Appendix B - Modbus Guide; Introduction; Setting Up; Modbus Commands Supported - ABB SM1000 User Manual

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SM1000
Appendix B – Modbus Guide
B.1 Introduction
A Modbus/RS485 serial communications link is available as an
option.
The instrument can be configured to act as either a Remote
Terminal Unit (RTU) slave when communicating with a master
(host) system, or as a Modbus master device, enabling it to
collect data from RTU slaves into its 24 Comms Analog and 24
Comms Digital Channels.
B.2 Setting Up
To set up the instrument on a Modbus network:
1. Connect the instrument to a Modbus link – see Section
5.5, page 101.
2. Set the RS485 configuration parameters – see Section
4.8.4, page 91.
3. Add the instrument to the link configuration on the host
system – refer to information supplied with the host
system.
B.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 analog
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 writable.
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
writable. 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
received.
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 writable.
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 writable.
106
Note.
Negative numbers
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.
B.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.
Code Name
01 Illegal Function
Illegal Data
02
Address
03 Illegal Data Value
Negative
07
Acknowledgment
Memory Parity
08
Error
Table B.1 Modbus Exception Responses
B.5 Operating Mode Modbus Coils
Tables A2.2 to A2.14 detail the contents of each Modbus coil.
Each coil is assigned a register that can have one of two values:
0000 and 0001.
Modbus Coil
Analog (Read Only)
Input 0 = Input OK
1 = Input failed
A1 0001
A2 0002
A3 0003
A4 0004
A5 0005
A6 0006
Table B.2 Analog Input Fail States
Appendix B – Modbus Guide
are represented in
Definition
The message function received is not
an allowable action
The address reference in the data field
is not an allowable address
The value referenced in the data field
is not allowable in the addressed slave
Received message error
Parity check indicates an error in one
or more of the characters received
Modbus Coil
Analog (Read Only)
Input 0 = Input OK
1 = Input failed
B1 0007
B2 0008
B3 0009
B4 0010
B5 0011
B6 0012
Reserved 0013 to 0050
IM/SM1000 Issue 11
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