Chuck, my hand-drawn schematic for the TPC shows a 2.0Kohm resistor in series with the optoisolator's LED. Unfortunately, my data sheet for the H11L1 does not give a nominal "ON" current, just the worst case of 1.6mA. The LED voltage is probably about 1.1V. I use 3.5V as the minimum for the serial drive line, and this would provide 1.2mA of current, not enough for the "worst case" parts.
On the other hand, the Transfer Characteristics graph on the data sheet indicates an "ON" current of .9 mA, presumably a nominal value. This would require an input of 2.9 volts.
For Don's case of 14 devices, if we assume that they are all like the TPC and use the nominal sensitivity of .9 mA, that would be 12.6 mA of drive current. I tried to build an equivalent simulated load by using a 150 ohm resistor (2000/14 = 143 ohms) and two germanium diodes that gave a forward drop of about .9V at 12 mA. For my Legacy Base with this load the output voltage sagged to 2.1V, with only 1.4V across the resistor. That is .7mA per device. That would be marginal at best.
