Quickstart Guide
ABB PLC and drives integration using Modbus RTU
Modbus
RTU
‐
6
‐
PLC
and
drives
integration
Hardware
physical
connection
In
the
following
chapter,
physical
connection
of
the
hardware
will
be
explained.
Fieldbus
grounding
For
AC500,
which
has
opto
‐
isolated
COM
interfaces,
it
is
recommended
not
to
connect
the
ground.
For
AC500
‐
eCo,
where
the
COM
interfaces
are
not
opto
‐
isolated,
it
is
recommended
to
connect
the
ground
at
one
place,
typically
in
the
bus
master.
For
connection
examples,
see
chapter
“Installation
examples”.
Fieldbus
shield
The
shield
should
always
be
connected
in
a
Modbus
RTU
line.
The
purpose
of
the
shield
is
to
protect
against
EMC
disturbances
and
reflection
on
the
bus
line.
For
connection
examples,
see
chapter
“Installation
examples”.
End
termination
Purpose:
To
avoid
signal
reflections.
Resistor
size:
120
Ohms,
¼
W.
Where
to
connect:
One
resistor
in
each
end
of
the
bus
line
(first
and
last
node).
Pull
up/pull
down
resistors
Purpose:
To
keep
a
defined
high
level
on
the
bus
while
there
is
no
data
exchange.
Resistor
size:
Depends
on
bus
master
(COM)
voltage
level.
470
Ω
for
AC500
(5
V
output)
and
270
Ω
for
AC500
‐
eCo
(3.3
V
output).
Where
to
connect:
Activate
pull
‐
up
and
the
pull
‐
down
resistors,
but
only
once
on
every
bus
line,
at
the
bus
master.
