Human-Machine Interface

Figure 2-9: Hardware to Display the Human-Machine Interface

PLCs interact with people for the purpose of configuration, alarm reporting or everyday control. A Human-Machine Interface (HMIClosed"Human-machine interfaces " Also known as computer-human interfaces (CHI), and formerly known as man-machine interfaces, they are usually employed to communicate with PLCs and other computers, such as entering and monitoring temperatures or pressures for further automated control or emergency response) is employed for this purpose. A simple system uses buttons and lights to interact with the end-user. Text displays are available as well as graphics on the touch panels.

Most modern PLCs can communicate over a network to some other systems, such as a computer running a SCADA system.

The communication between the HMI and the PLCClosed"Programmable Logic Controller" A Programmable Logic Controller, PLC, or Programmable Controller is a digital computer used for automation of industrial processes, such as control of machinery on factory assembly lines. Used to synchronize the flow of inputs from (physical) sensors and events with the flow of outputs to actuators and events is based on ModbusClosedModBus is a serial communications protocol and is now the most commonly available means of connecting industrial electronic devices. ModBus is often used to connect a supervisory computer with a remote terminal unit in supervisory control and data acquisition (SCADA) systems. Versions of the ModBus protocol exist for serial port and Ethernet (it is widely used with TCP/IP over Ethernet) over TCP/IP (Modbus TCP is the EthernetClosedEthernet is a large, diverse family of frame-based computer networking technologies that operate at many speeds for local area networks (LANs) version of Modbus) by means of a standard Ethernet cable that connects the two devices.

This communication is done in the background, asynchronously, at the cycle time (20-1000 milliseconds) specified in the Controller Properties (see "Configure the Controller"). Variables defined in the HMI to describe the interface (see Map Variables to HMI) are passed to the AKD PDMM/PCMM this way. This means there is no data coherency in the data exchange because the variables read by the Modbus do not come from the same PLC cycle. As this data has a rather low priority and is interpreted by human feedback, it should never be noticed by the user.


  • Please note that Kollmorgen HMIs are limited to communicating no more frequently than every 100 milliseconds.

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