AWC-608 & PPI

I've never been overly impressed with the performance of the AWC-608 when using PPI (with vector in particular). I always suspected that they implemented it differently than some of the other implementations described here on buildlog.
I decided to put my scope on it to examine the pulse length on the TTL line with various PPI settings. I was hopeful that it stayed constant with different feed rates, but it's not. Here's what I measured. Maybe someone can help me figure out the math behind their implementation. The pulse width is the width of the portion of the square wave that is low. The pulse train appears to be symmetrical, so I'd guess they just determined the time of the move and width of pulse they'd need to get the desired PPI.
100 PPI
Feedrate Pulse Width (ms)
7 21.6
8 19
9 17
10 15.2
200 PPI (it halved which I expected)
7 10.8
8 9.2
9 8.4
10 7.6
800 PPI
7 5.6
8 4.8
9 4.4
10 4
1000 PPI
10 2.4
At a Feedrate of 1 mm/sec and 100 PPI, pulse width was 152 ms
dirktheeng original postings about PPI, he found that a 2-3ms pulse width gave the best overall performance which if true, points to specific PPI and feedrates.
I decided to put my scope on it to examine the pulse length on the TTL line with various PPI settings. I was hopeful that it stayed constant with different feed rates, but it's not. Here's what I measured. Maybe someone can help me figure out the math behind their implementation. The pulse width is the width of the portion of the square wave that is low. The pulse train appears to be symmetrical, so I'd guess they just determined the time of the move and width of pulse they'd need to get the desired PPI.
100 PPI
Feedrate Pulse Width (ms)
7 21.6
8 19
9 17
10 15.2
200 PPI (it halved which I expected)
7 10.8
8 9.2
9 8.4
10 7.6
800 PPI
7 5.6
8 4.8
9 4.4
10 4
1000 PPI
10 2.4
At a Feedrate of 1 mm/sec and 100 PPI, pulse width was 152 ms
dirktheeng original postings about PPI, he found that a 2-3ms pulse width gave the best overall performance which if true, points to specific PPI and feedrates.