Computer Solutions
specification for a PLC, which led to the development of Modicon and the Modbus protocol, or the CAN chipset specified by Bosch for in-vehicle electrics.
Manufacturing Automation Protocol “The first time I was involved with Profibus it was using Fieldbus Messaging System (FMS), which is a subset of the Manufacturing Automation Protocol (MAP) conceived by Japanese the automotive industry and adopted by General Motors. Fieldbus only ran at 500 Kbits with FMS and was superseded by Profinet when Profibus decentralised peripherals (DP) came out running at 1.5 Mbits.”
Lane is deputy chairman of Profibus UK, and his employer Wago supports a multiplicity of automation protocols, some of which have now lapsed, such as Firewire, Apple’s implementation of the IEEE 1394 high speed serial bus protocol for PCs, which has been overtaken by the even faster USB3 protocol.
Before the days of high-speed serial communications, PLCs had to be wired in parallel to cope with update times, and before the days of serial or digital communications, analogue 4-20mA current loop communications ruled the roost for measuring variables such as pressure, temperature, flow and pH.
Although not as prominent nowadays as digital communications, current loop is still holding sway in process environments, particularly with the HART protocol, and is still favoured over the voltage analogue method of 0-20V, which cannot detect a broken wire.
Lane continues: “Another problem is where people tie zero volts down to earth because if you get an earth fault, you will actuallyconduct a signal, which is dangerous.
“At the other extreme, with high-speed Ethernet communications originating
from the IT environment, some plant and factory managers were initially suspicious that the IT department would take over control of shopfloor networking.
Some factory people do not understand the fundamentals of Ethernet networks with their MAC addresses, unique IP addresses and subnet masks. The earlier issues of whether Ethernet is deterministic enough no longer apply today because with EtherCAT and now Profinet, you get response times down to 31.25 microseconds.
Wireless mesh networks One trend that is driving the adoption of industrial Ethernet is the Internet of Things (IoT) as more customers demand connectivity for every device. Instrument manufacturers such as Endress & Hauser now connect devices via Ethernet and in some cases via wireless mesh networks.
In the case of 4-20mA analogue measurement, HART users are increasingly converting current loop signals to digital form for Ethernet. Rockwell Automation for example offers analogue cards with HART compatibility to pick up analogue signals and put them in digital form on an EtherNet/IP bus.
“Once converted, these signals can be throughout the system, whether they feed into a real-time digital controller or are captured for historical data collection and analytics,” says Mike Loughran, solution architect at Rockwell Automation. ”
Loughran quotes a Cisco report that identified that there would be $40 trillion worth of opportunities created around the IoT by about 2020, of which 27% will be in manufacturing automation. He claims over half of the devices in the field will be Ethernet-enabled and will not be automation devices such as PLCs or I/O blocks, but items such as weighing scales, RFID, barcode readers, cameras and mobile devices. l
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