DESCRIPTION AND OPERATION
DIGITAL I/O
I-E96-315B
2 - 9
DIGITAL I/O
If the HSS loses communication with the MFP, the manual
control circuitry enables the operator to manually change
actuator position. The HSS provides two +24 VDC isolated dig-
ital inputs as raise/lower commands from a manual station
(pushbuttons, contacts, etc.). These inputs connect to the
microcontroller. The microcontroller outputs a position
demand to the servo valve when the operator activates the con-
tacts. Trip bias is the third digital input. The trip bias input
signal is sent to the position error circuit. See Figure
. Acti-
vating the trip bias input drives the actuator to the 0 percent
position (fully closed) for emergency shutdown. Emergency
manual is an isolated output that provides a connection for an
external indicator (LED) or alarm. The indicator notifies the
operator that the module is in the emergency manual mode.
Figure
shows a schematic of a typical isolated digital input
circuit.
Figure 2-5. Typical Digital Input Circuit
TP25015A
HSS DIGITAL INPUT
SYSTEM POWERED
24 VDC FROM TDI
TERMINATION
UNIT
PUSHBUTTON
OR CONTACTS
0.1 µF
4.99 K
3 A
+24 VDC
–
+
4
5
3
1
2
