Measurement Computing PC104-DIO48 - Manual (Page 9)

In other words, if you connect bit A0 and not bit A1, do not be surprised
if A1 stays low, stays high or tracks A0. It is unconnected and so, is not
specified. The 82C55 is not malfunctioning. In the absence of a
pull-up/down resistor, any input which is unconnected is unspecified.
You do not have to tie input lines, and unconnected lines will not affect
the performance of connected lines. Just make sure that you mask out
any unconnected bits in software.
2.6 CONNECTOR DIAGRAM
The connector accepts female 50-pin header connectors, such as those
on the C50FF-2, a 2-foot cable with connectors.
If frequent changes to signal connections or signal conditioning is
required, please refer to the information on the CIO-TERM100,
CIO-SPADE50 and CIO-MINI50 screw terminal boards.
Figure 2-2 I/O Connector
5
B
A
SE
+
0,
1,
2
BAS
E
+
4,
5,
6