QMS 2425 Reference page 156

Printer-host communication is done via asynchronous bidirectional
eight-bit transfer. A return to compatibility mode is not required.
Printer-host communication is done in nibbles (four bits; one-half
byte) with the low order nibble sent first. A transfer of two nibbles is
required for each byte of information.
Your QMS 2425/2425 TURBO Print System supports PS Protocol, a
new protocol for communication between the printer and a host com-
puter over the parallel, optional serial, and optional network inter-
faces. This binary communications protocol (BCP) allows any 8-bit
binary value (0-255) to be treated as data, while allowing a few of the
values to function as special control characters. When communicat-
ing 8-bit binary data in binary or binary fixed mode, the printer uses
the quoting mechanism of the binary communications protocol to dis-
tinguish between the special control characters and print job binary
data.