Introduction; Electrical Installation; Host Computer Serial Communications; Two-Wire And Four-Wire Connection - ABB SM3000 User Manual

1 INTRODUCTION

This Operating Guide describes the Modbus serial data
communications option for the Multipoint Videographic
Recorder and must be used in conjunction with the User Guide
(part no. IM/SM3000) supplied with the instrument.
Information.
The Modbus option provides the following facilities:
• Standard RS422/485 communications.
• Modbus RTU protocol – for master (host computer) to
slave (Multipoint Videographic Recorder) system.
• 500V d.c. isolation from external connections to the
instrument.
• Two-wire or four-wire communication.
• 1200, 2400, 4800, 9600, 19200, 38400, or 115200 baud
transmission rate.
• Parity-checking – odd, even or none.
The instrument is designed to act as a Remote Terminal Unit
(RTU) slave when communicating with a master (host) system.
Terminal Block G
16
17
Tx+/Rx+
18
Tx–/Rx–
19
COM
20
A – 2-wire Connection
comply with local safety standards (IEC 60950, EN601010–1).
2
Host Computer
'B'
'A'
+5V
Tx+
1.8kΩ Pull-up
Tx–
Resistor
Rx+
'A'
Rx–
'B'
1.8kΩ Pull-down
Resistor
GND
0V
Caution. All equipment connected to the instrument's terminals must
Fig. 2.1 Two-wire and Four-wire Connection

2 ELECTRICAL INSTALLATION

This section describes the connection of serial data cables
between the master (host computer) and slave instrument on a
Modbus serial link. All connections other than those used for
serial communication are shown in Section 5 of the User Guide.

2.1 Host Computer Serial Communications

The serial interface option module has been designed to operate
using the Modbus Remote Terminal Unit (RTU) Master/Slave
protocol.
An appropriate RS422/485 communications driver must fitted
to the host (Master) computer. It is strongly recommended that
the interface has galvanic isolation to protect the computer from
lightning damage and to increase signal immunity to noise
pick-up.
2.2 Two-wire and Four-wire Connection – Fig. 2.1
Modbus serial communications must be configured as either
two-wire or four-wire serial links – see Fig. 2.1. Two-/four-wire
operation must also be selected in the the instrument's
Configuration Level – see Section 3.1.
The instrument must be added to the link configuration on the
host system – refer to information supplied with the host system.
Terminal Block G
Tx+
16
Tx–
17
Rx+
18
Rx–
19
COM
20
B – 4-wire Connection
Host Computer
+5V
1.8kΩ Pull-up
0V
Resistor
'B'
'A'
1.8kΩ
Pull-down
+5V
Resistor
1.8kΩ 0V
Pull-up
Resistor 'B'
'A'
1.8kΩ
Pull-down Resistor
0V