Intersil – How dual-protocol transceivers ease the design of industrial interfaces

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The trend in the design of industrial PCs towards smaller form factors and more versatile communications capabilities is driving the development of modern bus transceivers. In the latest industrial PCs, new transceivers are preferred to legacy designs for several reasons:
• they are highly integrated
• they offer a dual-protocol capability, supporting both the RS-232 and RS-485 standards
• they are easily configurable
This Design Note discusses the reasons for the requirement for dual-protocol capability, and reviews the functionality and features of a dual-protocol transceiver, the ISL33334E from Intersil.

The RS-232 standard
RS-232 is a single-ended full-duplex interface, suitable for point-to-point communication only. This means that one driver connects to one receiver and vice versa. The interface requires a ground wire connection between the driver and receiver grounds to provide a common reference for the Transmit and Receive signals, as shown in Figure 1.

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Fig. 1: The RS-232 protocol provides a single-ended, point-to-point data link

The means by which an RS-232 bus gains noise immunity is through the use of signals with a high amplitude. The standard uses inverse logic and specifies a logic zero at a bus voltage from +3V to +15V, and a logic one from -3V to -15V. The range from +3V to -3V is unallocated.

Most of today’s RS-232 designs use one or two data channels, with each channel consisting of a Transmit and Receive signal pair. Two-channel transceivers can use one channel for data transmission and the other for handshake control. Single-channel devices must resort to software flow control.

The RS-232 standard specifies a maximum signal rate of up to 19.8kbits/s and a maximum slew rate of 30V/μs. By reducing the amplitude of the bus voltage, however, modern transceivers can support data rates of up to 1Mbit/s without violating the slew-rate specification. Although not specified in the standard, the maximum cable length is limited in practice to around 30m.

The RS-485 standard
Developed in the early 1980s, the RS-485 bus protocol offers much more robust data transmission in noisy environments and over long distances. The standard uses differential signalling across a signal pair of two conductors, A and B. It specifies a differential bus voltage swing between the two conductors of 1.5V minimum when carrying a 54Ω differential load.

The RS-485 standard provides for up to 32 unit loads to be networked together via a multi-point bus topology. Bus nodes are daisy-chained, as shown in Figure 2, to one another through twisted-pair cable. The recommended characteristic cable impedance of 120Ω requires termination resistors at both cable ends with values which match the cable impedance.

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Fig. 2: A typical RS-485 network with daisy-chained bus nodes and terminated cable ends

As the receiver inputs use ground as their internal reference, a separate ground connection between drivers and receivers is not required, provided that the receiver input voltages do not exceed the specified common-mode voltage range of -7V to +12V.

RS-485 supports cable lengths up to 4,000ft (1,200m) and data rates up to 10Mbits/s, but not simultaneously, as shown in Figure 3 on the next page.

RS-485 supports both half-duplex and full-duplex multi-point topologies which allow each bus node to either transmit or receive data. A half-duplex bus uses two wires across, where one node may transmit data while another node receives data. In a full-duplex bus, two signal pairs (four wires) are used. One pair connects the driver of the master node to the receivers of multiple slave nodes, and the other pair connects the drivers of the slave nodes to the receiver of the master node.

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Fig. 3: The maximum data rate is reduced when cable length is extended

This topology allows the master to either broadcast data to all slaves or address a specific slave node, while simultaneously receiving data from the slave nodes, one slave at a time. A full-duplex bus increases data throughput but is substantially more expensive than a half-duplex bus due to the higher cost of wiring and installation, as shown in Figure 4.

Fig. 4: Half-duplex and full-duplex multi-point bus topologies in RS-485

Fig. 4: Half-duplex and full-duplex multi-point bus topologies in RS-485

Dual-protocol transceivers
Modern transceivers are capable of supporting the designs both of new industrial PCs and of RS-232-to-RS-485/RS-422 interface converters. Converters are needed in existing RS-232 equipment, such as legacy PCs, instrumentation equipment and industrial machinery, in which interfaces must either be connected to a single network, or be extended over long distances.

The ISL33334E from Intersil is an example of such a dual-protocol transceiver, as shown in Figure 5. It incorporates two RS-232 Transmit and Receive channels and one full-duplex RS-485 transceiver. The transceiver features a flow-through pin-out, with bus pins on one side and logic pins on the other. This allows for easy routing of signal traces to the local controller, and offers a great advantage over legacy transceivers, the pin-outs of which require the crossing of signal traces from the bus to the controller side and vice versa.

When operating the bus systems independently, each RS-232 port can support data rates of up to 400kbits/s without exceeding the maximum slew rate.

The RS-485 section allows for the selection of a 20Mbits/s high-speed mode and a 115kbits/s slew rate-limited mode. In high-speed mode the driver output is not slew-rate limited. This mode should only be applied over distances shorter than 100ft (30m). The high-speed mode also requires the implementation of termination resistors at both cable ends, and the resistor values must match the characteristic cable impedance of either 120Ω for RS-485 cable or 100Ω for CAT-5 cable.

Modern transceivers must be able to operate efficiently at a low supply voltage. In the ISL33334E this is achieved through the use of an optimised charge pump which creates the bipolar power supplies for the RS-232 drivers, while only requiring four small 0.1μF capacitors. Two capacitors are used for the action of the charge pump, converting the initial supply voltage of 3.3V into +5V for V+ and -5.3V for V-. The other two capacitors are used for buffering V+ and V- to ensure sufficient supply current for the RS-232 driver during switching action.

While the total supply current drawn by the transceiver is already low at less than 4mA, even more power can be saved by driving the entire chip into shut-down mode. In shut-down mode, the operation of the charge pump is stopped, and the remaining supply current only consists of the leakage currents flowing into the logic inputs. Total leakage current depends on the device’s configuration but can be as low as 40μA.

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Fig. 5: The ISL33334E includes one RS-485 transceiver and two RS-232 transceivers

When re-enabling the device, the charge pump takes up to 25μs to become stable. RS-232 communication is not possible during this time. Since the charge pump does not supply the RS-485 transceiver, RS-485 communication can start just 2μs after re-start. This is much quicker than in legacy transceivers, which require the charge pump for all modes of operation.

Conclusion
Modern dual-protocol transceivers such as the ISL33334E simplify the design of industrial interfaces due to their high level of integration, combined support of RS-232 and RS-485 protocols, programmable data rates and power-saving features. To support system engineers in their industrial networking designs, Intersil provides a wide range of fixed and programmable, single- and dual-channel multi-protocol transceivers.

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