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RS-485 Transceiver IC Offers Enhanced Speed, Protection, Connectivity

May 14, 2024
An advanced IC from STMicroelectronics gives the “very old dog” of RS-485 interfacing some very new tricks.

Yes, you read that right: The decades-old RS-485 interface standard—the enhanced, differential, multidrop extension to the legendary, classic RS-232 interface—is alive and well.

If you assume RS-485 is so “yesterday” and all new industrial designs are going with standards such as Ethernet, including the relatively new single-pair Ethernet (SPE) standard (10BASE-T1S and 10BASE-T1L) under IEEE Std 802.3cg-2019… well, that’s not necessarily the case. (By the way, if you also think RS-232 itself is “dead,” think again: It’s included on some recently introduced instrumentation I’ve seen for leading-edge electro-optic test applications.)

The reality is that RS-485, formally called TIA/EIA-485, is still widely used both in legacy applications. Meanwhile, its simplicity, ease of troubleshooting (don’t overlook that important attribute), flexibility, and low cost make it a good fit for new designs.

Transceiver Takes RS-485 to a New Performance Level

Recognizing this situation, STMicroelectronics introduced the ST4E1240 RS-485 transceiver offering 40-Mb/s speed, PROFIBUS-compatible output, and transient and hot-swap protection, yielding functionality and performance that goes far beyond that original RS-485 specification (Fig. 1). These transceivers offer robust and reliable RS-485 signaling for modern, performance-hungry industrial applications.

Not only do these transceivers support data speeds far above the original RS-485 standards, but they also enable multipoint connections over extended cable runs with more than 64 transceivers on the bus (Fig. 2). Typical uses include programmable logic controllers (PLCs), robotics, telecom infrastructures, smart-building controls, servo drives, optical networking devices, grid infrastructure, data-acquisition systems (including smart meters), and backplane buses.

The ST4E1240 fulfills all TIA/EIA-485 standards and ensures the output differential voltage exceeds 2.1 V (with a 5-V supply) for compatibility with the popular PROFIBUS fieldbus standard. In addition, a shutdown mode is controlled via external enable pins to reduce the quiescent current below 3.5 µA for use in energy-conscious applications.

Lower-Cost Built-In Protection

The built-in protection meets IEC 61000-4-2 up to ±12-kV contact discharge, allowing the devices to be used without external ESD protection. There’s also class-A fast-transient protection up to ±4 kV that meets IEC 61000-4-4, ensuring data integrity and bus stability. Together, these attributes yield additional bill-of-materials (BOM) and real-estate savings.

Hot-swap circuitry prevents unwanted states on the bus during device switch-on or plug-in. It’s capable of discharging up to 100 pF of parasitic capacitance. Furthermore, the transceiver integrates fail-safe protection that drives the output high to avoid an indeterminate state upon detecting an open, shorted, or idle bus. Other features include 250-mA short-circuit protection and thermal shutdown.

Even basic, nominally simple devices and their users benefit from a comprehensive datasheet. The ST4E1240 is supported by a 30-page datasheet with specifics from various perspectives, including tables and charts characterizing static and dynamic performance, test circuits and conditions, packaging, and more. The devices are in production now and available in a 4.9- × 3.91-mm SO-8 package, from $0.81 each (1,000 pieces).

References

IEEE Standard 802.3cg-2019 for Ethernet, “Amendment 5: Physical Layer Specifications and Management Parameters for 10 Mb/s Operation and Associated Power Delivery over a Single Balanced Pair of Conductors.”

Tritek, “Do You Need RS485 Communication in Lithium Batteries?

CUI Devices, “RS-485 Serial Interface Explained.”

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