Thanks for the suggestion, RB. I found a PDF of the Passport MIDI that I'm going to study. It tells about the PTM and the ACIA (so many acronyms 8-)

Just for fun, I was curious how many times the AlphaSyn reads the keyboard per second.

Let's throw some code into READ8_MEMBER(apple2e_state::c080_r)

Code
static int readcounter[16];
static double lastinterval;

if (machine().time().as_double() > lastinterval) {
lastinterval=machine().time().as_double()+5.0;
printf("%f  : ",lastinterval);
for (int i=0;i<16;i++) {printf("%d=%3d ",i,readcounter[i]); readcounter[i]=0;} printf("\n");
}
if ((offset>=0x70) && (offset<=0x7f)) readcounter[offset-0x70]+=1;

which produces this output:
Code
306.151057  : 0=7634 1=7634 2=7634 3=7634 4=7634 5=7634 6=7634 7=7634 8=7633 9=7633 10=7633 11=7633 12=7633 13=7633 14=7633 15=7634 
311.151146  : 0=7623 1=7623 2=7623 3=7623 4=7623 5=7623 6=7623 7=7623 8=7624 9=7624 10=7624 11=7624 12=7624 13=7624 14=7624 15=7624 
316.151269  : 0=7634 1=7634 2=7634 3=7634 4=7634 5=7634 6=7634 7=7634 8=7634 9=7634 10=7634 11=7634 12=7634 13=7634 14=7634 15=7634

It looks like about 7600 times per 5.0 second interval or 1520 times per second.

7600/5 = 1520

5.0 / 7600 = 0.00065789473 seconds per read which is 25x faster than checking it every 1/60 of a second.

so I think it could definitely detect how quickly the switches change for velocity sensing.