Nice catch RICH.
From that manual:
"Steps LED Flickers when activity is detected on pins that may
be used for step signals. The PMDX-126 does not know which
pins are being used as “step” signals, which are used as “direction” signals or which are used
for some other function. The PMDX-126 assumes that all of the signals listed in Table 14
could possibly be step signals and monitors them for activity.
The PMDX-126 also does not know the polarity of the step signals, so it looks for a rising edge on
any of the signals listed in the table and calls that “step activity”.
This has the side effect that changing a direction signal may cause the “Steps” LED to flicker.
Likewise, any activity on parallel port #1 pin 1 or parallel port #2 pins 1 or 16 will cause the “Steps” LED to flicker."
"When using outputs “1”, “14”, ‘16”, 17”, “A”, “B”, “C” or “D” for step and direction
signals, connect the “step” signal to pins “1”, “16”, “A” or “C” in order for the
activity monitor to function correctly."