10
4.2.1 RS-485 Connection
It is recommended that screened twisted-pair cable be used for RS-485 connec-
tion in order to minimize signal errors due to noise. An optional third wire, connec-
tion the common (0V) at each unit, is recommended for optimum performance.
The cable screen should be connected to the connector housing (ground) at the
host (PC) only. To reduce cable reflections over long distances, RS-422/485 sys-
tems require line termination. This is achieved by fitting two 120
⍀
terminating
resistors as shown in figure 4.2. One resistor should be fitted at the Host input/
output buffer and the other at the buffer of the most remote device.
Figure 4.4. RS-485 Multi-Drop Connection
4.3
RS-422
The RS-422 communication option enables connection of up to 128 meters (247
with repeaters) on two pairs of wires (4-wire bus). One pair is used for transmis-
sion and the other for reception.
This connection is more commonly used in full duplex communications systems
where host and slave can simultaneously transmit/receive data. In this instance
however, the Modbus protocol itself ensures half duplex operation in RS-485 mode
with data direction control. The RS-422 standard enables reliable communication
over a maximum distance of 1200 meters. Standard line repeaters may be in-
stalled to increase the maximum distance of an RS-422 network and/or the num-
ber of devices that may be connected.
23
Meter Response
The example shows a replay of 6 bytes (3 integers) as:
3-ph kW = 570
(02 3A Hex)
3-Ph kVA = 1884 (07 5C Hex)
3-Ph kvar = 1794 (07 02 Hex)
4.5.9 Function 06 Preset a Single Register
Description
This function allows a single integer register in a meter table to be changed by the
host. This command is commonly used to program meter parameters or to reset
energy registers to zero. When broadcast (address = 0), all meters on the network
are addressed together, but none reply.
E
T
Y
B
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L
P
M
A
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1
N
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2
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4
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2
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