Yes, "blocking commands" is exactly the word. I figured that out while lying in bed with the LOGIC TOOL image in my head. I could see that the length of the pulse was being added to the interrupt time. I could not find any information on that in any of my documents. Please tell me where in which .INC file is the ASM code which is activated by PULSOUT. Without access to those details, PBP is just guess work.

My plan is to insert counters into the interrupt service routine. Every four interrupts (20millisec) do a PULSOUT. Every 25 interrupts, TRIGGER one SONAR or the other alternatively. My TRIGGER is built of HIGH and LOW commands totaling 10 microseconds (or so). These are not blocking.

I'll give this a try and see how the car reacts (if at all.....)

Thanks again for the support.

Ken