How to connect into this device via serial?

Other Hardware Device Hacking
1

Hello,

I picked up a VFD display + controller off eBay and thought it would be a fun project to figure out the protocol and see what I can do with it.

I've been able to tap in to the TTL serial port on the microcontroller and have a small conversation, but I do not understand how the high voltage side of the serial works on this thing...

The device is opto-isolated. The power to run the optocouplers comes from the host, 13V.. Where does this come from?

I thought maybe it can come from the RS232 converter itself, by taking the T2 in pin and putting it low, I get 8V or so on the T2 out. , put that on the V+ line, -- ok, I got one of the activity LEDs to stay on... but I cannot talk to the device like I can on the TTL side.

I've actually found the schematic. Can someone please explain to me what I am missing here? It has to be old school serial.. :slight_smile:

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image1027×795 36 KB

Let me explain...
On the far right we have COMM2, COMM3, COMM4, COMM1, COMM0.

COMM2 Positive logic voltage, ?
COMM3 Receive (Data coming to this display)
COMM4 GND logic voltage
COMM1 Transmit (Data FROM this display)
COMM0 goes to reset pin on the micro

9bit but I have that figured out, thanks to reading the forum.

2

No one has any clue?

Looking at the diagram above; can someone tell me on Q4, leg would be TRANSMIT and which is GND?

3

A schematic is like code, posting part of it isn't very helpful.

Looking at the piece of schematic you've posted, one can determine the direction of the signals, but not what the signals actually are.

It's likely that COM4 is the GND.

If the VFD is from an old POS unit, then there was likely a laptop style power block to power it.

4

That is pretty much the schematic. It shows everything up to the UART pins on the micro...

Since this posting I've discovered the isolated voltage for the RS232 (COMM2 and COMM4) is 13V.
I also see this optocoupler design in a number of different google searches. Yes, COMM4 is isolated GND. Thank you for confirming that.

At 13V, it seems a TTL to Serial converter would work to communicate to the device. But, when I add this, the LEDs DS1 and DS2 will stay lit, depending on how I connect the RX/TX pins of the Serial converter and am unable to communicate with the device using the these pins. I can communicate using the UART pins on the mirco... Just not the 'right' way, through this isolated connection.

As I type this, it may be that I was confused on the isolated ground. I was using USB to power the Arduino. Not the Isolated 13V power supply. Perhaps by using the isolated power supply it will complete the circuit. :slight_smile:

My goodness. All the google searching shows there are a few solutions out there by chip manufacturers. Some even have examples, circuit design considerations, etc..

One such chip is the ADM3252E .. Amazing chip. HUGE cost!!! Like, $30.00USD -- for a Serial chip.

[https://www.analog.com/en/analog-dialogue/articles/designing-robust-isolated-rs-232-data-interfaces.html](https://Read more about the chip here)

Looking for a cheaper alternative, I thought I found one.. And, I guess, I did, but it requires off-chip components, optocouplers, transformer, .. Might as well not use the chip at all... MAX250 / MAX251 I think it is. Yes. The datasheet says, "The MAX252 has all the required components for RS232 communication in a single package." Geeze. Ok. Great. Let's take a look at the MAX252....

Wow! This chip also comes in a 40 pin dip. Let's order a bunch to experiment with...
Not a bad price, found them on eBay for about $9.00 USD with shipping.

Ok, let's make the circuit board... ...... reading reading ... NOOO
Page 6 -
"For applications requiring higher isolation ratings or transmission rates greater than 9600 baud, Maxim recommends the MAX250 and MAX251 device set that uses external optocouplers and transformer."

Cancel that order! I need 19,200 baud. Feeling defeated, went to bed.

Is there a reasonably priced (~ $8.00 USD) chip out there that does isolated RS232 at a speed of 19,200 baud?

I'll try powering the arduino with the isolated power supply instead of the USB port some day..

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