C. PIA System Setup Considerations
140
Power Supply Sequence Mismatch
A common application is to interface to a 24-position ZT 2226, Opto 22, or equivalent
I/O module rack. Vcc and ground are provided from the ZT 8904 through connector J4
with Vcc protected by a 1 A fuse. This application is illustrated in Figure 1 below. In this
application, no power supply sequence mismatch exists because the power supplying
the input circuitry within the PIA is applied before or at the same time as the power
supplying the external signals. Proper system operation will result.
Power
Supply
16C50A
PIA
ZT 8904
Interface Cable
24-Position
or
Custom
Application
Vcc
1Amp
24
Figure 1. I/O Rack Vcc and Ground Supplied through Interface Cable
Correct Power Supply Sequence, Signal Level Matched
However, if a power source other than that supplying the PIA is used to power the
external signals, then a power sequence mismatch could occur, resulting in a latchup
condition. An external power source might be required if the external circuitry requires
more than the 1 A supplied by the cable or if a custom interface is being designed.
Refer to Figure 2 below for an example.
Power
Supply
16C50A
PIA
ZT 8904
Interface Cable
External
Power
Supply
24-Position
or
Custom
Application
Vcc
24
Figure 2. I/O Rack Vcc and Ground Supplied Externally
Potential Power Supply Sequence Mismatch, Signal Level Mismatch