So here is how I connected the printer to the Raspberry Pi:
Here you see that the black wire is connected to GND, the yellow one goes to the receive (RX) and green goes to the transmit (TX) line. As we do not need to receive any data from the printer we do not need to connect the green (TX) line:
And on the Raspberry Pi we need to connect now the black (GND) with GND of the raspberry and the yellow/RX from printer to TX from the raspberry as can be seen below:
The Raspberry Pi outputs/accepts 3.3V levels signals and the printer outputs/accepts 5V signals. So by connecting the TX from the Raspi to RX from the printer the printer only sees there 3.3V signals. I’ve tested that with 2 different printers and both seem to accept and successfully detect the 3.3V signal (that has something to do with with TTL/CMOS logic level ranges/limits).
Please do not connect the TX from the printer to RX of the Raspberry Pi as you will feed 5V to the 3.3V Input Pin of the Pi which will most likely destroy the Pin or more when no level converter is used!