Javelin D
Ɣ
Model PD644 Voltmeter
Instruction Manual
14
Serial Communication
The PD644 can be connected directly to an RS-485 data bus of up to
32 nodes. For systems consisting of more than 32 nodes RS-485 re-
peaters are necessary. Serial communication wiring connections to the
PD644 are made via the three position screw terminal connector labeled
RS-485. Please refer to standard EIA-485 for complete specifications
for balanced digital multi-point communications. The use of shielded
twisted pair cable is recommended to protect the data signals from elec-
trical interference. For long wire runs it is good practice to terminate the
bus at both ends with a resistor connected between D+ and D- that
matches the characteristic impedance of the cable. A typical value for
termination resistors is 120 .
RS-485
CONNECTOR
1
3
2
G
D-
D+
Data Common
Data
-
Data
+
Shield
Twisted-Pair
RS-485
CONNECTOR
1
3
2
G
D-
D+
To other meters
or RS-485
devices
Data Common
Data+
-
Data
-
Figure 5. RS-485 Data Bus Connections
For non-multiplexed serial communications with RS-232 data communi-
cations equipment such as a personal computer, an RS-232 to RS-485
converter such as the Precision Digital PDA7485 is required. If using a
PDA7485, connect the PD644 according to the PDA7485 wiring dia-
gram for a two wire RS-485 application.
Relay Output Connections
Relay connections are made to a six-terminal connector labeled
RELAY1 RELAY2.
COM NO
NO
NC
NC COM
RELAY2
RELAY1
4
3
6
5
2
1
Figure 6. Relay Output Connections
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