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 ... +15	+3 ... +25
Mark state (1)	-5 ... -15	-3 ... -25
Undefined	-	-3 ... +3

Achievable distances

The maximum distance that RS323 can go varies depending on Bit 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 quick chart showing theoretical limits as various data rates for RS232 cable length according to Texas Instruments

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

External Links

• Introduction into RS232 specifications
• RS-232 Spec form Wikipedia