Electronic Design

Common-Mode Transceivers Extend RS-485 Network Range

RS-485 networks have been around for over 20 years, serving automated factories, chemical/petroleum plants, building HVAC installations, test and measurement systems, and medical and security applications. This ANSI TIA-EIA physical-layer standard defines a differential twisted-pair bus with up to 32 half-duplex multipoint drops. The maximum range is 1.2 km, with a maximum signaling rate of 50 Mbits/s. Obviously, maximum range and data rate can’t be achieved simultaneously, since data rate is strictly a function of bus length. Receiver sensitivity is ±200 mV.

Texas Instruments’ SN65HVD2x extended common-mode transceivers let designers implement a Super RS-485 network with extended range, higher speed, an increased number of nodes, improved noise immunity, and fault protection.

The HVD20 offers 25 Mbits/s up to 50 meters, the HVD21 provides 5 Mbits/s up to 150 meters, and the HVD22 supplies 0.5 Mbits/s up to 1200 meters. The HVD20 can achieve these conditions with up to 64 connected nodes. The HDV21 and HDV22 can operate with up to 256 nodes thanks to reduced unit load represented by the devices.

Two additional models, the HDV23 and HDV24, offer receiver input equalization for improved jitter performance and extended data rate and range. These devices can produce data rates of 25 Mbits/s up to 160 meters and 3 Mbits/s up to 500 meters, respectively.

All five versions boast an extended range of common-mode voltage from -20 to +25 V (which is more than the original standard specification), I/O bus electrostatic-discharge protection to 16 kV, 100 mV of receiver hysteresis, and a low standby supply current of 1 µA maximum. These transceivers also incorporate a receiver fail-safe detection and correction circuit that identifies open, short, or bus idle conditions so no ambiguous states are produced regardless of the line status.

A special low-cost version of the HDV20-22 devices is available. Optimized for speeds up to 200 kbits/s, the SN65HVD3082E suits low-speed industrial applications like energy meter networks, power inverters, and chassis-to-chassis interconnects.

These transceivers are made with biCMOS and operate from +5 V. The HDV20, 21, and 22 cost $1.93 in 1000-unit quantities. The HDV23 and 24 will be available beginning in May.

Texas Instruments Inc.

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