Installation guide

12 PV Clinics Installation Manual 21/07/11 01.00
• Use a GND connection: Connect GND to the shield if a shielded cable is used. Using devices like
the Transclinic, which have isolated RS485 outputs, will reduce problems with common mode noise.
• Use unique addresses: assign a unique Modbus slave address to each Modbus Slave device on
the network. Although Modbus allows for 0-255 Modbus slave addresses, you can still only connect
32 unit loads (including the PV Plant PC).
1.5.3 Modbus RTU RS232
Using an RS232 connection to a device may be useful for short distance point to point communications, i.e. if
you only have a single device to connect. Unless special low-capacitance cables are used, transmission dis-
tances are limited to about 50m.
1.5.4 Virtual COM ports
Since modern computers do not often come with RS232 serial ports (COM ports), you will probably use vir-
tual serial/COM ports to connect to any serial devices. Virtual serial ports are not simple physical converters
but require driver software on the PC. These drivers are normally supplied with the virtual serial port device
or load automatically (for Windows compatible Plug and Play devices). Typical virtual serial port devices are:
• Serial Expansion cards
• USB to RS485 Converters
• Ethernet to Serial converters
• USB Dongles usually present a GSM modem interface as a virtual com port
To identify the COM port that has been assigned to Plug and Play devices go to the Device Manager in
Windows Control Panel.
1.5.5 Modbus TCP
Ethernet is rapidly overtaking serial communications for use in Industrial applications. The use of rugged in-
dustrial Ethernet products means that this is frequently the preferred option for communications in spite of
the increased cost over RS485 and other technologies.
Modbus TCP is a version of Modbus for use over Ethernet networks. Except for the communication between
devices it operates in a similar way to Modbus RTU in that devices have documented memory maps that you
can use to access and interpret data.
Copper Ethernet Links
Copper Ethernet is the most commonly used system. It is limited to 100m cable lengths so anything longer
will require optical fibre or radio links. There are a wide range of Ethernet products available to build your
network.
Fibre-optic Ethernet links
Fibre optic links can extend a Copper Ethernet network over several kilometres. You need a media converter
at each end to change between types. Optic fibre communications have another big advantage over Copper
networks because they are completely electrically isolated so problems with common mode noise and in-
duced voltages are eliminated.
Modbus Gateways
You can put a Modbus RTU network at the end of an Ethernet network using a suitable Modbus Gateway
like the SL-MOD-GW. This can greatly simplify cabling if you already require Ethernet to that location. It also
allows you to connect distant parts of the network using a single fibre-optic link.