C. PIA System Setup Considerations
141
One solution is to switch the external signals' power supply with an output that is
controlled by the computer. In this manner, if the computer is off, so is the external
power supply. This solution is illustrated in Figure 3 following.
Power
Supply
16C50A
PIA
ZT 8904
Interface Cable
External
Power
Supply
Custom
Application
Vcc
1Amp
S
24
Figure 3. Computer-Switched External Power Supply
Correct Power Supply Sequence, Potential Signal Level Mismatch
A simpler solution is to power the relay controlling the external power supply directly
from Vcc and ground supplied by the interface cable.
Another solution is to utilize the same switch to control the computer's power supply and
the external signals' power supply, as illustrated in Figure 4. This is an acceptable
solution for power supply sequence mismatches as long as the computer supply ramps
up faster than the external power supply. This ensures the PIA input circuitry is powered
before the external signal circuitry.
Power
Supply
16C50A
PIA
ZT 8904
Interface Cable
External
Power
Supply
24-Position
or
Custom
Application
Vcc
24
Figure 4. Computer and External Power Supply with Common Switch
Correct Power Supply Sequence, Potential Signal Level Mismatch