, as I recall there is plenty of empty space inside, especially if you don't have the relatively rare battery option. I see no reason to build anything custom, just hack an off the shelf adapter inside.

author: SL4P
author_rank: Super Contributor
timestamp: January 19, 2018, 08:55:33 pm
content: Having both chip connected at the same time may give you problems.... idlevstates conflicting levels.You might want to add a couple of schottky diodes to isolate the drivers.

author: mdijkens
author_rank: Regular Contributor
timestamp: January 19, 2018, 10:01:51 pm
content: @SL4PCan you elaborate a bit more on that?Sounds important to understandWhat kind of idlevstates conflicting levels?Where should I put schottky diodes?

author: SL4P
author_rank: Super Contributor
timestamp: January 19, 2018, 10:10:30 pm
content: Well, for example if one interface is connected, and the other isn’t, you need to ensure the common pin junction aren’t being pulled in different directions.Low will almost always win voltage-wise, but a forced high level could fry an output.Current limiting is your friend - after Google.Lose the RS232There is no point in TTL — RS-232 —TTL — USB and back again.And check the logic levels are/not inverted as you need them.(it’s worth reading and understanding the origins of serial cooms- there are good reasons why they did what they did back in the 50s, 60s & 70s)

author: mdijkens
author_rank: Regular Contributor
timestamp: January 19, 2018, 11:17:15 pm
content: Alright,understand it a bit nowBut losing the double level conversion would mean soldering on smd components on the PCB of the Fluke, which I am not really capable off I thinkAlso, everyone using the Fluke nowadays with a RS232 to USB converter basically does the same double level conversion (once inside the Fluke and once in the external converter)I did do extensive tests connecting only one side or the other, and connecting/disconnecting in any order and no current above 15mA is drawn from either USB-side nor RS232 side and nothing gets hot ....As far as I know I tried every possible config and order for short and long times and it all looks goodI am mainly concerned about interfering with the Fluke circuitry in ways I have no idea off (noise, filtering, static, HF, and more I know nothing about)

author: SL4P
author_rank: Super Contributor
timestamp: January 19, 2018, 11:23:45 pm
content: So, as suggested earlier, go buy a $5 USB-RS232 converter, plug it into your PC, and the problem’s gone.No dodgy hacks, or questionable wiring, it all works like factory because it is factory.If you don’t want an external adapter, find an RS232 interface that can live inside your computer... PCMCIA, or slot based. I have an 8-port ethernet serial adapter on my workbench. More ports than i use at any given time.

author: IanB
author_rank: Super Contributor
timestamp: January 19, 2018, 11:35:07 pm
content: I use USB-RS232 converters and find this to be the simplest and most robust solution. I get virtual comm ports on my PC that logging software can connect to.

author: mdijkens
author_rank: Regular Contributor
timestamp: January 19, 2018, 11:42:20 pm
content: I have ordered some USB RS232 converters on eBay to test and experiment with. But in the end I'd like to put the (standard) converter inside the FlukeThen I can just pick any standard micro-USB cable I have laying around everywhere and plug the Fluke to my laptopEspecially since I don't have a workbench where I can keep everything setup and ready for use.Whenever I work on something, I have to get everything out of the drawers onto the kitchen-table and at the end of the day I have to put it all away againJust want to approach it carefully

author: mdijkens
author_rank: Regular Contributor
timestamp: March 02, 2018, 03:20:19 pm
content: Yes.. I've done it :-)I've been experimenting with several small boards that directly convert RS232 to USB and are powered by the USB side.I've tested no power is drawn from the Fluke side when:- Fluke turned on, nothing connected to usb- Fluke turned on, when pc connected to usb, then disconnect pc- Fluke turned on, then connect pc, then disconnect pcAll sweet and dandy :-)I've also created a small little windows application that automatically connects when the fluke is found on any usb-port and displays all the data from the Fluke display on my monitor. It also logs everything to a .log and .csvIf anyone is interested, just let me know...

author: mdijkens
author_rank: Regular Contributor
timestamp: March 07, 2018, 01:59:06 pm
content: I have created a small youtube video about it

author: bitseeker
author_rank: Super Contributor
timestamp: March 08, 2018, 12:19:33 am
content: I like how the microUSB panel-mount connector fits perfectly in the old DB-9 opening.

author: Huluvu
author_rank: Regular Contributor
timestamp: March 08, 2018, 02:32:48 pm
content: Hi mdijkens,Thanks for the video and the good explanationsI also own a fluke 45 and it would be great if you would share your softwareKind regardsArne

author: glarsson
author_rank: Frequent Contributor
timestamp: March 08, 2018, 02:44:34 pm
content: DE-9

author: mdijkens
author_rank: Regular Contributor
timestamp: March 08, 2018, 03:05:49 pm
content: Sure. Just send me an emailbtw: The software also works with the regular RS232. In the end it's all (virtual) serial ports...

author: mdijkens
author_rank: Regular Contributor
timestamp: March 10, 2018, 01:04:21 pm
content: Because of all the requests I getI have created a small download page for the software:mdFluke download pagePlease let me know what you think of it.I'd also like to know if it works with other (Fluke) equipment or not.