By exchanging information across the Web with open application protocols such as HTTP, SMTP, and FTP, people across the world can share and collaborate. These protocols rely on the physical layer of Ethernet. Engineers and managers in industrial automation are also using protocols based on Ethernet, of which one example is Modbus TCP/IP. New protocols including Ethernet/IP and PROFINET highlight a clear trend where industrial protocols are in transition to the Ethernet physical layer.
Open communication protocols
Modbus TCP/IP is an open industrial protocol based on standard Ethernet. Modbus provides connectivity to programmable automation controllers (PACs), PLCs, and legacy devices to preserve hardware and software investments. According to the ARC Advisory Group, Modbus TCP/IP was the leading industrial bus for devices shipped in 2004 .
Modbus TCP/IP is a client/server-based architecture. The general Modbus frame, which contains a function code, a data packet, and error checking, is referred to as an Application Data Unit (ADU). The Modbus TCP/IP ADU is an extension of the existing Modbus protocol, which contains a dedicated 7 byte header called the Modbus Application Protocol header (MBAP), a Modbus function code from the Modbus specification, and the data packet which from the Protocol Data Unit (PDU).
Figure 1: Modbus TCP/IP Application Data Unit over TCP/IP.
Technologies for adding determinism or synchronization to Ethernet-based networks
Several technologies are being developed to add either synchronization or different levels of determinism to the standard Ethernet physical layer. There are two basic requirements for determinism:
Software scheduling to ensure that packets are transferred without loss
Hardware clock or custom ASIC for scheduling packet transfers
Deterministic Ethernet-based networks are being developed based on a wide range of technologies including software-based time triggering, IEEE 1588, and custom hardware ASICs
Time-Triggered networking is a software-based technology that schedules packet transfers on TCP/IP. It does not rely on additional hardware to implement a clock source, and is limited by the speed of the Ethernet network and accuracy of the processor clock.
One implementation is the time-triggered network feature in National Instruments LabVIEW Real-Time. This protocol uses a private Ethernet network between nodes, and all traffic is predetermined by a schedule. Developers use software to define when each node can send data messages. The size of each data message is fixed prior to execution to ensure that the schedule is met. This configuration requires a private subnet and places implementation requirements on the system developer.
The IEEE 1588 precision time protocol (PTP) is a technology for sharing clocks between distributed systems. IEEE 1588 does not provide determinism unless it is combined with a protocol that handles software scheduling. Technically, IEEE 1588 provides a distributed time base used to timestamp data with submicrosecond precision. Synchronization skew depends on the resolution of the clocks at each device, the synchronization rate, and network topology.
Hardware ASICs provide determinism by setting priorities of packets or by inserting messages into the TCP/IP network frame at the hardware level, only adding a few nanoseconds of delay. Depending on the implementation and whether the ASICs is implemented on the client, master, or the networking equipment, there are advantages and disadvantages for ASICs.