Serial, RS232

Overview

Communication as defined in the RS232 standard is an asynchronous serial communication method. The word serial means, that the information is sent one bit at a time. Asynchronous tells us that the information is not sent in predefined time slots. Data transfer can start at any given time and it is the task of the receiver to detect when a message starts and ends.

Voltages

The signal level of the RS232 signal can have two states. A high bit, or mark state is identified by a negative voltage and a low bit or space state uses a positive value. This might be a bit confusing, because in normal circumstances, high logical values are defined by high voltages also. The voltage limitations are shown below.

RS232 voltage values

Level	Transmitter capable (V)	Receiver capable (V)
Space state (0)	+5 Vdc up to +15 Vdc	+3 Vdc up to +25 Vdc
Mark state (1)	-5 Vdc up to -15 Vdc	-3 Vdc up to -25 Vdc
Undefined	-	-3Vdc up to +3Vdc

Pinouts

View - looking into Male connector	View - looking into Female connector

Pin No.	Name	Dir	Notes/Description
1	DCD	IN	Data Carrier Detect. Raised by DCE when modem synchronized.
2	RD	IN	Receive Data (a.k.a RxD, Rx). Arriving data from DCE.
3	TD	OUT	Transmit Data (a.k.a TxD, Tx). Sending data from DTE.
4	DTR	OUT	Data Terminal Ready. Raised by DTE when powered on. In auto-answer mode raised only when RI arrives from DCE.
5	SGND	-	Ground
6	DSR	IN	Data Set Ready. Raised by DCE to indicate ready.
7	RTS	OUT	Request To Send. Raised by DTE when it wishes to send. Expects CTS from DCE.
8	CTS	IN	Clear To Send. Raised by DCE in response to RTS from DTE.
9	RI	IN	Ring Indicator. Set when incoming ring detected - used for auto-answer application. DTE raised DTR to answer.

Achievable Distances

The maximum distance that RS-232 can go varies depending on Baud Rate and also the capacitance of the wire itself. Texas Instruments states that RS-232 maximum cable length is 50 feet, or the cable length equal to a capacitance of 2500 pF. Depending on the cable such as Category 5 Twisted pair the maximum achievable distance at 19200 is 157 feet.

Here is a chart showing theoretical limits as various data rates for RS-232 cable length according to Texas Instruments.

Baud rate	Maximum cable length (ft)
19200	50
9600	500
4800	1000
2400	3000

Advantages

RS-232 is without doubt the best known interface, because this serial interface is implemented on almost all computers available today.

Disadvantages

One of the main problems with RS-232 is the lack of immunity for noise on the signal lines.

However RS-232 suffers from three main problems:

• 1) it requires multiple ground points and can create potential ground loops.
• 2) it picks up noise easily and
• 3) it allows only short communication distances.

In order to overcome some of these issues RS-422 or RS-485 is the preferred choice.

Interfacing to Microcontrollers

In order for RS232 to communicate with a Microcontroller such as a Pic16f88 or Atmel, level conversion must first be done because of the voltage levels on TTL is 5 Vdc max and normal RS232 far exceeds voltage limitations and damage will occur.

Devices such as a MAX232 will make the necessary voltage conversion possible for communication to and from TTL devices.

External Links

• Introduction into RS232 specifications
• Selecting and Using RS-232, RS-422, and RS-485 Serial Data Standards
• RS-232 Spec form Wikipedia
• Telecommunications Industry Association