| A |
S AN ELECTRICAL ENGINEER IN THE PAPER INDUSTRY, I’ve designed and commissioned control systems for high-speed finishing machinery. The length of some machines could be measured in city blocks, with auxiliary equipment located hundreds of feet away.
To reduce installation cost and time, control systems are designed with distributed I/O that requires networked I/O racks. Fifteen years ago, there were only a few network technologies to choose from. Today, not only are there more of them to choose from, but there are no universal performance measures to compare the capabilities of one network to another. Without a way to categorize capabilities, it is difficult to match a control strategy to a network.
One vital way to categorize them is the International Standard Organization's Open System Interconnect (ISO/OSI) model, which describes a network communication architecture with seven layers. Most LANs only use layers one, two, and seven. The first layer is the physical layer that deals with the cabling, topology and electrical characteristics. The second layer is the data link layer that deals with transmitting data reliably across the network. The seventh layer is the application layer, which is the software used directly by the engineer.
The OSI layer with the greatest effect on network performance is layer two where Medium Access Control (MAC) is implemented. MAC refers to the arbitration of each node’s access to the wire (a node can be a programmable controller, a PC, an I/O adapter, a drive, etc.). Networks use one or more of these common MAC methods: contention, reservation, and token passing.
With a contention-based MAC, collisions are allowed between two or more nodes trying to access the network. When the line drivers in the network-interface hardware try to control the signal level on the wire at the same time, the result is an incorrect level that is detected by the hardware. This can make troubleshooting difficult because other phenomena may also cause signal strength variations. When troubleshooting a contention-based network in an industrial environment, collisions can indicate electrical noise or incorrect cable termination, as well as heavy network traffic.
Carrier Sense Multiple Access with Collision Detection (CSMA/CD) used with Ethernet, and Carrier Sense Multiple Access with Arbitration on Message Priority (CSMA/AMP) used with DeviceNet, are examples of contention schemes. When a collision occurs on an Ethernet network, the transmitting nodes stop communicating and wait for random periods of time before attempting to transmit again. This is why Ethernet is not considered deterministic. When a collision occurs with DeviceNet, the message with the highest priority is given access to the network.
With a reservation-based MAC, access to the network is divided into time slices. Each node gets a slice in which it has exclusive access to the wire. When its time is up, it releases the network and the next node gains access to the wire for its share of time. Concurrent Time Domain Multiple Access (CTDMA) used with ControlNet is a specific type of reservation MAC that allows multiple nodes to access the network in predefined time slices. Foundation fieldbus also is a reservation-based MAC network.
With token passing, a permission is passed from node to node. When a node has the “token,” it is allowed access to the wire for a set period. When its time is up, it passes the token to the next node. ArcNet, Profibus, and A-B Remote I/O are token-passing networks.
Contention works well with soft, real-time applications (it just has to be fast enough) such as sequential logic. Token passing and reservation are useful for hard real-time applications (update needs to be guaranteed) such as closed-loop control over the network. One drawback to token-passing is that adding nodes changes how often the existing nodes can access the network. This is particularly important when upgrading an existing machine where a feedback control loop is closed over the network. The extra time delay decreases the phase margin and makes the loop tend towards instability. Adding nodes to a reservation-based network does not change the update as long as network capacity is not approached. That the update can be specified by the engineer and remains fixed makes this network ideal for closed-loop control systems.
A token-passing network and a contention-based network are generally easy to set up. Node numbers need to be assigned and that's about it. A reservation network is generally more complex because the engineer needs to pre-schedule update times for each node. In addition, a fairly sophisticated network controller is needed.
