Serial intercept with logic analyzer

Using Arduino Networking, Protocols, and Devices
1

Hi everyone,
I have an old fuel dispenser board (FD), which had a port name SERIAL with 2 pins. These 2 pins connect to another "decoder board" - it's just a name. From this decoder board, they connect to a Adafruit receipt printer (Mini Thermal Receipt Printer : ID 597 : $49.95 : Adafruit Industries, Unique & fun DIY electronics and kits)
I guessed what happen to the signal: The fuel dispenser board send fuel volume and price to decoder board, decoder board added the other information like: station name, address, time (The seller told me that, if I want to change the station name in the receipt, I have to bring the decoder board to them and they will change it, so it must be in decoder board) and then: The decoder board send VIA UART to the receipt printer. And they work flawlessly.

I guaranteed the signal from decoder board to printer was UART serial. I took TX line from decoder board - which connect to the printer - connected it to Arduino then run "Adafruit Thermal Printer Library" Printer test at 19200 baud rate and it works - a test paper printed.
However, I had an issue with the signal from fuel dispenser to decoder board. There are only 2 wire with SERIAL label in fuel dispenser board, so I think it was GND and TX. I used a multimeter and measure the resistance of these 2 pins to the screw holes. And I found the GND pin was on the left so the TX pin would on the right.
The GND and TX pins on the fuel board must be connected to the GND pin and RX pin on the decoder board, right? GND -> GND; TX -> RX. However, the GND pin on the decoder board doesn't seem a GND. I have 3 reasons for it:

  • It has the 0v label in decoder board. Does 0v mean GND?
  • When I use the multimeter measures the resistance between it and negative wire of input power, it shows very high resistance.
  • The signal, when I captured with Saleae Logic Analyzer, doesn't seem to be a UART serial signal.
    Now I am very confusing: Do they use UART? Does the SERIAL label mean another "serial protorol"?
    I uploaded my logic analyzer data, with GND pin on fuel dispenser connected to GND pin on Logic, Tx to CH0; the fuel volume was 0.30 liter, price per liter was 14560, payment price is 4400. Does it really UART serial? Do I miss somethings to reveal it?

8 MHz, 800 M Samples [3].zip (107 KB)

2

Now I am very confusing: Do they use UART? Does the SERIAL label mean another "serial protorol"?

UART (or serial) doesn't have to be TTL level. There are other standards that are often used for such stuff (RS-232 and RS-485 probably used most often). Look at it with a scope. You will see immediately if it's a differential signal.

3

Hi pylon, thank you for your hints.
However, it seems the data was TTL level. I used the analog input of logic analyzer and the maximum high and low voltage was 5v and 0v.

4

However, it seems the data was TTL level. I used the analog input of logic analyzer and the maximum high and low voltage was 5v and 0v.

Is that input able to recognize negative voltages? Or does it simply show a voltage no matter if it was positive or negative?