Tonghui TH1942 - Fitting Native RS232 port - Page 1
Forum section: electronics / testgear

author: picitup
author_rank: Regular Contributor
timestamp: October 10, 2015, 10:31:48 am
content: Hi AllI just bought a Tonghui DMM from Magnus who packed it very well and made it an easy transaction, so thanks Magnus :-)The user guide and spec sheets are here:https://www.globalmediapro.com/att//a/2/6/l/a26lx8/th1942-manual.pdfhttps://www.globalmediapro.com/att//a/2/6/l/a26lx8/tonghui_th1942.pdfI did a bit of reading and querying before my purchase and found that it had no physical RS232 port on the back, but had a USB port. Magnus confirmed that this was actually a USB/serial port. Great for connecting to a PC but not so great when connecting to a micro controller. I did spot that even though there was no connector, there was a cut-out for an RS232 port (IMG3_Rear) and looking at the picture from another review, it seemed the PCB area for the RS232 existed, but was unpopulated.To take the meter apart, all you need to do is remove the two screws from the rear of the unit (IMG3_Rear) and slide the cover backwards. If you want to separate the front panel, you just need to peel off the grey surround from the front and there are 2 screws either side of the front panel.IMG5_RS232Perspective shows the RS232 and USB interface area of the PCB and you can clearly see the cut-out for the RS232 connector which has just been covered up by the rear sticker. There's a great temptation to cut this back out with a scalpel, but hold on a second, I don't know if I can get it working yet lol.The picture IMG5_RS232Plan shows a top view of the USB/RS232 area. I've not put any great thought or testing into this, but I'm guessing the missing chip U125 is a MAX3232 or something. I'm not sure what chip U120 is, but may be a multiplexer to switch between USB/Serial and RS232.There's a couple of 6N136 optocouplers so I guess these are output isolation for TX and RX. If you look just under the rectifier, there's a wide horizontal gap in the PCB copper which is only bridged by the opto isolators. The data sheet is here:http://www.vishay.com/docs/83604/6n135.pdfThere's also a Silicon Labs CP2102 USB/UART chip (U135). Data sheet is here:https://www.silabs.com/Support%20Documents/TechnicalDocs/CP2102-9.pdfOn the left, you can see a bridge rectifier, a fat electrolytic capacitor and a plan view of a 7805 so I guess this is the 5v isolated power supply for the interfaces.On the centre right is a place for a connector (X111). I'm not sure what this is for until I trace out the PCB.So..... Before I unleash the full power of my intellect on the problem (about 10mW lol), has anyone got one of these meters with an RS232 port fitted? Would you be OK to send me a piccy to make this a true 5 minute job?As an aside, I was playing with my power supply, the Hantek PPS2320a and noticed this also has a USB port at the back which is a USB/Serial converter so I might move onto this at a later date.Thanks for reading.....Steve

author: picitup
author_rank: Regular Contributor
timestamp: October 10, 2015, 10:32:55 am
content: Rest of piccies.....

author: picitup
author_rank: Regular Contributor
timestamp: October 10, 2015, 03:43:01 pm
content: OK so I waited the obligatory 10 minutes for someone to give me the answer but got impatient so I buzzed the tracks out. The missing chip is a MAX232 which is the 5v version of the RS232 driver. I've traced out the circuit (IMG7_RS232Schematic) and all I need is the chip in a SOIC16 format and a few 1uF 0805 caps.The fly in the ointment is that the TX output from the CP2102 is connected to the R1 OUT of the MAX232. As both outputs will try to drive each other, it's become clear that the design of this interface is to either populate chips on the USB/Serial side, or the RS232 native side but not both.I was sort of hoping there would be a link here to avoid the conflict but I guess that would cost another 0.001 pence so wasn't included.I could de-solder the CP2102 or just cut the TX track but it would be nice to have both as an option, so if you look at IMG6_RS232PlanCutTracks I've drawn where the tracks need to be cut. Gosh that track A is small.Now, if the tracks are cut and track C is connected to track A, the function is USB/Serial and if C is connected to B, it's native RS232. Depending on how lazy I feel I'll either hot glue some link pins on the board or fit a c/o switch on the back.Well, all that needs to be done is to order the parts and see if it works. I'll post back up when I've soldered the parts in and (hopefully) it's all working. This has been quite a bit easier than I'd thought. It was a good day today.Thanks for reading.....Steve

