P
RO
V
U
™ PD6088/PD6089 Modbus
®
Scanner
Instruction Manual
43
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, and/or 4 external
relays via expansion modules. 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 8
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 8 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:
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
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:
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
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):
Latching and Non-Latching Relay
Operation
The relays can be set up for latching (manual reset)
or non-latching (automatic reset) operation.
Relay terminology for following tables
Terminology
Relay Condition
On
Alarm (Tripped)
Off
Normal (Reset)
Ack
Acknowledged
The On and Off terminology does not refer to the
status of the relay’s coil, which depends on the fail-
safe mode selected.
•
In latching relay mode, if Fail-Safe is off, latched
relays will reset (unlatch) when power is cycled.
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
