1 Introduction

RS-232 is prevalent in low-cost, low-speed, point-to-point industrial applications. RS-232 is typically used for factory equipment diagnostics or programming. This interface utilizes large signal amplitudes to increase signal-to-noise ratio and work in harsh industrial environments. RS-232 is a full duplex wired interface and a communication port design can be implemented with a single driver and single receiver, or the interface can have multiple drivers and receivers to account for handshake control signals to control the data flow between the sender and the receiver.

The RS-232 standard uses inverse logic in which a high-level digital signal from the microcontroller (MCU) is translated to negative analog signal (less than or equal to -5V). Conversely, a low-level digital signal is translated to positive analog signal (greater than or equal to +5V). Receiver thresholds are defined as ±3V so there is some margin between lowest RS-232 driver output and receiver thresholds. From data rate perspective, RS-232 standard asks for up to 20kbps signaling rate with a max limit on slew rate of transmitted signal to control emissions. Modern day devices can support up to 1Mbps data rate while keeping slew rate in check. Maximum data rate is subjected to maximum capacitive loading that arises from the cable capacitance between the sender and the receiver.

RS-485 has been the most used wired communications interface in industrial applications for more than two decades now. Balanced differential signaling of RS-485 allows for reduced emissions and rejection of common mode noise increases immunity. Communications over long distances in noisy industrial environments is feasible. RS-485 allows multipoint network where multiple communicating nodes can talk through same bus, reducing cable cost. RS-485 networks can be implemented in either half duplex (2 wire) or full duplex (4 wire) fashion. Both networks are terminated at farthest ends- termination matching to cable characteristic impedance reduces reflections and improves signal quality.

RS-422 is similar to RS-485, except that RS-422 allows multidrop network with only one driver and multiple receivers. RS-485 compliant transceivers are designed in a manner to be compliant to RS-422 as well.

Multiprotocol transceivers, as the name suggests, can support multiple wired protocols or interfaces. MP transceivers combine RS-232 and RS-422/RS-485 interface into one package. Thus, a communication port in an end application can be configured in any of the below fashions:

• Point-to-point RS-232 interface
• As a multidrop single driver-multiple receivers RS-422 interface
• As a multipoint multiple drivers-multiple receivers RS-485 network