1.6 Industrial interface transceivers  (Page 19/3)

Transceiver positioning within the network hierarchy

Industrial interface (IIF) transceivers operate on the device level within the industrial network hierarchy. Here, they transmit and receive digital data between network nodes that build the input/output for programmable logic controllers (PLCs), distributed control systems (DCSs), human-machine interfaces (HMIs), motor drives, valves, process analyzers and other instruments.

Figure 1 shows the positioning of IIF transceivers within the industrial network hierarchy.

In the case of a sensor/actuator interface node (the left half of Figure 2, for example), analog sensor data is conditioned by the amplifier and converted into a synchronous, digital data stream by an analog-to-digital converter, then further processed by a microcontroller. The UART interface of the MCU commonly feeds the driver with asynchronous data that is transmitted across a differential bus toward the destination node. In the opposite direction, a network node receiving asynchronous data needs to convert this data into a synchronous stream, which upon conversion by a digital-to-analog converter provides an analog output signal to drive a motor.

The lion's share of network designs favor a half-duplex topology: bus nodes connected in a daisy chain using unshielded twisted pair (UTP) cable with a characteristic impedance of Z ₀ = 100 Ω to 120 Ω (the right half of Figure 2). Because signal propagation along the bus is significantly longer than a driver’s rise and fall times, the bus cable is treated as a transmission line, thus requiring termination resistors at both cable ends whose values must match the characteristic cable impedance, R _T = Z ₀ .

Functional principles

The most popular interface standard in industrial automation and process control applications is EIA/TIA-485, better known as RS-485. Another emerging standard is the controller area network (CAN) standard, initially developed for automotive applications only.

For decades, RS-485 has been – and still is – the industry’s workhorse. The reasons for this lie in the robust output capability of the driver and the high noise immunity of the receiver. The output stage of an RS-485 transceiver (Figure 3, left) consists of two push-pull stages driving a bridge-tied load, R _L . R _L constitutes the sum of the termination resistors and the differential input impedance of the bus receivers. The resulting output (Figure 4, left) is a true differential bus signal with a specified minimum of V _OD = ±1.5 V across a maximum load of R _L = 54 Ω.

One drawback of RS-485 is that it does not allow multiple drivers to access the bus at the same time. Therefore, in order to prevent bus contention, communication between bus nodes requires a data frame to include start and stop bits, as well as address information for individual nodes.