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C. PIA SYSTEM SETUP CONSIDERATIONS
The 16C50A Parallel Interface Adapter (PIA) device used on the ZT 8904 is designed
by Ziatech to offer bidirectional I/O signals with or without event sense capability. This
device features low power, high speed, wide temperature operation achievable only by
utilizing CMOS technology.
Although CMOS technology offers many advantages, you must observe a few cautions
when interfacing to any CMOS parts.
CMOS inputs and outputs can exhibit latchup characteristics. These inherent
characteristics of any CMOS technology can result in the formation of a Silicon-
Controlled Rectifier (SCR) that appears between Vcc and ground when voltages greater
than Vcc or less than ground are applied to inputs or outputs.
When this happens, Vcc is effectively shorted to ground. The only way to remove the
latchup condition is to shut off the power supply. If a large current is allowed to flow
through the chip, its operating temperature may increase, it may exhibit intermittent
operation, or it may be damaged.
CMOS inputs must be protected from slow rising signals and inductive coupling on their
inputs. Failure to do so will allow a potentially large current to flow through the chip,
damaging the chip.
The purpose of this appendix is to illustrate precautions you should take to prevent
latchup conditions and protect inputs.
PREVENTING SYSTEM LATCHUP
The most common causes of latchup are:
•
Input signals applied before the input circuitry is powered, resulting in a signal to
power supply sequence mismatch
•
Input signals greater than Vcc or less than ground, resulting in a signal level
mismatch
Each of these conditions is covered in the following topics.
