IBM RT Series Hardware Reference Manual page 455

Bit 7 Not used
Bit 6 Not used
Bit 5 Not used
Bit 4 + IRQ Enable-A 1 in this position allows an interrupt
to occur when '-ACK I changes from true to false.
Bit 3 +SLCT IN-A 1 in this bit position selects the printer.
Bit 2 -INIT-A 0 starts the printer (50-microsecond pulse,
minimum).
Bit 1 +AUTO FD XT-A 1 causes the printer to line-feed
after a line is printed.
Bit 0 +STROBE-A O.S-microsecond minimum, high, active
pulse clocks data into the printer. Valid data must be
present for a minimum of 0.5 microsecond before and
after the strobe pulse.
Printer Status - Address X79, X7D
Printer status is stored at this address to be read by the
microprocessor. The following are bit definitions for this byte.
Bit 7 -BUSY-When this signal is active, the printer is busy
and cannot accept data. It may become active during
data entry, while the printer is offline, during printing,
when the print head is changing positions, or while in an
error state.
Bit 6 -ACK-This bit represents the current state of the
printer's I -ACK I signal. A 0 means the printer has
received the character and is ready to accept another.
Normally, this signal will be active for approximately 5
microseconds before I-BUSY I stops.
Bit 5 + PE-A 1 means the printer has detected the end of
paper.
Serial/Parallel Adapter 21