CN0416

RS-485 Bus Standard

The RS-485 bus standard is one of the most commonly used physical layers in industrial applications needing local networks capable of multidrop communication links and long haul data transfer over distances up to 1200 m. A data rate of 10 Mpbs is achievable for cable lengths up to 12 m, dropping to 100 kbps at 1200 m, as shown in Figure 2.

Table 1 shows the some of the specifications of the RS-485 bus standard, as well as the improved specifications that the ADM2682E and LTC2865 provide.

For the full details of each transceiver device, refer to their data sheets. The extended specifications of the ADM2682E and LTC2865 improve the robustness of systems that are nominally compliant but may occasionally experience events that break the standard. For example, two equipment cabinets in a factory may both be tied to earth ground, with a few volts ac between grounds. However, an ESD strike to one cabinet may momentarily drive its common-mode voltage to 20 V. An LTC2865 rides through such an event, while a transceiver that just meets the standard may produce corrupt data or fail entirely.

The ADM2682E greatly extends common-mode tolerance, allowing for 25 kV/μs transient immunity, with up to 5000 V potential difference between devices (1 minute, per UL1577). Refer to the ADM2682E data sheet for details on safety and regulatory approvals.

The inclusion of the isolated and nonisolated transceivers on EVAL-CN0416-ARDZ aids in determining the actual requirements and implementation of the end application.

The circuit has four on-board Ethernet jacks, one pair for isolated and one pair for nonisolated communications. Whereas Ethernet cable is not intended for RS-485 applications, and the impedance is 100 Ω vs 120 Ω for RS-485, the ubiquity of common CAT5 and CAT6 cables provides a convenient way to build prototype systems during development. Also, each pair of Ethernet ports have crossover pin configurations, allowing the use of straight-through Ethernet cables for multinode systems. Connection points to the RS-485 signals are provided, facilitating the use of other connectors, or direct wiring to theRS-485 cable of the application.

RS-485 Bus Node Capacity

Both the ADM2682E and LTC2865 have 1/8 unit loading of 96 kΩ minimum, allowing up to 256 nodes in a single bus system. (The RS-485 standard specifies a receiver input impedance of 12 kΩ and a maximum of 32 nodes.)

Termination

A common guideline for termination is to terminate lines transmitting signals with rise/fall times less than four times the propagation delay of the cable. A standard CAT5 cable has a propagation delay of 4.8 ns/m to 5.3 ns/m. The ADM2682E and LTC2865 have maximum rise/fall times of 15 ns. Thus, for lines greater than 0.78 m in length, it is recommended to use termination. For best practice, properly terminate all RS-485 systems.

The circuit shown in Figure 3 can configure the RS-485 line termination to either a differential resistance or left open via S7 for isolated communications and via S6 for nonisolated communications. The full-duplex RS-485 standard requires a termination at the master node and the farthest slave node.

For a half-duplex connection, terminate both ends of the transmission cable. The value of the termination resistance should be equal to the characteristic impedance of the cable. For CAT5 cables, this is 100 Ω. By default, the circuit uses a 100 Ω termination resistance but can be configured through solder links to use the standard RS-485 termination resistance, which is 120 Ω. Table 2 shows the advantages and disadvantages of open and terminated RS-485 communications.

Half Duplex Operation

The circuit shown in Figure 4 can be set to operate in halfduplex mode for both isolated and nonisolated communications via Switch S5 and Switch S4, respectively. The driver noninverting output is shorted with the noninverting input of the receiver, whereas the inverting output of the driver is shorted with the inverting input of the receiver. This configuration allows for a 2-wire RS-485 network, but the bus can only accommodate data transmission in one direction at a time. The driver and receiver enable pins are required to assure that only one driver is enabled to send data at a time. Figure 5 shows the connection setup for a three-slave node half-duplex isolated RS-485 communication. Slave nodes are connected through similar Ethernet jacks. For example, Slave A P5 is connected to Slave B P5 using a straight-through CAT5 cable. The master node is connected to a slave node via an opposite Ethernet jack. For example, Slave A P5 is connected to P4 of the master node using a straight-through CAT5 cable.

Full-Duplex Operation

The circuit shown in Figure 5 can be set to full-duplex mode for both isolated and non-isolated communications via S5 and S4, respectively. In full-duplex mode, the circuit uses all receiver and driver inputs and outputs, respectively. This allows for a 4-wire RS-485 network and can accommodate simultaneous data transmission in both directions between master and slave nodes. Figure 6 shows the connection setup for a three-slave node full-duplex non-isolated RS-485 communications. Slave nodes are connected through similar Ethernet jacks when using straight-through Ethernet cables. For example, Slave A P6 is connected to Slave B P24 using a straight-through CAT5 cable. The master node is connected to a slave node via an opposite Ethernet jack. For example, Slave A P6 is connected to P24 of the master node using a straight-through CAT5 cable.

True Fail-Safe Receiver Inputs

In multinode systems, a bus idle condition occurs when all nodes are in receive mode. In the bus idle condition, the differential input voltage is 0 V. Previous generation RS-485 receivers required external fail-safe termination and biasing to prevent the bus idle condition from producing erroneous data. Both ADM2682E and LTC2865 tolerate bus idle conditions without external circuitry.

The ADM2682E has true fail-safe receiver inputs with a differential input threshold voltage to −200 mV to −30 mV. Thus, the 0 V differential input voltage is received as a defined logic level and prevents random erroneous data from entering or impacting the system.

When a device requires an adjustment to the input threshold voltage, the adjustment causes duty cycle asymmetry at the receiver output, which worsens at low input signal levels and slow input edge rates. The LTC2865 has a full fail-safe operation without adjusting the input threshold voltage. Instead of adjusting the input threshold voltage, the LTC2865 uses an internal window comparator to determine if the input voltage falls between the positive and negative thresholds. If this failsafe condition persists for more than 3 μs, the fail-safe condition asserts and the signal is forced to a high state.

Signal and Power Isolation

The CN-0416 circuit can be configured via Switch S2 to use either isolated communications through the ADM2682E or non-isolated communications through the LTC2865. 

When using a straight-through Ethernet cable to connect an isolated Ethernet jack to a non-isolated Ethernet jack between two slave nodes, connect P4 to P24 and connect P5 to P6. In contrast, when connecting between a master and a slave node in the same situation, connect P4 to P6 and connect P5 to P24. When the same electrical system is used to power the supplies of all nodes, connecting them all to the same earth ground may reduce the noise produced by ground currents flowing between nodes at different ground potentials. For the single earth ground configuration, the non-isolated RS-485 communication is most applicable. 

When different nodes are powered by different electrical systems, especially when they are situated in different buildings, may increase the impedance of the earth ground, which in turn, increases the likelihood of ground currents between nodes. For potential ground current situations, isolated RS-485 communication is required. The ADM2682E has 5 kV rms signal isolation using an Analog Devices, Inc., iCoupler^® data channel. The ADM2682E also has 5 kV rms power isolation implemented using an Analog Devices isoPower^®, integrated, isolated dc-to-dc converter.