RS232 and RS485 Protocol

Updated on 2017/10/03 18:37

RS232 and RS485 Protocol

RS-232

What is RS-232 What is RS-232

  • RS-232 is a popular communications interface for connecting modems and data acquisition devices (i.e. GPS receivers, electronic balances, data loggers, ...) to computers.
  • RS-232 can be plugged straight into the computer’s serial port (know as COM or Comm port).

RS-232 Signals

Architecturally RS-232 is a bi-directional point to point link.

DB-9 Connector

RS-232.png

PinSignal
1Data carrier detect
2Received Data
3Transmitted data
4Data Terminal Ready
5Signal Ground
6Data Set Ready
7Request to Send
8Clear to send
9Ring Indicator

(serial port - PC side)

  • Two independent channels are established for two-way (fullduplex) communications.
  • RS-232 can also carry additional signals used for flow control (RTS, CTS) and modem control (DCD, DTR, DSR, RI).

RS-232 Line Driver

  • Unbalanced Line Drivers
  • Each signal appears on the interface connector as a voltage with reference to a signal ground.
  • The “idle” state (MARK) has the signal level negative with respect to common whereas the active state (SPACE) has the signal level positive respest to the same reference.

RS-232-Interface-circuit.png

RS-232 Speed

 How fast can RS-232 be?

  • The maximum speed, according to the standard, is 20kbit/s. However, modern equipment can operate much faster than this. (i.e. Lynx can reach 115200 baud.)
  • The length of the cable also plays a part in maximum speed. The longer the cable and the slower the speed at which you can obtain accurate results. A large wire capacitance and inductance limits the maximum length of the cable and/or the maximum speed; Moreover higher is the capacitance of the cable higer is the interference between two   adjacent signal wire. 50 feet (15m) @ max baudrate is commonly quoted as the maximum distance. It is not specified in EIA standard but it’s recommended respect these values.

RS-485

What is RS-485?

  • RS-485 is a EIA standard interface which is very common in the data acquisition world
  • RS-485 provides balanced transmission line which also can be shared in Multidrop mode.
  • It allows high data rates communications over long distances in real world environments.

RS-485 speed

How fast can RS-485 be?

  • RS-485 was designed for greater distance and higher baudrates than RS-232.
  • According to the standard, 100kbit/s is the maximum speed and distance up to 4000 feet (1200 meters) can be achieved.

RS-485 Line Driver

  • Balanced Line Drivers
  • Voltage produced by the driver appears across a pair of signal wires that transmit only one signal. Both wires are driven opposite.
  • RS-485 driver has always the “Enable” direction control signal.
  • Differential system provides noise immunity, because much of the common mode signal can be rejected by the receiver. So ground shifts and induced noise signals can be nullified.

RS-485 Network

  • RS-485 provides Half-Duplex, Multidrop communications over a single twisted pair cable.
  • The standard specifies up to 32 drivers and 32 receivers can share a multidrop network
  • Terminator resistors avoid reflected signal

RS-485 Half-duplex

  • Datalogic uses Half-Duplex configurations for Data Collecting and Master/Slave layouts.
  • Usually we talk about Multidrop network (i.e. MX4000,DPS9000)
  • Datalogic uses Half-Duplex configurations for Data Collecting and Master/Slave layouts.
  • Usually we talk about Multidrop network (i.e. MX4000,DPS9000)

RS-485 Full-duplex

Potentially RS-485 interface can also use 4-wires to comunicate in multidrop mode but...
 

Comparison between RS-232 and RS-485

 RS-232RS-485
Mode of OperationSINGLE-ENDEDDIFFERENTIAL 
 Total Number of Drivers and1 DRIVER
Receivers on One Line 1 RECEIVER
32 DRIVER
32 RECEIVER 
Maximum Cable Length50 FEET4000 FEET 
Maximum Data Rate @Max length20kb/s100kb/s 
 Driver Output Signal Level (Loaded Min.) Loaded +/-5V to +/-15V+/-1.5V 
Driver Output Signal Level (Unloaded Max) Unloaded +/-25V+/-6V 
Driver Load Impedance3kΩ to 7kΩ54Ω 
Max. Driver Current in High Z State   Power Off+/-6mA @ +/-2v+/-100uA 
Slew Rate (Max.)30V/µSN/A 
 Receiver Input Voltage Range+/-15V-7V to +12V 
 Receiver Input Sensitivity+/-3V+/-200mV 
 Receiver Input Resistance3kΩ to 7kΩ≥ 12kΩ

References

  • Created and Edited by Prof. Sujit wagh, SKNCOE,Pune
  • WikiNote Foundation
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Created by Sujit Wagh on 2017/09/29 11:26
Translated into en by Sujit Wagh on 2017/09/29 11:33