Hi. If possible, I would like some clarification on the MIDI electrical specifications.
The calculations presented in both the original and revised 2014 spec to calculate the loop current seem to assume the output buffer is able to drive down to 0 volts. In the event the output buffer has a higher logic low level (for example, 0.4V TTL logic low), the current in the loop will be reduced significantly below the required 5mA minimum. As the 2014 specification quite rightly points out, there are MIDI devices which require the full 5mA worst-case to turn on.
I have looked at a number of commercial implementations of MIDI output buffers, and they vary enormously in their logic low level. Some devices use a PNP emitter follower, which could produce a logic low level as high as 1.1V. Other devices use an NPN common-emitter configuration with a low level around 0.1V.
What is the recommended course of action to compensate for a non-zero logic low level? Should the 220 ohm resistors (or 33 ohm in a 3.3V circuit) be reduced to maintain a 5mA absolute minimum current? Or is any compliant MIDI receiver expected to be able to accept loop currents lower than 5mA which might result from a 0.4-0.8V logic low level? Is there a defined required minimum logic low level?