Measurement Computing CIO-PDMAxx - Manual (Page 11)

When the CIO-PDMA board is used as an output device to send data to another TTL
device over a standard ribbon cable, it may be necessary to add balancing resistors
(typically 220 Ohms) as close to the board as possible.
If the CIO-PDMA board is used as an input device, it may be necessary to add
resistors in series (220 Ohms typically) with the data lines, at the source of the signal.
Failure to observe these precautions may result in spurious signals being interpreted
by the receiver as valid data.
7