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Host Request
M
E
T
E
R
A
D
D
R
E
S
S
F
U
N
C
I T
O
N
T
A
B
L
E
N
U
M
B
E
R
A (
d
d
e r
s s
H
g i
h
B
t y
) e
T
A
B
L
E
O
F
F
S
E
T
A (
d
d
e r
s s
L
o
w
B
t y
) e
D
A
T
A
V
A
L
U
E
(
H
g i
h
B
t y
) e
D
A
T
A
V
A
L
U
E
L (
o
w
B
t y
) e
C
H
E
C
K
S
U
M
(
H
g i
h
B
t y
) e
C
H
E
C
K
S
U
M
L (
o
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B
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The example above shows a value of 200 (00H C8H), written to the CT Primary
register (Data Table 14, offset 0). The meter accessed has a Modbus ID of 25
(19H).
Meter Response
M
E
T
E
R
A
D
D
R
E
S
S
F
U
N
C
I T
O
N
T
A
B
L
E
N
U
M
B
E
R
A (
d
d
e r
s s
H
g i
h
B
t y
) e
T
A
B
L
E
O
F
F
S
E
T
A (
d
d
e r
s s
L
o
w
B
t y
) e
D
A
T
A
V
A
L
U
E
(
H
g i
h
B
t y
) e
D
A
T
A
V
A
L
U
E
L (
o
w
B
t y
) e
C
H
E
C
K
S
U
M
(
H
g i
h
B
t y
) e
C
H
E
C
K
S
U
M
L (
o
w
B
t y
) e
The replay format is a copy of the command confirming its validity:
Note:
This Modbus command is limited to writing 2-byte data only. Long inte-
ger registers may be written, but the meter assumes sets upper bytes
automatically to zero.
B
Y
T
E
E
X
A
M
P
1
1
9
H
2
0
6
H
3
0
E
H
4
0
0
H
5
0
0
H
6
C
8
H
7
8
9
H
8
6
C
H
B
Y
T
E
E
X
A
M
P
1
1
9
H
2
0
6
H
3
0
E
H
4
0
0
H
5
0
0
H
6
C
8
H
7
8
9
H
8
6
C
H
24
4.1.2
Data Format
The device uses a fixed data format for serial communications:
L
E
1
S
a t
t r
B
The 8 data bits are always transmitted least significant bit first. This data byte is
binary coded.
The baud rate is programmable as 4800, 9600 or 19200 baud. This is carried out
in programming mode as described in section 3.2.
4.2
RS-485
The RS-485 communication option enables connection of up to 128 meters on a
single pair of wires (247 with repeaters). The pair is used for transmission and
reception with each meter (and the host) automatically switching data direction.
The host should be fitted with an RS-485 driver (or converter) capable of opera-
tion in two-wire mode (half duplex).
PC operation in RS-485 two-wire mode usually requires software control of the
data direction. This controls the line drivers connected to the bus at the host serial
port. This direction control requires high-speed operation and may be problem-
atic under certain operation systems such as Windows. It is advisable to check
L
E
with the software vendor before selecting RS-485 as the mode of operation.
If software data direction control is not suitable, RS-232/RS-485 converters are
available for standard PCs, which carry out automatic hardware control. For more
information on these, contact your distributor.
Each Modbus serial transaction is preceded by a device address allowing the host
to temporarily connect with any meter on the bus. Certain commands allow the
host to transmit commands or data to all meters simultaneously. These com-
mands are known as broadcasts and use address '0.' The RS-485 standard en-
ables reliable communication over a maximum distance of 1200 meters.
Standard line repeaters may be installed to increase the maximum distance of an
RS-485 network and/or the number of devices that may be connected.
t i
8
D
a
a t
B
s t i
9
1
S
o t
p
B
t i
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