ProtEX PD8-6080/PD8-6081 Explosion-Proof Modbus
®
Scanner Instruction Manual
52
Relay Operation Details
Overview
The relay capabilities of the scanner expand its usefulness beyond simple indication to provide users with
alarm and control functions. These capabilities include front panel alarm status LEDs, as well as either 2 or
4 optional internal relays. Typical applications include high or low temperature, level, pressure or flow
alarms, control applications such as simple on/off pump control, and pump alternation control for up to 4
pumps. There are four basic ways the relays can be used:
1. High or Low Alarms with Latching or Non-Latching Relays
2. Simple On/Off Control with 100% Adjustable Deadband
3. Sampling (Based on Time)
4. Pump Alternation Control for up to 4 Pumps
Relays Auto Initialization
When power is applied to the scanner,
the front panel LEDs and alarm relays
will reflect the state of the input to the
scanner after the first response from
the slave device. The following table
indicates how the alarm LEDs and
relays will react on power-up based on
the set and reset points:
Fail-Safe Operation (
failsf
)
The following table indicates
how the relays behave based
on the fail-safe selection for
each relay:
Note: NO = Normally Open, NC = Normally Closed. This refers to the condition of the relay contacts when
the power to the scanner is off.
Front Panel LEDs
The LEDs on the front panel provide status indication for the
following:
The scanner is supplied with four alarm points that include front
panel LEDs to indicate alarm conditions. This standard feature is
particularly useful for alarm applications that require visual-only
indication. The LEDs are controlled by the set and reset points
programmed by the user. When the display reaches a set point for a high or low alarm, the corresponding
alarm LED will turn on. When the display returns to the reset point the LED will go off. The front panel LEDs
respond differently for latching and non-latching relays.
For non-latching relays, the LED is always off during normal condition and always on during alarm condition,
regardless of the state of the relay (e.g. Relay acknowledged after alarm condition).
For latching relays, the alarm LEDs reflect the status of the relays, regardless of the alarm condition. The
following tables illustrate how the alarm LEDs function in relation to the relays and the acknowledge button
(Default: F3 key assigned to ACK):
Alarm #
HI or LO
Alarm
Set
Point
Reset
Point
Power-Up
Reading
Relay
& LED
1
HI
1000
500
499
Off
2
LO
700
900
499
On
3
LO
250
400
499
Off
4
HI
450
200
499
On
Fail-Safe
Selection
Non-Alarm State Alarm State
Power Failure
NO
NC
NO
NC
Off
Open
Closed
Closed
Open
Relays go to
non-alarm state
On
Closed
Open
Open
Closed
Relays go to
alarm state
LED
Status
LED
Status
1
Alarm 1
5
Alarm 5
2
Alarm 2
6
Alarm 6
3
Alarm 3
7
Alarm 7
4
Alarm 4
8
Alarm 8
