Break Through: Fieldbus Protocols

"Also, as the wireless standards mature and more functionality is pushed out to the field, we're likely to see an even greater distributed architecture with all the primary control done at the field-device level with wireless linking devices resident in the field junction boxes that communicate back to a central control room. This would be a significant change from our current design practices. However, the potential cost savings are tremendous due to reduced home-run cables, cable trays, process interface buildings and other equipment."

Ironically, once integrators and users begin to eliminate point-to-point hardwiring, some users also start question how much twisted-pair fieldbus networking they need, especially if more data processing and intelligence is migrating to the field anyway. "We're just saying it's helpful to distribute controllers closer to the I/O points and other devices that they're hosting," says Karie Daudt, senior product manager for Turck Inc.'s (www.turck.com) networks and interface division. "For example, we have a programmable gateway for EtherNet/IP that is like a mini PLC. It has both smart I/O and distributed PLC capabilities, and so it can run control programs locally for distributed I/O points, and then just pass the resulting process data up to the asset management system. In fact, one of these programmable gateways has been doing PID control for a glass manufacturer, and it can keep that process running locally, even if communications are lost with the initial master PLC."   

Jim Montague is Control's executive editor.

DeviceNet, ControlNet, EtherNet/IP

• DeviceNet, ControlNet, EtherNet/IP are based on the upper-layer Common Industrial Protocol (CIP), and are supported by ODVA (www.odva.org) and ControlNet International (www.controlnet.org), which co-manage EtherNet/IP.
• DeviceNet uses linear, trunkline/dropline topology; ControlNet has a linear, tree, star or combination; and EtherNet/IP has active star with devices connected to an Ethernet switch
• DeviceNet uses twisted-pair physical media for signal and power; ControlNet uses coaxial or fiberoptic cable; EtherNet/IP typically uses 10/100Base T or 1 Gbps with Cat 5E, twisted-pair cable
• DeviceNet allows 64 nodes maximum; ControlNet allows 99 nodes maximum; EtherNet/IP has no practical device limit
• Maximum distance is 500 m at 125 kbps for DeviceNet, depending on data rate; ControlNet's maximum distance is 1 km via coax with two nodes, 3 km over fiber with 99 nodes, 30 km over fiber or coax with repeaters up to 99 nodes; EtherNet/IP has no distance limit
• DeviceNet and ControlNet's communication method is producer/consumer with peer-to-peer and master/slave options
• Data rate is 500 kbps, 250 kbps or 125 kbps for DeviceNet; 5 Mbps for ControlNet; 10/100 Mbps and 1 Gbps for EtherNet/IP
• Data packet size is 0-8 bytes variable for DeviceNet; 0-510 bytes variable for ControlNet; 0 to 65,511 bytes variable for EtherNet/IP    

Foundation fieldbus

• Foundation fieldbus H1 and High-Speed Ethernet (HSE) are supported by the Fieldbus Foundation (www.fieldbus.org)
• H1 typically uses twisted-pair wiring in a star or bus topology; HSE uses twisted-pair or fiberoptic media in a star topology
• Maximum devices for H1 is 16 devices per segment, depending on performance needs, and up to 65,000 segments; HSE has no device limit due to IP addressing
• H1's maximum distance is 1.900 km at 31.25 kbps and 100 m at 100 Mbps on twisted-pair; HSE's maximum distance is 2 km at 100 Mbps using full-duplex, fiber-optic cable
• H1 and HSE use client/server, publisher/subscriber, event notification communication methods
• Data packet size for H1 is 128 octets, with 256 maximum overhead, but this dimension varies for HSE because it uses TCP/IP
• H1's cycle time usually is less than 500 msec, depending on individual applications; HSE's cycle time is less than 100 msec

Modbus

• Modbus Serial (RTU/ASCII), Modbus Plus (proprietary to Schneider Electric), and Modbus TCP/IP were developed by Modicon and Schneider, and are supported by Modbus-IDA (www.modbus.org)
• Uses twisted-pair, RS-232 and RS-485 wiring in linear, line, star and tree topologies with segments
• Modbus Serial can address a maximum of 247 devices; Modbus TCP/IP allows unlimited addressing
• Maximum distance is 100 m between switches for Modbus TCP/IP via Cat5 copper wiring
• Modbus uses a master/slave or client/server communication methods
• Transmission properties include 1 Mbps for Modbus Plus; 300 bps-38.4 kbps for RTU/ASCII; 100 Mbps for TCP/IP
• Data packet size is variable for Modbus Plus; 0-254 bytes for RTU/ASCII; 1,500 bytes for TCP/IP

HART, Wireless HART

• HART (highway-addressable, remote transducer) field communications and Wireless HART protocols are supported by the HART Communication Foundation (www.hartcomm.org)
• Employs 4-20 mA wiring in point-to-point topology for simultaneous analog/digital communication, point-to-point for digital only, multidrop for digital only (up to 32 devices), and intrinsically safe with appropriate barriers
• One device maximum for point-to-point or 16 devices maximum for multi-drop, which is derived by the noise calculation.
• A network can have more than 16 devices if it meets the specification's noise calculations, but those that exceed these limits might have degraded communications.
• Maximum distance for twisted-pair is 10,000 ft for one device and 5,000 ft for multiple devices
• Traditional analog, 4-20 mA communications, as well as simultaneous encoded digital, FSK based on the Bell 202 telecomm standard, PSK, high speed HART-ITU (CCITT) V.27
• 1,200 bps transmission speed, and 9,600 bps for high-speed HART
• Data packets include one start bit, eight data bits, one odd-parity bit, one stop bit, as well as two-dimensional error checking with device status in every reply
• Two or three transactions per second cycle time for diagnostic information and non-primary variables; three or four transactions per second in "burst mode," and instantaneous analog transmission of primary variables

Profibus, Profinet, ProfiSafe

• Profibus-PA, Profibus-DP, Profinet, Profinet for Motion Control, and ProfiSafe were developed by Siemens AG and are supported by the Profibus Nutzerorganisation e.V. and the Profibus Trade Organization (www.profibus.com)
• Uses twisted-pair or fiberoptic cabling in line, star, ring or bus topologies
• Maximum devices are generally 127 nodes in four segments with three repeaters, plus three masters; Profibus-PA is limited by current restrictions to 15 devices per segment; Profinet allows unlimited devices; and Profisafe has application-specific limits
• Maximum distance is typically 100 m between segments at 12 Mbps, or 12 km with fiber
• Master/slave, peer-to-peer communication methods
• Transmission properties include 1.5 or 12, Mbps for Profibus DP; 31.25 kbps for Profibus PA; 100 Mbps for Profinet
• 256-byte data packets, including 244 bytes of content data
• Configuration-dependent cycle times of less than 2 msec, as well as 1 msec and 10 msec for Profinet versions