Modbus-IDA, whose members are primarily users and suppliers in the auto-
mation industry.
Modbus is an application-layer messaging protocol for client–server com-
munication between devices connected on different types of buses or networks.
It can be implemented over an Ethernet network as an asynchronous serial
transmission such as RS-232 or RS-485, or as a high-speed, token-passing net-
work call Modbus Plus. For Ethernet and Modbus Plus the message created
by the Modbus protocol is “tunneled” or imbedded into the frame or packet
structure that is used on an Ethernet or token-passing network.
The most common implementation of Modbus uses the serial RS-485 phys-
ical layer with either Modbus RTU (a binary representation of the data) or
Modbus ASCII (human readable). The Ethernet implementation option uses
Modbus/TCP.
Modbus, like other communication protocols, defines a message structure
and format for message fields; how a controller requests access to another
device, how to respond to requests, how errors will be detected and reported,
how to identify devices, how to recognize a message to a device, and so on.
Modbus versions have different functionality. For example, the basic Mod-
bus protocol is a master–slave arrangement that does not provide for a “slave”
device to report to the master unless it is polled by the master. In the Modbus
implementation over Ethernet, Modbus/TP devices can report to a master.
Typical Modbus implementations are limited to 247 devices, although in a
Modbus/TP implementation, no such limit exists.
Modbus is a simple yet effective protocol. Typical problems that the
designers have to overcome include high latency and timing problems.
OPC
OPC is somewhat of an acronym embedded in an acronym. OPC stands for
“OLE for process control”; in turn, OLE stands for “object linking and
embedding.” OPC is based on Microsoft’s OLE/COM technology which
essentially allows Windows programs to communicate with hardware devices.
OPC operates in a client–server approach. The OPC server converts the
communications protocol of a hardware device into the OPC protocol.
The OPC client software uses the OPC server to obtain data or send com-
mands to the hardware device. The OPC client software, typically a human–
machine interface (HMI), allows the client to communicate to the hardware
device.
OPC is an open standard. Software vendors simply include OPC client cap-
abilities in their products and they become compatible with hardware devices.
24 Smart Building Systems for Architects, Owners, and Builders