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!
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