Appendix 2 - Modbus Guide; A2.1 Introduction; A2.2 Setting Up; A2.3 Modbus Commands Supported - ABB SM1000 User Manual
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APPENDIX 2 – MODBUS GUIDE
A2.1 Introduction
A Modbus/RS485 serial communications link is available as an
option.
The instrument is designed to act as a Remote Terminal Unit
(RTU) slave when communicating with a master (host) system.
A2.2 Setting Up
a) Connect the instrument to a Modbus link – see Section 5.5.
b) Set the RS485 configuration parameters – see Section 4.4.5
c) Add the instrument to the link configuration on the host
system – refer to information supplied with the host system.
A2.3 Modbus Commands Supported
The following Modbus commands are supported:
01 Read Coil Status – reads the on/off status of 16
consecutive digital states, starting at a specified address.
The instrument returns zeros for points which do not contain
defined data
03 Read Holding Registers – reads 8 consecutive anolog
values, starting from a specified address. The instrument
returns zeros for registers which do not contain defined data.
05 Force Single Coil – sets the value of a single coil (digital
signal) at the specified address. The data value must be
FF00
to set the signal ON and zero to turn it OFF. The
Hex
instrument returns an exception response if the register is
not currently writeable.
06 Preset Single Register – sets the value of a single register
(analog value) at the specified address. The instrument
returns an exception response if the register is not currently
writeable. Limits defined in configuration are applied to the
value before storage.
08 Loopback Diagnostic Test – used to test the integrity of
Modbus transmissions. The instrument returns the message
recieved.
15 Force Multiple Coils – the instrument carries out updates
that are valid and returns an exception response if any of the
coils are not currently writeable.
16 Preset Multiple Registers – the instrument carries out
updates that are valid and generates an exception response
if any of the registers are not currently writeable.
Information.
• Negative numbers are represented in '2's complement'
format, e.g 1000 = 03E8 (Hex), –1000 = FC18 (Hex)
• The instrument cannot accept a new message until the
current message has been processed and a reply sent to
the master (maximum response time 50ms).
• The instrument monitors the elapsed time between
receipt of characters from the host. If the elapsed time
between two characters is 3.5 character times, the slave
assumes the second character received is the start of a
new message.
76
A2.4 Modbus Exception Responses
If the instrument detects one of the errors listed in Table A2.1
while receiving a message from the host system, it replies with a
response message consisting of the instrument's Modbus
address, the function code, the error code and error check
fields.
C
o
d
e
N
a
m
e
0
1
I
e l l
g
a
F l
u
n
c
o i t
n
I
e l l
g
l a
D
a
a t
0
2
A
d
d
e r
s s
0
3
I
e l l
g
l a
D
a
a t
V
a
u l
e
N
e
g
i t a
e v
0
7
A
c
n k
o
w
e l
d
g
m
e
t n
M
e
m
o
y r
P
r a
y t i
0
8
E
r r
r o
Table A2.1 Modbus Exception Responses
A2.5 Operating Mode Modbus Coils
Tables A2.2 to A2.7 detail the contents of each Modbus coil.
Each coil is assigned a register that can have one of two values:
0000 and 0001.
A
n
a
o l
g
n I
p
t u
R
a e
d
A
1
A
2
A
3
A
4
A
5
A
6
B
1
B
2
B
3
B
4
B
5
B
6
R
s e
r e
e v
d
Table A2.2 Analog Input Fail States
D
f e
n i
t i
o i
n
T
h
e
m
s e
a s
g
e
u f
n
c
o i t
n
r
c e
i e
e v
d
a
n
a
o l l
w
a
b
e l
c a
o i t
n
T
h
e
a
d
d
e r
s s
e r
e f
e r
n
e c
n i
h t
e
d
a
s i
n
t o
a
n
a
o l l
w
a
b
e l
a
d
d
e r
s s
T
h
e
a v
u l
e
e r
e f
e r
n
e c
d
n i
h t
e
d
a
a t
n
t o
a
o l l
w
a
b
e l
n i
h t
e
a
d
r d
s e
e s
d
s
R
c e
i e
e v
d
m
s e
a s
g
e
e
r r
r o
P
r a
y t i
c
h
e
k c
n i
d
c i
t a
s e
a
n
e
r r
r o
n i
r o
m
o
e r
f o
h t
e
c
h
r a
c a
e t
s r
r
c e
i e
e v
M
o
d
b
u
s
C
o
l i
O
n
. y l
0
=
n I
p
t u
O
, K
1
=
n I
p
t u
a f
0
0
0
1
0
0
0
2
0
0
0
3
0
0
0
4
0
0
0
5
0
0
0
6
0
0
0
7
0
0
0
8
0
0
0
9
0
0
1
0
0
0
1
1
0
0
1
2
0
0
1
3
o t
0
0
5
0
s i
n
t o
a t
e i f
d l
e i f
d l
s i
a l
e v
o
n
e
d
e l i
d
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