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Phototronics serial port

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mdanh2002

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Hi,

I have an Appa 207 multimeter which outputs its measurements via a RS-232 phototronics serial port at the back, according to the manual. I do not have the RS-232 to USB cable for this multimeter so I was trying to find some way to make it work. According to the manual, the transmission via RS232 is one-way (multimeter to PC only). I disassemble the unit and found 2 LED inside this phototronics serial port. By setting the multimeter to transmit its data and probing at the TX pin on the board I can see some sort of serial data stream on my oscilloscope. However, they are clearly not pure RS232 to me, more likely some customized waveforms to light up the LED.

I tried to use the TFMS5400 40kHz infrared receiver to see if it's infrared but nothing is received. I also tried with a cheap cellphone camera to see if the LEDs light up during transmission but they also don't light up in the viewfinder. Maybe they are not infrared LED.

Any ideas how I could receive the data from this port? I can possibly trace it back to the processor board where the RS232 signal is originally generated and extract it from there but maybe there is an easier way.

Attached are the photos of the 2 LED and the waveform.
 

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Can you post a picture of the optical interface on the rear of the meter? I want to see what the optical interface connector looks like.

Appa does show the accessories for your meter **broken link removed**. Even with a work around for the optical cable, they show a IC-300 RS-232 Interface Cable (APPA 100/200/300 Series), IC-300U USB Cable(APPA 100/200/300 Series) for cables as well as CD-300 WinDMM 300 software (APPA 300 Series, 207).

The cables like this, RS232 and USB that I have worked with are all IR (Infra Red) and even though your meter is unidirectional the cables are normally bidirectional. Even with the cable you would likely need software. Possibly you could use a USB to RS232 dongle on the PC and then use Hyper Terminal to read the data.

Ron
 
Can you post a picture of the optical interface on the rear of the meter? I want to see what the optical interface connector looks like.

Appa does show the accessories for your meter **broken link removed**. Even with a work around for the optical cable, they show a IC-300 RS-232 Interface Cable (APPA 100/200/300 Series), IC-300U USB Cable(APPA 100/200/300 Series) for cables as well as CD-300 WinDMM 300 software (APPA 300 Series, 207).

The cables like this, RS232 and USB that I have worked with are all IR (Infra Red) and even though your meter is unidirectional the cables are normally bidirectional. Even with the cable you would likely need software. Possibly you could use a USB to RS232 dongle on the PC and then use Hyper Terminal to read the data.

Ron

Hi,

I attached the photo of the port to this thread. The RS232 phototronics port is highlighted in red. The 2 IR LEDs can be seen through the two holes on this port.

I have the software CD for this multimeter - just not the RS232 cable. I still think the port uses infrared, but perhaps the transmitted signal is modulated at different frequency - that's why my TFMS 5400 IR receiver (demodulates at 40 kHz) cannot receive any signal. But I don't understand why the infrared LEDs, if they are indeed IR, don't show on my cellphone camera (my remote controller IR LED does)

The board has 4 pins, RXD, GND, TXD and VAL so I think it's made for bidirectional communication. But where is the receiver part - I only saw 2 IR LEDs.

I would like to get this working without purchasing the cable - perhaps by making a circuit which would demodulate the TX signal directly into normal RS232 to be fed to a computer.
 

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I have the software CD for this multimeter - just not the RS232 cable. I still think the port uses infrared, but perhaps the transmitted signal is modulated at different frequency - that's why my TFMS 5400 IR receiver (demodulates at 40 kHz) cannot receive any signal. But I don't understand why the infrared LEDs, if they are indeed IR, don't show on my cellphone camera (my remote controller IR LED does)

It won't work with a TSOP, as it's not a remote control - why would you imagine it would need modulation like a remote control?. It will just be an LED (and quite often visible, not IR) stuck on the data output - which might bear little resemblance to RS232.
 
It won't work with a TSOP, as it's not a remote control - why would you imagine it would need modulation like a remote control?. It will just be an LED (and quite often visible, not IR) stuck on the data output - which might bear little resemblance to RS232.

