MULTI GAUGE CONTROLLER
COMMUNICATION
. .
MODBUS RTU
The serial communication for the Modbus RTU is make through EIA
, EIA
(formerly RS
and RS
) or optionally Pro iNet (as encapsulation of the data frame) transmission. The EIA stan-
dards have been introduced by the Electronics Industry Association and describe the electrical
characteristics of communication networks. Table . includes the summary of different physical
layers described by these two standards.
EIA Standard
EIA
C
EIA
-wire
Transmission type
From point to point,
nom Volt (min
V, max V)
One or Two Pairs of wires. Differential
Mode. Half duplex - communication oc-
curs in both directions but not at the
same time. Typically once a unit be-
gins receiving a signal it must wait for
the transmitter to stop sending before
it can reply.
Electrical connections
wires:
Tx,
Rx,
GND(common)
wires: A, B, C(common)
Number of devices in the
line
transmitter, receiver
receivers, transmitter
Maximum data rate
, Kb/s
Mb/s
Maximum cable length
m
m
Table . : EIA Standard
Note
: The EIA
C standard enables connecting
one
device to a computer, PLC controllers or
similar devices with a cable max.
m
long.
Note
: The EIA
standard enables connecting
at least one
device to a computer, with a cable
max.
m
long. In this way up to
receivers (SLAVE) and one transmitter (MASTER) can be
connected. EIA
is a balanced two-conductor transmission system, which means that informa-
tion is carried out by the difference in voltage between two conductors, instead of the voltage in
relation to the common conductor or ground, as it is done in EIA
C. One voltage polarity indi-
cates logic “ ”, reverse polarity indicates logic “ ”. The difference must be at least +
mV. The
EIA
differential transmission is less vulnerable to in luence of external signals and if the de-
vice is in an environment with substantial interference, it should be used instead of the EIA
C.
Even though the EIA
standard is commonly described as a two-conductor connection, a chassis
ground/screen two-conductor connection as a “common” signal should be used to ensure addi-
tional protection against interference.
Note
: The MG works in a half-duplex system, which does not enable transmission and reception
of data in the same time. Data are transmitted interchangeably, once query, once response.
Note
: Most PC computers is equipped with an EIA
port for communication. A limit of
devices may be bypassed by dividing most networks into segments, which are electrically isolated.
When communication with more than
devices on the same bus is require, a PC computer with a
special EIA
attachment is required for buffering EIA
networks. It can also be used to change
a -conductor EIA
to a -conductor EIA
.
Note
: For EIA
connection between the MG
devices and a MASTER device, in particular
at greater distances and transmission speeds (>
bps, > m) it is recommended to use a -
conductor twisted pair, preferably, with an additional screen. Remember also to add a terminator
(
Ω...
Ω resistor connected between lines A and B) at the beginning and the end of the EIA
bus.
User manual rev.
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