author: bitslice
author_rank: Frequent Contributor
timestamp: October 10, 2015, 10:48:34 pm
content: I thought it might have been a MAX232 when I saw the row of cap pads.You could always use VBUS on the USB to detect if anything is plugged into that port, then use a relay to isolate the RS232 port?The cp2102 can be put into suspend mode, maybe that was the intent if ever a RS232 was fitted

author: picitup
author_rank: Regular Contributor
timestamp: October 11, 2015, 09:10:04 am
content: Yes I think that was a give away.That's a good idea to use a relay powered from the VBUS power. I'll try and find a small 5v reed CO relay.I did look into possibly suspending the cp2102 and funnily enough there is an led (D301) connected to the inverted SUSPEND pin (11) see pic IMG7_SuspendLED. This would help by disabling the TX output, but I would then still need to disable the MAX232 output so I think the relay idea is probably the best and simplest so thanks for that.I've found a small axicom 5v DPCO relay part V23079-A1001-B301 so I've ordered it and will fit that when it arrives.CheersSteve

author: picitup
author_rank: Regular Contributor
timestamp: October 11, 2015, 01:43:30 pm
content: Hi PeterThanks very much for the photo. I see yours uses the SP232 which is pin compatible with the MAX232. I see it also has pin 26 (TXD) of the CP2102 directly wired to the SP232. I would have though this would create two competing drivers. I also see there is a multiplexer chip the 4052.Can I ask - if you have native RS232 and and USB/Serial, how do you switch between the two interfaces? Is there a menu option?ThanksSteve

author: picitup
author_rank: Regular Contributor
timestamp: October 12, 2015, 07:59:30 pm
content: Thanks again for the info. As your config has the two outputs from the USB/UART and the MAX232 connected together (the same as mine), I'm going to populate the board and not cut the tracks as I described earlier and see if it works. It is possible the suspend mode on the USB/Uart chip becomes active when the RS232 port is used, but as I say, I'll solder the parts in and give it a try.I'll update again when the parts for the RS232 section arrive.CheersSteve

author: picitup
author_rank: Regular Contributor
timestamp: October 20, 2015, 03:31:44 pm
content: Well sorry for the slow reply, but work keeps getting in the way of having fun.The good news is that once populated, the RS232 port now works. All I've done, is to add a MAX232, some 1uF 16v 0805 ceramic caps and a 22uF 16v electrolytic. The electrolytic is surface soldered as the ground track is very large and my soldering iron wouldn't melt the track. There was no need to cut any tracks, so I now have a spare relay.The default baud rate for the Tonghui is 9600,n,8,1 and I sent the command *IDN? and it came back with:TH1942 Digital Multimeter, Ver 1.1.11.11.23I've attached a photo of the PCB and the output in Teraterm.Well that was easier than I thought it would be!CheersSteve

author: bitslice
author_rank: Frequent Contributor
timestamp: October 20, 2015, 03:38:27 pm
content: Glad it worked out

author: picitup
author_rank: Regular Contributor
timestamp: October 20, 2015, 06:06:36 pm
content: Thanks for your good wishesJust before I put the case back together I thought I would try out the USB/Serial port. I connected the port to my computer and it auto-installed as a Silicon Labs CP210x on COM2. at first, the suspend LED was lit and I couldn't get it to work, but after unplugging and replugging the USB cable, the LED went out and it worked!The only mystery is that there are 2 competing outputs connected together on the MAX232 and CP2102. My only guess is that the MAX232 outputs can stand a short circuit and I read on the CP2102 data sheet that outputs can manage 500mA so I guess the CP2102 overrides the MAX232 and the MAX232 doesn't mind.Any road up, no nasty track cuts and relays lol.Job done - when I get some spare time I'll look at the pps2320a power supply and see if I can get a native RS232 on that device. I've bought a quad USB/RS232 device based on the MCS7840 chip from fleabay in China. Someone local is offering an XP desktop for free so hopefully if I get that, I can set up an equipment controller pc.I'm not quite sure where all this is going, but it really is funThanks for reading...Steve