On CPC, the printer is operated as follows: to send a byte, use port &EF00 by sending the value 3 times (1st: Bit7=0; 2nd: Bit7=1; 3rd: Bit7=0), with bit7 used to validate the byte. For this reason, only the 7 least significant bits were taken into account.
On Plus machines, the 8th bit of the printer port is on bit3 of register 12 of the emulated [[CRTC]]. This bit must have "seemed" unused to Amstrad engineers. Unfortunately, they didn't know about the Overscan Bits.
== Data / Strobe ==