6.5.3 RS485 interface
The electric interface RS485 is a symmetrical two-wire interface (half duplex) and is
designed according to TIA/EIA-485 / ITU-T V.11. The data protocol is the M-Bus
protocol. The following parameters can be transmitted via the RS485 interface:
-
Manufacturer ID, optional/freely selectable
-
Medium
-
Primary and secondary address of M-Bus
-
Energy values
-
Instantaneous
value
P
Sum
-
Error
status
The RS485 interface, which is galvanically separated from the meter, is located on the
additional terminals 14 (A) and 16 (B).
Properties
Number of connected meters
up to 32
Max. cable length
up to 1,000 m
Data transfer rate
300 ... 9600 Baud
Signal according to TIA/EIA-485 / ITU-T
V.11
logical “1”
-0.3 V to -6 V
logical “0”
+0.3 V to +6 V
Table 7: Specification of RS485 interface
RS485 standard bus
Up to 32 meters can be operated on one RS485 bus. Normally the
first
and
last
meters
in the bus system are terminated with a terminating resistor between wire "A" and "B" to
eliminate reflections.
Bus layout:
Unit 1
Unit 2
Unit 32
see detail
B
A
B
A
. . .
RS485 two-wire bus
see detail
B
A
max. 5 m
max. 1000 m
Figure 6: System layout – RS485 two-wire bus
Detail:
Important:
RS485
14 16
A
B
R
TERM
= 120
Ω
Connecting
terminals
The terminating resistor can be mounted
only on the first and last meter in the
bus.
Figure 7: Detail drawing – RS485 terminating resistor
18
