C. PIA System Setup Considerations
144
Typically, optical isolators are used to help remove electrical noise while providing for
different grounds. Separate grounds are achieved through the use of an additional
power supply for the optocoupler rather than using the computer's power supply. If the
computer's power supply powers the optocouplers, electrical isolation is defeated. An
example of one such circuit is illustrated in Figure 8 below. The circuit can be altered to
allow for design considerations.
Assuming a Vil of 1 V maximum for the 16C50A PIA, the HP Dual Optocoupler must
have a Vol of less than or equal to 1 V over the operating temperature. Using a TTL-
compatible optocoupler gives a Vol of .6 V maximum with rise and fall times (50 ns and
10 ns, respectively) that are easily compatible with the PIA, given a 1k
Ω
pullup.
Ziatech
+
+
2.2K
+
24V
-
10mA
1K
HCPL-2630
16C50A
PIA
Figure 8. PIA-to-Optocoupler Interface Example
Inductive Coupling
Inductive coupling on I/O lines can cause noise to be coupled into the chip, resulting in
intermittent operation. This situation occurs when the PIA I/O signals are routed with
other signals within a wire bundle. One way to filter inductively coupled noise, or any
noise for that matter, within a system with the same ground (not using optocouplers) is
illustrated in Figure 9.
Ziatech
+
+
1K
50
@
Ω
39pf
LCA05
.25A
I/O
74S1053
5V
16C50A
PIA
Figure 9. PIA-to-Filter Interface Example
In the above circuit, the Texas Instruments 74S1053 Schottky diode clamps limit a
transient to ±1 V above +5 V or below ground. The ferrite bead has a 50
Ω
impedance
at the frequency of interest. As the diodes begin to clamp and current flows through
them, the voltage across the LCA05 5 V bidirectional TransZorbs
®
increases, causing