RS-232, RS-422 and RS-485 Interfaces Information
RS232 interface chips, RS422 interface chips, and RS485 interface chips are interconnection devices for the transmission of serial data. They are used in notebook and portable computers, battery-powered and handheld devices, and communications equipment. RS232 is a serial data standard that supports point-to-point connections and provides asynchronous communication capabilities. Hardware flow control, software flow control, and parity checking are other important features. RS422 is a serial interface that supports longer transmission distances than RS232, but provides less signal line. RS485 is compatible with the RS422 data standard and provides a 2-wire bus topology. Typically, RS485 serial interfaces are used in applications where a single controller needs to control multiple devices.
Specifications and Features
RS232 interface chips, RS422 interface chips, and RS485 interface chips differ in terms of product specifications and features. The number of drivers, number of receivers, data rate, slew rate, operating temperature, and supply voltage are important specifications to consider. Electrostatic discharge (ESD) protection is critical for equipment reliability. Interface chips with integral slew rate circuitry minimize radio frequency interference (RFI) and eliminate the need for external capacitors. Features for RS232 interface chips, RS422 interface chips and RS485 interface chips include low-power operation and latch-up protection. Charge pumps are often used in low-voltage devices. Short circuit protection enables RS232, RS422 and RS485 interface chips to withstand an accidental short circuit from a conductor in the interconnecting cable to one of the package outputs.
Logic Family and Integrated Circuit Type
RS232 interface chips, RS422 interface chips, and RS485 interface chips differ in terms of logic family and integrated circuit (IC) package type. Examples include complementary metal-oxide semiconductor (CMOS), transistor-transistor logic (TTL) and low voltage differential signaling (LVDS). CMOS uses a combination of p-type and n-type metal-oxide-semiconductor field effect transistors (MOSFET). TTL also uses transistors, but as digital switches. LVDS products provide low skew, differential signaling, and relatively high speeds. IC package types for RS232 interface chips, RS422 interface chips, and RS485 interface chips include small outline integrated circuit (SOIC) and quad flat package (QFP).
RS232 interface chips, RS422 interface chips and RS485 interface chips conform to serial data standards from the Electronics Industries Association (EIA) and the Telecommunications Industry Association (TIA). There are different standards for RS232 interface chips (EIA/TIA-232), RS422 interface chips (EIA/TIA-422) and RS485 interface chips (EIA/TIA-485.) The EIA also publishes Data Terminal Equipment (DTE) to Data Communications Equipment (DCE) standards that describe the electrical, mechanical and functional interface between terminals such as cathode ray tubes (CRT) and communications equipment such as modems.