HP 7470A Interfacing And Programming Manual page 26

The P-mask value specifies which of the status—byteconditions will
result in a logical 1 response to a parallel poll over the HP—IB interface.
P-Mask Status Bit
Bit Value Number Meaning
1 0 Pen down
2 1 P1 or P2 changed
4 2 Digitized point available
8 3 Initialized
16 4 Ready for data; pinch wheels down
32 5 Error
For example, a P—mask value of 48 specifies that only bits 4 and 5 (16 +
32) of the status byte can cause the plotter to respond to a parallel poll
with a logical 1 on the appropriate data line.
The plotter, when set to default values or initialized, automatically sets
the E—mask to 223, the S—mask to 0, and the P—mask to 0. An IM
command without parameters or with invalid parameters also sets the
masks to the default values 223,0,0.
1-14 GETTING STARTED