The trend in industrial PC designs towards smaller form factors and more communication versatility is driving the development of modern bus transceivers. New transceivers are favored over legacy designs because of their high level of integration, dual-protocol capability supporting the RS-232 and RS-485 standards, and ample configuration features.
Developed in the early 1980s, the RS-485 protocol is now greatly improved, providing for robust data transmission in noisy environments and across long distances. The protocol uses differential signaling 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 loaded with a 54Ω differential load.
RS-485 supports networking of up to 32 unit-loads via a multipoint bus topology. Bus nodes are daisy-chained to one another via twisted-pair cable, as shown in Figure 1. The recommended characteristic cable impedance of 120Ω requires termination resistors at both cable ends, the values of which should match the cable impedance.
Fig. 1: Typical RS-485 network with daisy-chained bus nodes and terminated cable ends
As the receiver inputs are internally referenced to ground, a separate ground connection between drivers and receivers is not required. This is true, as long as 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. The maximum applicable cable length for a given data rate is shown in Figure 2.
Fig. 2: RS-485 cable length versus data-rate characteristic
RS-485 supports multipoint topologies in which each bus node can either transmit or receive data. Two types of multipoint buses exist: half-duplex and full-duplex, as shown in Figure 3. A half-duplex bus uses two wires to connect nodes: one node may transmit data while another node receives data.
Fig. 3: Half-duplex and full-duplex multipoint bus topologies in RS-485
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. 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 wiring effort.
Modern transceivers are capable of supporting the designs of new industrial PCs and the designs of RS-232 to RS-485/RS-422 interface converters. The latter is 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.
Figure 4 shows the block diagram of a dual-protocol transceiver. The device incorporates two RS-232 Transmit and Receive channels, and one full-duplex RS-485 transceiver. Notice the transceiver’s 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 provides a great advantage over legacy transceivers, as shown in Figure 1, 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 specified maximum slew rate.
Fig. 4: Dual-protocol transceiver incorporating one RS-485 and two RS-232 transceivers
The integration of one RS-485 and two RS-232 transceivers into one IC makes the interface design of an industrial PC versatile, as the local controller can either drive the various bus systems independently, or act as an interface converter, as shown in Figure 5. When operated as an RS-232-to-RS-485 converter, the RS-232 signals of either channel 1 or channel 2 or both can be converted to logic levels, and then transmitted via the RS-485 bus. Using address coding, the controllers on both sides of the RS-485 link can distinguish between two RS-232 data streams.
Fig. 5: Networking multiple pieces of RS-232 equipment via RS-232-to-RS-485 converters
To extend the data link between two RS-232 interfaces via a point-to-point link over long distance, the dual-protocol transceiver is configured as a standalone RS-232-to-RS-485 converter. Two converters are needed, one at each cable end to convert RS-485 bus signals into RS-232 data and vice versa. The configuration is simple, as the Enable inputs for driver and receiver can be wired to their respective voltage rails to keep the transceiver constantly active, as shown in Figure 6a.
Fig. 6a/b: Networking multiple pieces of RS-232 equipment via RS-232-to-RS-485 converters
Networking multiple pieces of RS-232 equipment over a full-duplex RS-485 bus requires a minor configuration change for the converters in slave nodes. The driver and receiver in the master node (PC) can remain active all the time, and so can the receivers in the slave nodes. The drivers in the slave nodes, however, must be closely controlled to prevent two or more slaves from accessing the bus at the same time.
For this purpose, the driver of the second RS-232 channel is used to enable and disable the RS-485 driver with the RTS flow control signal, as shown in Figure 6b. Note that, within the converter, the RTS must be looped back to the CTS input of the controller. This is known as a null-modem configuration.
Networking multiple pieces of RS-232 equipment over a half-duplex RS-485 bus requires the configuration shown in Figure 7a. Here the RTS signal controls the Enable functions of both driver and receiver. This configuration is required in all nodes, master and slaves, because a half-duplex bus can only pass data in one direction at a time.
Fig. 7a/b: Networking multiple pieces of RS-232 equipment via RS-232-to-RS-485 converters over a half-duplex RS-485 bus
In some equipment the RTS and CTS control signals can be up to 10ms out of synchronization with the data to be transmitted. In this case it is best to make the Enable signals data-driven. This is accomplished by implementing an inverter function between the Driver Input (DI) and the Enable pins (DE485 and RE485).
This puts the transceiver in transmit mode when DI = low, and in receive mode when DI = high. In receive mode the driver outputs are high-impedance, and the low-impedance termination resistors reduce the bus voltage to 0V. Since the RS-485 receiver is a full fail-safe device, all dual-protocol transceivers on the bus will indicate a zero bus voltage as a logic high at the receiver output, RO. Thus toggling a driver output between active low and high impedance will still generate a low-to-high transition at another receiver’s output.
Modern dual-protocol transceivers 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 configuration features. A wide range of fixed and programmable, single and dual-channel, multi-protocol transceivers from Renesas may be used in the applications described in this Design Note.