IBM 5251 Series Maintenance Information Manual page 186

When updating the cursor, the controller actually places
the cursor three positions to the left of the column
address it received. This is to compensate for an
average three position delay in the sensing of the
character.
The contents of the first two frames are as follows:
Frame one (data byte and control bits)
3 4 5 6
7
8
9
10 11 12
13
o
1 2
~
Fill Bits
(always
000)
l ~
Parity ID
l
- - . . . . --. . . .
R-o-w~:dress
Reserved (0 - 25)
Ti p Switch Activated
Light Pen Installed
Bits 0 through 15 are used to send light pen data from
the 5251 Models 1 or 11.
Bits 7 through 14 are used internally on the 5251
Models 1 or 11 as a data byte.
Bit 7
o
=
Light Pen is not installed
1
=
Light Pen is installed
Note: If bit 7 is 0, all remaining bits of this frame
(except bit 15) will also be O.
Bit 8
o
=
Tip switch is not activated
1
=
Tip switch is activated
Bits 10 through 14
The binary value of bits 10 through 14 is the row
address of the selected field. The decimal values of the
bits are:
16 8 4 2
Example:
1 0 0 1 0
=
a row address of 18.
5-6
14