[GreenKeys] Nagle's loop driver incompatible with Model 15 with RF filter.

[John Nagle]

     My board has a problem with Teletype Model 15 machines
equipped with keyboards with the 92222 FILTER. This is
an RF noise filter with a capacitor.  It's mounted
on a bracket above the keyboard's camshaft.

     The RF filter has enough capacitance to keep the
keyboard signal at MARK too long, which results in
keyboard data errors.

    The board doesn't put enough load on the keyboard
input. The filter cap has enough energy storage to
drive the opto-isolator for a few bit times, I think.
U6 probably needs a resistor between pins 1 and 2 so the
keyboard input draws a bit more current.  More on this later.

    I have one Model 15 with a 92222 FILTER, and one without.
The one without it works fine with that board, as do the
Model 14 machines.

				John Nagle