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hardware used for Modbus RTU, such as radios, must transfer data as entire packets without
injecting delays in the middle of Modbus messages.
l
Modbus ASCII — Used in serial communications, data is transmitted as an ASCII
representation of the hexadecimal values. Timing requirements are loosened, and a simpler
longitudinal redundancy check checksum is used.
l
Modbus
TCP/IP
or
Modbus
TCP
—
Used
for
communications
over
TCP/IP
networks.
The
TCP/IP
format
does
not
require
a
checksum
calculation,
as
lower
layers
already
provide
checksum
protection.
The
packet
structure
is
similar
to
RTU,
but
utilizes
a
different
header.
Devices
labe
l
led
as
Modbus
gateways
will
convert
from
Modbus
TCP
to
Modbus
RTU.
Campbell
Scientific
dataloggers
support
Modbus
RTU,
Modbus
ASCII,
and
Modbus
TCP
protocols.
If
the
connection
is
over
IP,
Campbell
Scientific
dataloggers
always
use
Modbus
TCP.
Modbus
slave
functionality
over
other
comports
use
RTU.
When
acting
as
a
master,
the
datalogger
can
be
switched
between
ASCII
and
RTU
protocols
using
an
option
in
the
ModbusMaster()
instruction.
7.2.3
Understanding
Modbus
Terminology
Many
of
the
object
types
are
named
from
using
Modbus
in
driving
relays:
a
single-bit
physical
output
is
called
a
coil,
and
a
single-bit
physical
input
is
called
a
discrete
input
or
a
contact.
Information
is
stored
in
the
slave
device
in
up
to
four
different
tables.
Two
tables
store
on/off
discrete
values
(coils)
and
two
store
numerical
values
(registers).
The
coils
and
registers
each
have
a
read-only
table
and
read/write
table.
7.2.4
Connecting
Modbus
devices
Dataloggers
can
communicate
with
Modbus
on
all
available
communication
ports.
Consideration
should
be
given
to
proper
surge
protection
of
any
cabled
connection.
Between
systems
of
significantly
different
ground
potential,
optical
isolation
may
be
appropriate.
For
additional
information
on
grounds,
see
10).
The
common
serial
interface
used
for
Modbus
RTU
connections
is
RS-485
half-duplex,
or
two-
wire
RS-485.
This
connection
uses
one
differential
pair
for
data,
and
another
wire
for
a
signal
ground.
When
twisted
pair
cable
is
used,
the
signal
can
travel
long
distances.
Resistors
are
often
used
to
reduce
noise.
Bias
resistors
are
used
to
give
a
clean
default
state
on
the
signal
lines.
For
long
cable
lengths,
termination
resistors,
which
are
usually
120
ohms,
are
needed
to
stop
data
corruption
due
to
reflections.
Signal
grounds
are
terminated
to
earth
ground
with
resistors
to
prevent
ground
loops,
but
allow
a
common
mode
signal.
The
resistors
to
ground
are
usually
integral
to
the
equipment.
The
resistive
ground
is
label
l
ed
as
RG
on
Campbell
Scientific
equipment.
7. Communications Protocols
85
Содержание CR1000X
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Страница 16: ...Appendix A Glossary 176 Appendix B Index 214 Table of Contents viii ...
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