Hi Nigel,

I did not see any visible light from the LEDs during the transmission. What do you think is a possible receiver circuit for this? If it's not some simple modulation/demodulation, then I am guessing the RS232 cable for this multimeter would have some dedicated electronics to convert the data stream from the LED back to RS232 to be fed to the PC. Any suggestions?
 
The light has about 0.1 inch to jump in a very dark environment. There is no need to 40khz modulation.
The two "LEDs" could be a LED and a photo transistor. (data in, data out)
 
The light has about 0.1 inch to jump in a very dark environment. There is no need to 40khz modulation.
The two "LEDs" could be a LED and a photo transistor. (data in, data out)

Hi ronsimpson,

Thanks for your input

What do you think about the data stream I probed at the TX pin? Any hope of getting the actual data from that, or should I simply forget about this and just purchase the RS232 cable?
 
via a RS-232 phototronics serial port at the back, according to the manual
The manual may be a bit loose with its terminology.
RS232 defines the ELECTRICAL interface, the voltage levels and also the connector, it does not define the format of the data on the lines.
99.9% of the time the data format is asynchronous serial data, but not always.

The cable (which you don't have) for the multimeter obviously has some electronic circuits in it to convert whatever comes out of the photo transistor to voltage levels.

Looking at your oscilloscope waveforms, I am not sure that you are seeing the correct waveform, it could be that the sampling speed is a bit low, hence the fast pulses are one height, and the slower pulses are of greater height (in the negative direction) (I know what I mean!:rolleyes:).
Try speeding up the sweep if you can on that scope and scroll through the data stream rather that trying to put it all on the screen at once.

As a way forward in decoding this data stream, can I suggest that you:
Set the meter so that the display is all zeros.
Change the mode, ie DC volts, AC volts, DC current, AC current etc and see what changes in the data stream.
Now try changing the range and see if there is a change in the data stream. The decimal point should move.
Inject a voltage and change the digits one at a time, again looking for where the change is in the data stream.

While doing all this, bear in mind that if the output is "RS232" :rolleyes: for display on a PC or whatever, it will probably be coded in ASCII,
also, on the scope screen, the most significant bit of a byte of data will probably be on the right of the group with the least significant bit on the left.
Not MSB on the left and LSB on the right as it is usually displayed on the written page. (I have burned my brain with this many times over the years).

JimB
 
The light has about 0.1 inch to jump in a very dark environment. There is no need to 40khz modulation.
The two "LEDs" could be a LED and a photo transistor. (data in, data out)

That would be my guess. The cable I have here is an IR2 Optical Interface RS232-C and while your connector is different my guess is they are about the same thing. One is an emitter and one is a receiver. You can likely build a circuit using an IR photo transistor to find which is which. If you are correct you only need the receiver side. A Google of "ir phototransistor circuit" should give you a few possibles. Remember you only need the Rx and not the Tx (Emitter).

Ron
 
The waveform posted in #1 looks strange because you are using a voltage probe to look at a current waveform. The LED uses current not voltage to make light.
Lets say the LED is like a 3V Zener diode. At 3V you have 10mA and light. At 2.9V you have no light and no current.
Look at the voltage waveform. It slopes down slowly because the LED pulls no current below its operating voltage.
The flat top of the voltage waveform is when light is produced. As soon as the voltage drops the smallest amount the current goes to zero and the light goes to zero.
If you had a current probe you would see very sharp on/off edges.
 
Thanks all for the input. I do not have a current probe at the moment. I guess the only feasible way for me to attempt this is to purchase a few photo transistors from eBay and build the receiver circuit. Just curious, can I also attempt it with photoresistors? I have a few LDRs in my junk box.
 
Thanks all for the input. I do not have a current probe at the moment. I guess the only feasible way for me to attempt this is to purchase a few photo transistors from eBay and build the receiver circuit. Just curious, can I also attempt it with photoresistors? I have a few LDRs in my junk box.

No, you want photo transistors. You should be able to find some locally in Singapore. You can try a photo resistor but I would not be too optimistic.

Ron
 
Any number of them should work. You need to look at the data sheets. Here is the PT333-3C for example. Also note what RonSimpson noted:

The light has about 0.1 inch to jump in a very dark environment. There is no need to 40khz modulation.
The two "LEDs" could be a LED and a photo transistor. (data in, data out)

Ron
 
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