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.
Industrial Protocols Based on Ethernet Physical Layer
Ethernet/IP is a protocol developed managed by the Open Device Vendors Association (ODVA) that extends the Communication and Information Protocol (CIP) to Ethernet. Ethernet/IP is an application layer protocol that uses TCP for general messages and User Datagram Protocol (UDP) for I/O messaging and control. UDP is a multicast protocol that routes packets from the controller to multiple destinations. Although this is an efficient communication method for one controller to broadcast data to multiple receivers, it can create broadcast storms that consume network bandwidth. To prevent broadcast storms, managed switches with Internet Group Management Protocol (IGMP) capabilities are required.
PROFINET - is an application protocol that uses familiar concepts from PROFIBUS; however it is implemented on the physical layer of Ethernet. The PROFINET protocol uses proxy servers to provide connectivity to PROFIBUS devices and additional protocols such as DeviceNet, Modbus, and AS-i.
PROFINET Real-Time (RT) - relies on the physical layer of Ethernet but does not use either the TCP/IP or UDP/IP application layer. PROFINET uses information in the standard Ethernet frame header to identify packets that are real-time. Switches that implement Quality of Service (QoS) are then able to further prioritize packets.
Adding Technologies Such As IEEE 1588 or Custom ASICs to Industrial Protocols
Several Ethernet-based protocols implement software scheduling and additional features to make Ethernet well suited for industrial applications. New technologies such as IEEE 1588 and custom ASICs are also being added to these protocols to deliver hard real-time determinism for demanding motion and distributed control applications.
CIP Sync is an addition to Ethernet/IP that uses IEEE 1588 technology to distribute clocks and synchronize timebases. The level of determinism available with CIP Sync networks depends on the availability of an IEEE 1588 clock on Ethernet switches in the network and the implementation of the CIP Sync protocol on distributed devices. System determinism is highly dependent on system design and Ethernet throughput. Ethernet switches with IEEE 1588 clocks are not available as of February 2007; however, vendors including Hirschmann have announced plans to release commercially available IEEE 1588 switches.
PROFINET Isochronous Real-Time (IRT) uses the IEEE 802.3 physical layer and then adds IEEE 802.1Q and Enhanced, Real-Time Ethernet Controller (ERTEC) ASICSs that are defined in the ProfiNET specification and available for purchase from Siemens. These ERTEC ASICs are added to both the device and Ethernet switch to reserve bandwidth for high-performance traffic. Industrial switches that offer quality of service (QoS) are suitable for PROFINET non-Real Time (NRT) and Real Time (RT) However, to achieve the highest level of performance, similar to that of CIP Sync, use switches with the ERTEC ASIC to ensure bandwidth for deterministic communication with PROFINET Isochronous Real-Time (IRT), which enables clock rates of <1 ms and a jitter precision of <1 µs.
Table 1. Networking Requirements for Industrial Protocols
Open, Flexible Hardware and Software Platforms – Key to Ethernet Benefits
The trend toward Ethernet-based industrial protocols is clear. It is critical to take full advantage of the economies of scale of Ethernet open software and hardware platforms, with which system designers can integrate different industrial protocols. With the ability to connect different protocols using a single software API and migrate existing code forward, you can build hybrid systems that offer best-in-class hardware from different vendors.