HP QMS 1660 - Manual (Page 132)

PS Protocol
Option
QMS 1660 Print System Reference 5-32
EPP (Enhanced Parallel Port) Mode
Printer-host communication is done via asynchronous bidirectional
eight-bit transfer. A return to compatibility mode is not required.
Nibble Mode
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.
PS Protocol Option
Your QMS 1660 Print System supports PS Protocol, a new protocol
for communication between the printer and a host computer over the
serial, parallel, and optional interfaces. 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 communicating 8-bit binary data in binary or binary
fixed mode, the printer uses the quoting mechanism of the binary
communications protocol to distinguish between the special control
characters and print job binary data.