Appendix B - Modbus Guide; Introduction; Setting Up; Modbus Protocol - ABB SM3000 User Manual

SM3000
Multipoint Videographic Recorder
Appendix B – Modbus Guide
B.1 Introduction
A Modbus/RS485 serial communications link is available as an
option.
Note. The Modbus option provides the following facilities:
n Standard RS422/485 communications.
n Modbus RTU protocol – for master (host computer) to
slave (Multipoint Videographic Recorder) system.
n Modbus protocol
Videographic Recorder) to slave (RTU) system.
n 500 V DC isolation from external connections to the
instrument.
n Two-wire or four-wire communication.
n 1200, 2400, 4800, 9600, 19200, 38400, or 115200
baud transmission rate.
n Parity-checking – odd, even or none.
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 36 Comms Analog and 36
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
7.5, page 116.
2. Set the RS485 configuration parameters – see Section
6.7.4, page 105.
3. Add the instrument to the link configuration on the host
system – refer to information supplied with the host
system.
IM/SM3000–EN Rev. L
– for master (Multipoint
B.3 Modbus Protocol
B.3.1 Modbus Commands Supported
The following Modbus commands are supported:
01 Read Coil Status – reads the on/off status of up to 120
consecutive digital states, starting at a specified address.
The instrument returns zeros for points that do not contain
defined data.
03 Read Holding Registers – reads up to 120 consecutive
analog values, starting from a specified address. The
instrument returns zeros for registers that 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
FF00Hex to set the signal ON and zero to turn it OFF. The
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.
Note.
n Negative numbers
complement' format, e.g 1000 = 03E8 (Hex), –1000 =
FC18 (Hex).
n The instrument cannot accept a new message until
the current message has been processed and a reply
sent to the master (maximum response time 50 ms).
n 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.
Appendix B – Modbus Guide
are represented in '2's
131