Overview - ABB MRP31.0 Technical Description

Overview

This chapter contains a short description of the RS-485 standard and the MRP31 Modbus Interface module.
RS485 Standard
RS-485 is a serial interface standard for communication over a twisted-pair cable. Because the RS-485 signal transmission is
differential, it provides better protection against noise and longer transmission distances than the RS-232. RS-485 is a half-
duplex multi-drop network, which means that multiple devices may reside on line. Only one transmitter may be active at any
given time.The RS-485 standard specifi es only the electrical characteristics of the bus system.
The RS-485 transmission line consists of two wires, A and B. The signal transmission is based on the voltage difference between
the wires. The potential difference between the two wires determines the logic state bit: when B is at higher voltage than A, the
state is defi ned as bit 1 (data high) and when A is at higher voltage than B, the state is defi ned as bit 0 (data low).
Ground wire and cable shield should be connected to prevent common mode voltage between the network devices from drifting
outside the allowable limits. RS-485 bus cable should be terminated with a 120 Ohm resistor on both ends to prevent signal
refl ection.
The MRP31 Modbus Interface
The MRP31 Modbus communication interface is an optional device for the UMC100.3 which enables the connection of the
UMC100 to a RS-485 network. The MRP31 provides galvanic isolation between the UMC100.3 and the RS-485 network.
• Use UMC100.3 to set bus address, baud rate and timeout of the Modbus communction. Parity and frame lenght are
automatically detected
• MRP31 supports the RTU transmission mode only
• MRP31 is considered as a slave on the Modbus RTU network. Only one master can communicate with the MRP31 at a time.
Through the interface you can:
• give control commands to the UMC100.3 (for example Start, Stop, Fault Reset)
• read status information and actual process values from the UMC100.3
• read device diagnosis information
MRP31
Top View
Issue: 09.2014
Status
Indication
LEDs
X3:
Modbus
connection
X3:
Modbus
connection
X2: 24V DC X1: Communication
supply connection to UMC100.3
Side View
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Technical description MRP31.0