Difference between RS232 & RS485

SPECIFICATIONS	RS232	RS423	RS422	RS485
Mode of Operation	SINGLE -ENDED	SINGLE -ENDED	DIFFERENTIAL	DIFFERENTIAL
Total Number of Drivers and Receivers on One Line (One driver active at a time for RS485 networks)	1 DRIVER 1 RECVR	1 DRIVER 10 RECVR	1 DRIVER 10 RECVR	32 DRIVER 32 RECVR
Maximum Cable Length	50 FT.	4000 FT.	4000 FT.	4000 FT.
Maximum Data Rate (40ft. - 4000ft. for RS422/RS485)	20kb/s	100kb/s	10Mb/s-100Kb/s	10Mb/s-100Kb/s
Maximum Driver Output Voltage	+/-25V	+/-6V	-0.25V to +6V	-7V to +12V
Driver Output Signal Level (Loaded Min.)	+/-5V to +/-15V	+/-3.6V	+/-2.0V	+/-1.5V
Driver Output Signal Level (Unloaded Max)	+/-25V	+/-6V	+/-6V	+/-6V
Driver Load Impedance (Ohms)	3k to 7k	>=450	100	54
Max. Driver Current in High Z State (Power ON)	N/A	N/A	N/A	+/-100uA
Max. Driver Current in High Z State (Power OFF)	+/-6mA @ +/-2v	+/-100uA	+/-100uA	+/-100uA
Slew Rate (Max.)	30V/uS	Adjustable	N/A	N/A
Receiver Input Voltage Range	+/-15V	+/-12V	-10V to +10V	-7V to +12V
Receiver Input Sensitivity	+/-3V	+/-200mV	+/-200mV	+/-200mV
Receiver Input Resistance (Ohms), (1 Standard Load for RS485)	3k to 7k	4k min.	4k min.	12k

Serial RS485

RS-485 standard supports half-duplex data communication, this means that in order to send and receive data using only two cable channels. The specifications of this standard can support data communications from a number of devices and is also capable of supporting data communication in a distance up to 1200 meters. Connection from each device connected to the RS-485 carried out in parallel, so that connection and release device can be done without disrupting the whole network work.

To adjust the impedance of the cable used to install a resistor whose value is adjusted with the characteristic impedance of the cable (~ 120 Ohm). Communication network topology using the RS-485 data can be viewed at the following picture.

Here datasheet for RS485

Serial RS232

Serial Ports come in two "sizes", There are the D25 pin type connector and the D9 pin type connector, both of which are male on the back of the PC, thus you will require a female connector on your device. Below is a table of pin connections for the 9 pin and 25 pin D-Type connectors.

DB9	DB25	Abbrevation	Function
1	8	CD	Carrier Detect
2	3	RD	Receive Data
3	2	TD	Transmit Data
4	20	DTR	Data Terminal Ready
5	7	SG	Signal Ground
6	6	DSR	Data Set Ready
7	4	RTS	Request To Send
8	5	CTS	Clear To Send
9	22	RI	Ring Indicator

Abbrevation	Function	Description
CD	Carrier Detect	When the modem detects a "Carrier" from the modem at the other end of the phone line, this Line becomes active.
RD	Receive Data	Serial Data Input (RXD)
TD	Transmit Data	Serial Data Output (TXD)
DTR	Data Terminal Ready	This is the opposite to DSR. This tells the Modem that the UART is ready to link.
DSR	Data Set Ready	This tells the UART that the modem is ready to establish a link.
RTS	Request To Send	This line informs the Modem that the UART is ready to exchange data.
CTS	Clear To Send	This line indicates that the Modem is ready to exchange data.
RI	Ring Indicator	Goes active when modem detects a ringing signal from the PSTN.

Null Modems
A Null Modem is used to connect two serial communication devices. This is commonly used as a cheap way to network games or to transfer files between computers (using Zmodem Protocol, Xmodem Protocol etc). This can also be used with many Microprocessor Development Systems. It only requires 3 wires (TD, RD & SG) to be wired straight through thus is more cost effective to use with long cable runs.

LoopBack
It has the receive and transmit lines connected together, so that anything transmitted out of the Serial Port is immediately received by the same port. If you connect this to a Serial Port an load a Terminal Program, anything you type will be immediately displayed on the screen.

Here datasheet for RS232

Serial Port Overview

The Serial Port is harder to interface than the Parallel Port. In most cases, any device you connect to the serial port will need the serial transmission converted back to parallel so that it can be used directly. This can be done using a UART. On the software side of things, there are many more registers that you have to attend to than on a Standard Parallel Port.

So what are the advantages of using serial data transfer rather than parallel ?

1. Serial Cables can be longer than Parallel cables. The serial port transmits a '1' as -3 to -25 volts and a '0' as +3 to +25 volts where as a parallel port transmits a '0' as 0v and a '1' as 5v. Therefore the serial port can have a maximum swing of 50V compared to the parallel
   port which has a maximum swing of 5 Volts. Therefore cable loss is not going to be as much of a problem for serial cables than they are for parallel.


2. You don't need as many wires than parallel transmission. If your device needs to be mounted a far distance away from the computer then 3 core cable (Null Modem Configuration) is going to be a lot cheaper than running 19 or 25 core cable for paralel device.


3. Microcontroller's have also proven to be quite popular recently. Many of these have in built SCI (Serial Communications Interfaces) which can be used to talk to the outside world. Serial Communication reduces the pin count of these MPU's. Only two pins are commonly used, Transmit Data (TXD) and Receive Data (RXD) compared with at least 8 pins if you use a 8 bit Parallel method (You may also require a Strobe).