User's Manual
PMAC2A PC104 Hardware Reference Manual
Software Setup
16
If the CPU’s operational frequency has been determined by (a non-zero setting of) I46, the serial
communications baud rate is determined at power-up/reset by variable I54 alone according to the
following table:
I54 Baud Rate I54 Baud Rate
0 600 8 9600
1 900 9 14,400
2 1200 10 19,200
3 1800 11 28,800
4 2400 12 38,400
5 3600 13 57,600
6 4800 14 76,800
7 7200 15 115,200
For a saved value of 0 for I46, the serial baud rate is determined by the combination of I54 and the CPU
frequency as shown in the following table.
I54
Baud Rate for
40 MHz CPU
Baud Rate for
60 MHz CPU
Baud Rate for
80 MHz CPU
0 600 Disabled 1200
1 900* (-0.05%) 900 1800* (-0.1%)
2 1200 1200 2400
3 1800* (-0.1%) 1800 3600* (-0.19%)
4 2400 2400 4800
5 3600* (-0.19%) 3600 7200* (-0.38%)
6 4800 4800 9600
7 7200* (-0.38%) 7200 14,400*(-0.75%)
8 9600 9600 19,200
9 14,400*(-0.75%) 14,400 28,800*(-1.5%)
10 19,200 19,200 38,400
11 28,800*(-1.5%) 28,800 57,600*(-3.0%)
12 38,400 38,400 76,800
13 57,600*(-3.0%) 57,600 115,200*(-6.0%)
14 76,800 76,800 153,600
15 Disabled 115,200 Disabled
* Not an exact baud rate
Filtered DAC Output Configuration
The PMAC2 PC104 is a PMAC2 style board with default +/-10V outputs produced by filtering a PWM
signal. This technique has been used been for some time now by many of our competitors. Although this
technique does not contain the same levels of performance as a true Digital to Analog converter, for most
servo applications it is more than adequate. Many of our customers using this product have migrated over
from the PMAC1 style board with a true 16-bit DAC. This document is meant for explaining the
tradeoffs of PWM frequency vs. resolution in the PMAC2PC104 base configuration as well as a
comparison to the PMAC1 style 16 bit DACs.
Both the resolution and the frequency of the Filtered PWM outputs are configured in software on the
PMAC2PC104 through the variable I900. This I900 variable also effects the phase and servo interrupts.
Therefore as we change I900 we will also have to change I901 (phase clock divider), I902 (servo clock
divider), and I10 (servo interrupt time). These four variables are all related and must be understood
before adjusting parameters.