the 4550 basically has a loop...
read ADC1
if ADC1 is over the on trigger variable then turn on B1
If ADC1 is lower than off trigger var then turn off B1

For every time B1 is turned on increment pulse count var

If pulse count var = 4 read tmr1 and calculate RPM
Reset TMR1 and restart loop

The faster the cycle the better...

The addition would be to recieve a new ON and OFF trigger var from the 877A
The single byte 'commands' comming from the 877a are...
* Stall warning - return to predefined static settings
* Overspeed1 - stop pulsing
* Overspeed2 - engage emergency break (relay on B2)

The 877a will only be changing one var every 5 seconds or so...

The 877a will be doing all the work and the calculations and putting data on the LCD...

I don't know 'DTs instants' i'm guessing i go do a search for it... lol