As many of you know, we had a rash of bad behavior when the last 25 MIMA systems were installed. Turned out we got some bad DB-25 cables, that had extremely high (8-20 ohm) end to end resistance. The cables were replaced with some 10 foot DB-25 that had normal less than one ohm end to resistance, and we thought we were home free, but found that more strange behavior was still happening.
Some care full scope troubleshooting showed that the 20 KHZ MAMODE signals were not making it up to 5 V, because the rising edge was being rounded by the long cable impedance. The stock cables show some of this effect, but the much longer path and higher capacitance that the DB-25 presents was just enough to further slow the rise time so the car was only intermittently seeing the signals, as they did not rise into the high state. After two sets of cables, And the increasing difficulty in getting shorter and larger conductor cables, I found that simply adding a parallel pull up resistor to the 5V at the MCM brought the rise times back in line with the stock signals without the MIMA harness. All the problems went away, so this should fix the problem,and not let the high speed signals be so effected by small capacitance and resistance variables.