Line driver / transceiver (RS-485 serial bus transceiver)

01

Size data rate, cable length and node count

Texas Instruments' application report SLLA272 (The RS-485 Design Guide) lists the standard's key features: a balanced interface, multipoint operation from a single 5 V supply, a −7 V to +12 V bus common-mode range, up to 32 unit loads, a maximum of 10 Mbps at 40 ft and a maximum cable length of 4,000 ft at 100 kbps. It recommends daisy-chained (bus) topology over sheathed unshielded twisted-pair cable of 120 Ω characteristic impedance and 22–24 AWG. [1]

A unit load represents about 12 kΩ; standard drivers must drive 32 unit loads, and transceivers with reduced loading such as 1/8 unit load allow up to 256 transceivers. Because failsafe biasing can add up to 20 unit loads, the guide's equation gives a maximum of 96 devices with 1/8-unit-load transceivers. [1]

Editorial calculation: 4,000 ft is about 1,219 m and 40 ft about 12 m; with failsafe biasing taking 20 of the 32 unit loads, the remaining 12 unit loads divided by 1/8 per transceiver gives the guide's 96 devices. [1]

Give the supplier the bus standard (RS-485, RS-422, CAN), half or full duplex, data rate, cable length and type, number of nodes, supply voltage, isolation voltage needed, ESD and surge requirements, failsafe needs, operating temperature, package (IC or module) and certifications.

02

Terminate the bus, ensure failsafe and isolate where needed

The guide says lines should be terminated and stubs kept short: the trunk is terminated with 120 Ω resistors at each end, or in noisy settings with two 60 Ω resistors and a capacitor as a low-pass filter, matched preferably to 1%. Failsafe is the receiver's ability to assume a defined output when there is no signal (open, shorted or idle bus). For remote links it advises assuming large ground potential differences, which add to the signal as common-mode noise, and, for differences TI puts in the kilovolt range, galvanically isolating the transceiver's signal and supply lines. [1]

Mornsun lists RS-485 transceiver modules that integrate the transceiver, an isolation chip and an isolated DC/DC converter, with data rates from 9.6 kbps to 20 Mbps, supplies of 3.3–5 V, 32 to 256 nodes and isolation of 2,500–5,000 V DC (or 3,750 V AC / 5,000 Vrms by model); for example the TD041S485H at 20 Mbps, 3.3/5 V, 256 nodes and 3,750 V DC in a DFN package. [2]

Editorial note: check that the module's node count and the bus's failsafe biasing together stay within the 32-unit-load budget, and ask for isolation test voltage and duration.

03

Read a 3.3 V transceiver IC data sheet against the bus limits

MaxLinear's SP3485 data sheet (Rev. 2.0.2, 2021) describes a 3.3 V low-power half-duplex RS-485/RS-422 transceiver, interoperable with 5 V logic and pin-compatible with the 75176 pinout, rated to 10 Mbps under load with a −7 V to +12 V common-mode input range and up to 32 transceivers on the bus. With a 27 Ω RS-485 load it gives a minimum differential output of 1.5 V, a receiver threshold of −0.2 to +0.2 V with 20 mV hysteresis, and a receiver input resistance of 12 kΩ minimum. [3]

These values match the standard's limits TI lists: the same −7 to +12 V common-mode range, 32 unit loads and 10 Mbps maximum rate. [3] [1] Its ESD rating is ±2 kV (human body model); ask for the bus-pin ESD and surge ratings of any IC or isolated module you are offered, since Mornsun lists its modules by isolation voltage. [3] [2]

Editorial calculation: 32 receivers at 12 kΩ each in parallel load the bus with about 375 Ω; 1.5 V across 27 Ω is about 56 mA of differential drive.

Renesas' ISL317xE family shows the features a higher-specification 3.3 V transceiver adds. Its product page lists ±15 kV IEC 61000 ESD protection on the RS-485 pins and more than 7 kV HBM on the other pins; bus currents of +125 µA/−100 µA, which it describes as a true 1/8 unit load allowing up to 256 devices without exceeding the standard's 32-unit-load limit or using repeaters; full fail-safe receivers that give a logic-high output when the inputs are open, shorted or terminated but undriven; hot-plug outputs that stay high-impedance during power-up; data rates up to 20 Mbps, with slew-rate-limited versions for lower EMI; 800 µA maximum quiescent current and 10 nA shutdown current; the same −7 V to +12 V common-mode range; and half- and full-duplex pinouts. [4]

Editorial calculation: a 1/8 unit load is equivalent to about 12 kΩ × 8 = 96 kΩ of input resistance, so 256 such receivers in parallel present about 375 Ω, the same bus loading as 32 standard 12 kΩ receivers. Against the SP3485's ±2 kV HBM rating, the ISL317xE's ±15 kV IEC 61000 rating on the bus pins is a different test method as well as a higher figure, so ask suppliers to state ESD ratings with the test standard named. [4] [3] [1]

Ask for the unit-load rating, ESD rating on the bus pins, failsafe behaviour and supply voltage of the exact transceiver part fitted.

04

Compare what suppliers offer

Suppliers linked to this page list RS-485 transceivers or modules; each card links to the page reviewed.

The suppliers reviewed list transceiver ICs or isolated modules with digital isolators and power modules; ask each for unit load, data rate, common-mode range, failsafe, ESD and isolation ratings, and package.

05

Trace the transceiver functions

This is a functional reference built from the sources cited; it is not one maker's design.

06

Fill the specification fields with values for your bus

A transceiver quotation needs values for your bus. The table explains each field, gives published values where a reviewed source states them, and says what to obtain from the supplier.

07

Send the bus, length and isolation data together

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Bus: [RS-485; RS-422; CAN; duplex]
Rate: [bps]
Cable: [length; type]
Nodes: [number; unit load]
Isolation: [voltage; test]
Protection: [ESD; surge]
Package: [IC; module]
Environment: [temperature]

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