PD2-6262 Helios Large Display Analog Dual-Input Rate/Totalizer Instruction Manual
15
Modbus RTU Serial Communications
Serial communications connection can be made to the onboard RS485 terminal block or USB connector
shown in Figure 7. If RS232 is required, an RS485 to RS232 adapter (PDA7485) may be used. See
Ordering Information on page 5 for additional information.
Relay Connections
Relay connections are made to two six-
terminal connectors labeled
RELAY1
– RELAY4 on Figure 7. Each
relay’s C terminal is common only to the
normally open (NO) and normally closed
(NC) contacts of the corresponding relay.
The r
elays’ C terminals should not be
confused with the COM (common)
terminal of the INPUT SIGNAL connector.
Figure 12. Relay Connections
Switching Inductive Loads
The use of suppressors (snubbers) is
strongly recommended when switching
inductive loads to prevent disrupting the
microprocessor’s operation. The
suppressors also prolong the life of the
relay contacts. Suppression can be
obtained with resistor-capacitor (RC)
networks assembled by the user or
purchased as complete assemblies. Refer
to the following circuits for RC network
assembly and installation:
Figure 13. AC and DC Loads Protection
Choose R and C as follows:
R: 0.5 to 1
Ω for each volt across the contacts
C: 0.5 to 1 µF for each amp through closed contacts
Notes:
1. Use capacitors rated for 250 VAC.
2. RC networks may affect load release time of
solenoid loads. Check to confirm proper operation.
3. Install the RC network at the meter's relay screw
terminals. An RC network may also be installed
across the load. Experiment for best results.
Figure 14. Low Voltage DC Loads Protection
RC Networks Available from Precision Digital
RC networks are available from Precision Digital and
should be applied to each relay contact switching an
inductive load. Part number: PDX6901.
Note: Relays are de-rated to 1/14th HP (50 watts) with an inductive load.
Use a diode with a reverse breakdown
voltage two to three times the circuit
voltage and forward current at least as
large as the load current.
C
NO
NO
NC
NC
C
RELAY4
RELAY3
4
3
6
5
2
1
C
NO
NO
NC
NC
C
RELAY2
RELAY1
4
3
6
5
2
1
C
R
C
R
