Wiring an Omron Thermal Printer to a Pi Pico

Question

I'm trying to connect an old Omron thermal printer (PRT-1 Z, companion of the 705CP Blood Pressure Monitor) to a Raspberry Pi Pico. The idea is being able to print something using the Pico. The printer doesn't have a standard connector and instead has a proprietary mini-USB-like male connector.

What have I tried so far:

I tried disassembling the printer to have a look at the PCB. I notice the cable pinout is labelled "USB". Unfortunately, each wire is only labelled with its color (BK, R, etc.) and not with its function. I assumed it was the standard wiring (BK for GND, R for V+, WH for D- and G for D+), and tried wiring it to my Pico (black to pin 3, red to 40, white and green to GPIOs 15 & 16).

I naively tried to send random data to the printer to see if something happened. Nothing did.

Then, I tried to see if I had an incoming signal while pressing the buttons on the printer, which has 4 buttons (feed, to control paper feed; graph; all data; data/stop). The program I used to do so (MicroPython through Rshell & REPL) is the following:

from machine import Pin

d1 = Pin(15, Pin.IN)
while True:
   print(d1.value())

All I got was only a continuous series of 1 while the LED on the printer was blinking.

Any advice on how could I proceed to establish connection with this printer?

Edit: thanks to @jsotola's comment, I realized I first needed to identify the correct USB pinout. I opened a separate question on the Electrical Engineering Stack Exchange.

Answer 1

Looking at the printer manual, it has buttons such as 'print graph' and 'print all data'. This suggests that the printer may not be a straightforward USB-to-paper device, but may work by fetching and processing raw data from the main unit.

In view of this, it is unlikely that you'll be able to re-purpose the printer without some investigative work; the best approach could be to intercept & log the USB communications between the main unit and printer, in the hope of discovering that the main unit actually sends text to the printer, rather than just sending some pressure values.