Electronic Design

Run RS-485 Connections At 35 Mbaud

Older serial and parallel buses aren't going away. Instead, they are speeding up. It's now possible to get a 40-Mbaud RS-485 serial connection—that's 5 Mbytes/s. With those serial bus speeds, designers can upgrade their PROFIBUS and RS-485 systems or simply take advantage of a 35-Mbaud/s multidrop connection.

One such device, the IL485 from NVE Corp., implements a high-speed, differential-pair RS-485 bus transceiver. This bidirectional transceiver isolates the input lines from the functional interface.

The RS-485 standard defines a half-duplex connection. Earlier implementations used twisted-pair wiring for the RS-485 connection, with two- or four-wire implementations. The IL485 uses a differential pair, coupled with galvanic isolation, between the bus inputs and the chip functional outputs. The chip supplies one minute of 2500-V rms isolation. The differential pair supports 1-ns skew with a 25-ns propagation delay and ±60-mA output-drive capability.

The key to the high I/O isolation is NVE's patented galvanic-isolation technology. Instead of relying on external resistive, capacitive, or optical isolation components, galvanic isolation implements on-chip isolation in silicon. The technique integrates giant magnetoresistive (GMR) materials, microcoils, and silicon circuits to provide signal isolation.

Basically, the input current drives resistive planar coils. The coils generate a magnetic field, proportional to current, that's detected on the other side of an isolating, thick polymer dielectric film. The sensed magnetic field is amplified and used to replicate an isolated copy of the input signal. An on-silicon magnetic shield over the windings helps increase the circuit's magnetic effect while shielding the sense elements from external magnetic fields. With this transceiver isolation, RS-485 supports multiple connections to subsystems whose ground and voltage levels may be floating relative to the transceiver system's levels.

Housed in a small 16-pin SoC package, it costs $5.20 each in 1000-unit lots.

NVE Corp., 11409 Valley View Rd., Eden Prairie, MN 55344; (952) 829-9217; www.isoloop.com.

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