I/O bus

The bus for communication with the I/O modules is a 2-wire RS485, using Modbus RTU protocol. It is necessary to keep some basic rules for design, installation, and commissioning, to avoid communication dropouts or complete data loss.

Recommended cable

The total bus length should not exceed 1200 m. It is recommended to use line topology, or to bring the bus from one module to another without stubs. To terminate the bus properly, the first and the last device on the bus shall have set the BUS END switches ON. See also data sheets of the modules.

Recommended cable types are e.g.

  • JY(st)Y 2×0.8
  • LAM Datapar 1x2x0.8

When designing the cable layouts, it should be kept in mind that the EMC influences increase strongly with increase of the parallel cable run length and decreasing of the distance between cables. If there is a common cable tray for power lines and communication, it is advisable to separate both cable types using a shielding wall or kep them at least 20 cm apart. In fact, the communication is robust enough to work properly mostly even at parallel cable runs. Problems may arise at sites with unshielded photovoltaic inverters, variable speed drives etc.

It is not recommended to combine different cable types, there are signal reflections at the connecting points and it is very difficult if not impossible to balance the bus properly. This is relevant especially at higher bus lengths (hundreds of meters).

Overvoltage protection

Parallel cable runs within panels are irrelevant because of their low length.

If the bus is lead between the cabinets in a highly EMC critical environment, such as outdoor installations, PV plants, wiring between buildings, etc., it is recommended to use some of the commercially available overvoltage protections, or a galvanically separated RS485 repeater.

The bus is permanently short-circuit resistant. Of course the communication is down when the bus is short-circuited, but the modules will not be damaged.

Wiring rules

Note the correct polarity. Wrong bus polarity, or interchanging of the wires, leads to communication malfunction and may block communication with more modules or even with all modules on the bus (depending on the distances between the modules and their amount). The modules will not be damaged.

There is no maximum distance between the modules. The only limitation is the total bus length of 1200 m.

Tools for addressing of the modules

For hardware addressing, no special tools are necessary. The address is set by setting the DIP switches on the module, see module data sheets.

For software addressing, the I/O modules are addressed using

  • configuration software ModComTool, and
  • a RS232/RS485 converter (e.g. R012) or USB/RS485 converter (e.g. M080).

If the converter is not available, the modules may be addressed and configured using the process station switched into Ethernet / RS485 converter mode (commissioning modeat Merbon,converter modeat SoftPLC) – see help for Merbon IDE, or data sheets of SoftPLC process stations and Special functionsin SoftPLC IDE.