Line Drivers for the Field Bus

Classic serial data transmission methods are still widely used in order to link actuators and sensors on field bus level - which comes as no surprise, as they are proven and robust and provide fairly trouble-free handling. RS232_Timing_0.jpgProtocols such as RS422 and RS485 are the high runners, as they permit very reliable linking of numerous users across great distances, thanks to differential transmission and their multipoint capability. 

On the physical level, line drivers are thus critical components, as they determine immunity, bit error rates and operating safety in "noisy" EMI-environments.

For this application, the semiconductor manufacturer LSI/CSI has developed differential line drivers based on its HV-CMOS technology which position themselves as a better alternative to conventional x7272 components, offering high signal levels and strong driver currents. 

Superior signal-to-noise ratio with 30 V differential

LS7272_BlockDiagram_01_0.pngBoth quadruple differential line drivers, the LS7272 and LS7273, are short-circuit proof with an operating voltage from 4.5 to 30 volts (36 V peak), for both input & output levels and supply voltage. Transmission bandwidth reaches up to 4 MHz while supporting environment temperatures of between -40 to +120 °C, the component can therefore also be deployed in extremely hot conditions.

The difference between the LS7272 and LS7273 is their output configuration. The outputs of the LS7272 can be switched from "push-pull" to "open drain" with a configuration pin, while the LS7273 features only "open drain" outputs.

Both ICs are equipped with thermal protection against overloading/ short circuiting on the output side. However, this feature can be disabled with a configuration pin if required. Other features common to both line drivers:

  • Pin-compatible with OL7272 and function equivalent to ET7273
  • Symmetric 120 mA driver current for sink & source 
  • Dynamic peak output current of 1.5 A 
  • RS422A compatible
  • Enable input
  • CMOS/TTL compatible inputs with switching hysteresis

Both drivers are available in 16-Pin DIP, SOIC or TSSOP packages.

Specifications LS7272 und LS7273

Practical advice - CAT5 is ideal for field busses

CAT5-Connector_01_0.pngThe cabling of RS422/RS485 components is usually realized with DSUB-15 connectors, mainly for historic reasons, as the ANSI standard does not set any particular guidelines in this regard. Other connectors can be used as well - CAT5 network cables are particularly suited, especially the shielded SFP variant (Shielded Twisted Pair). These cables are readily available at low cost and in many designs and lengths, and provide excellent attenuation and shielding as well as extremely low crosstalk values. 

Cables and connectors must be fully assigned (i.e. 4 wire pairs) to enable simultaneous transmission of data and handshake-signals.