Help understanding an optocoupler circuit

Projects General Guidance
1

Hey all, hope the holidays are treating you well!

I have a unique situation I've been chasing for about a year, but I think I finally have maybe -just- enough info that someone more versed in optocouplers could help me fill in the missing pieces (or maybe I'm still missing too much).

I have been working with a piece of medical equipment that has a separate standalone display screen. The screen runs its own software, and communicates with the main unit using a 4 pin wire. Up until recently I could only say for sure that one was ground, I assumed there was an RS232 RX/TX and an unknown as everything else was RS232 but this didn't seem to be the case.

I want to use a set of ESP32 boards to send this signal via BLE so a wire does not need to be run. I have some code written that can communicate between the boards, but no idea how to interface the boards to the device or screen.

Recently I found a really old wiring diagram from the late 90s, and it shined no light except we're looking at RX.A, RX.K, and TX for the 3 remaining wires.

image
image324×140 7.85 KB

(I understand there's no ground here but I do have that)

I believed that RXA/RXK were Anode/Cathode, and some sort of optocoupler was needed, but was unsure of the protocol or if I could even interface it with an arduino/esp properly. Until recently, when I came across a device that converted the data to serial input on a computer. I was only able to open it for a second but I was able to take this pic:

image
image777×910 87.7 KB

The black/white connector can be ignored as it's just an LED that lights up whenever the Metro has power. The connector has 4 wires coming from it, given the pics it looks either Anode or Cathode is not connected, just one RX then TX, 5v, and GND. (GND comes from the shield and is soldered to a dummy pin before going to the board). (Can this work without both Anode and Cathode??)

My understanding is this is a 2 channel optocoupler, likely handling RXA/RXK and sending that data to the arduino, and then using the other channel to handle the TX signal back to the device. My belief is the device sends the screen the data of what it is to display, and the screen responds with a very simple "received" command. (the device will alert if it does not get a response from the chart). It appears they are using A0 and A3 for their TX/RX (not sure on the order but I think it is that order)

I figure this is basically exactly what I am after, but instead of serial data to a PC I just want to BT the data to a matching board which converts it back to the format which the screen will understand.

I am hoping someone may see these pictures, see the few traces they can see, and be able to go "oh yeah this is what is happening". It seems all I would need is an MCT6 and a few resistors for each side of things, as well as a power source for the ESP32 which is easy enough.

Any help would be amazing. If you can determine just this side I can likely come up with the other, but if you are feeling generous this holiday season and want to provide both this side and the matching side's diagram that would be swell.

Thanks!

Edit showing the pin not in use:

image
image602×866 44.9 KB

2

Or you'd get in trouble?

3

Nah, it was actively in use at the doctor's office. I couldn't take the system down, but if I can't get an answer here he is willing to let me come back after hours to properly photograph both sides and be able to trace the leads.

4

It could well be a simple serial connection between two devices using an optocoupler such as the examples here: serial - UART isolation with opto-coupler - Electrical Engineering Stack Exchange

What sensors are attached to the Metro and what does it do apart from sending data to a screen?

The PCB looks rather home made.

EDIT
The optocoupler appears to be marked MCT6 ONSM-S-A0003546810-1.pdf

5

It is using an MCT6 yeah. In this situation there's nothing else going to the Arduino. Basically they are running software similar to the stock screen on their computer, and this device just converts the signal from the unit into serial so the computer can display the correct screen in the software.

Looks like they blue papered the PCB themselves for sure, and it has options to work for different units and screens, but luckily I bumped into it configured pretty much exactly how I'm trying to do it.

6

Can you draw a high level diagram of all the main components in this system. I have difficulty seeing how it all fits together. Maybe there is some application on the PC which can talk to the Metro over USB but what is connected to that DIN socket ?

7

Unfortunately I don't have diagrams for any of it or it would likely have me much further along.

In a normal situation the main medical unit connects to a screen using a din7 cable. (Pinout from the chart in first post).

In the situations pictured instead of a screen they have the box with the metro that I'm trying to understand better. The device connects to that din7 instead of to a screen, and the metro both converts the data to serial, as well as sends the response packet to the main unit.

That serial data is sent over the USB to the computer for some random piece of screen emulation software to show the data the screen would normally show.

Since I'm trying to send the data between 2 different esp32s in order to make a screen essentially Bluetooth I figured that Metro board would be a great first stepping stone.

8

LOL. I had copied the same words…

Thinking of how fast some ppl can be in, um, certain circumstances. :expressionless:

a7

9

Optocouplers were used quite often with 20ma current loop communications.

10

I guess I would start by building the optocoupler circuit and connect it to an arduino and start playing with it. You only have to get the baud rate and other characteristics of the serial link right. You do not have to understand how the data is encoded which the serial link is transmitting. A logic analyser/oscilloscope can help. Optocouplers don't support a vary high baud rate so an application to transfer a serial connection over the air should be quite straightforward.

11

This topic was automatically closed 180 days after the last reply. New replies are no longer allowed.