This document describes how RS232 works at the physical level so you will
know what signals you can expect to see at the microcontroller pins.
It is a method (or
protocol - an agreed standard) that defines how to transfer data between two
devices using a few wires. It uses a serial
transmission method where bytes of data are output one bit at a time onto a
Data is only transmitted in one direction for each wire so for bi-directional communication (two directions) you need two wires.
These two along with a ground reference (total: three wires) make up the minimum configuration that you can get away with.
Note: For more reliable communication over long distances you may need to use other connections defined in the RS232 standard sush as DTR DCT etc. handshake signals etc.
More formally RS232 is an asynchronous communication protocol that lets you transfer data between electronic devices.
Basically it can transfer a single byte of data over a serial cable having between 3 to 22 signals and running at speeds from 100 to 20k baud. Common baud rates used are 2.4k, 9.6k, 19.2k, The cable length can be up to 50ft. Higher baud rates are used but not covered by the standard they still work though e.g. 38400,57600 Baud (bits/s).
To transfer a block of data individual bytes are transmitted one after another.
This section describes
how RS232 works in general without describing complex handshake methods - only
the simplest system is described - this it the most useful and the
most likely to work!.
Data is transmitted serially in one direction over a pair of wires. Data going out is labeled Tx (indicating transmission) while data coming in is labeled Rx (indicating reception). To create a two way communication system a minimum of three wires are needed Tx, Rx and GND (ground). Crossing over Tx & Rx between the two systems lets each unit talk to the opposite one.
Each byte can be transmitted at any time (as long as the previous byte has been transmitted). The transmitted byte is not synchronized to the receiver - it is an asynchronous protocol i.e. there is no clock signal. For this reason software at each end of the communication link must be set up exactly the same so that each serial decoder chip can decode the serial data stream.
Note: The signal level inversion (logic 1 is -12V and logic 0 is +12V).
This is simply
the transmission speed measured in bits per second. It defines the frequency of
each bit period.
For a baud rate of 2400 (2400 bps) the frequency is 2400Hz and the bit period is 1/2400 or 416.6us. This is the information that a receiver uses to recover the bits from the data stream.
To make it work
over long cables high voltages are sent from each transmitter since due to
cable resistance the voltage reduces the further the signal has to travel. The
output voltage specification isfrom +5V to +25V (transmitting a logical zero)
and -5V to -25V (transmitting a logical one).
Note: all signals in the cable have to generate the same voltage levels e.g. DTR, DSR, RTS, CTS. So you need a lot of level translator chips for a full interface but for very short distances you only need TX and RX and ground.
The receiver can accept minimum signal levels of ±3V.
The maximum voltage of ±25V does not have to be used and a common voltage in use is ±12V (output by MAX232 transceiver chip).
A mark (logical one) is sent as -12V and a space (logical zero) is sent as +12V i.e. the logic sense is inverted.
Note: The fact that high voltages exist at the serial port allows powering devices that you would not normally expect to find on it. But they must draw very little current.
At the receiver the input voltage levels are defined as ±3V i.e. to receive a logic zero the voltage must be greater than 3V and to receive a logic one the voltage must be smaller than -3V. This allows for losses as the signal travels down the cable and provides noise immunity i.e. any spurious noise up to a level of ±3V can be tolerated without it having any effect on the receiver.
The protocol is
described as asynchronous as there is no clock transmitted at all. Instead a
different method of clock recovery is used.
At the beginning of each transmission a start bit is transmitted indicating to the receiver that a byte of data is about to follow.
The start bit lets the receiver synchronize to the data bits. What this means is that the receiver can create its own sample clock at the middle of each bit. Note that once the start bit is found the receiver knows where the following bits will be as it is given the sample period (derived from the baud rate) as part of the initialization process.
Data bits follow
the start bit. There will be seven or eight data bits with the lsb transmitted
first. The reason you can choose between seven or eight is that ASCII is made
up of the alphabet within the first seven bits (as well as the control
characters). The eighth bit extends the character set for graphical symbols.
If you only want to transmit text then you only need 7 bits. This saves a bit and increases transmission speed when transmitting large blocks of data. Other data bit sizes are 5 and 6 bits. However bit length is usually ignored and a transmission size of 8 bits is commonly used.
Note: If you use RS232 to transmit raw data (binary data) then you will need 8 data bits.
The parity bit
is a crude error detection mechanism. You can use either odd parity or even
parity or none at all (in this case no parity bit is transmitted).
It simply evaluates all the data bits and for odd parity returns a logic one if there is an odd number of data bits that are set. For even parity an even number of data bits that are set, sets the parity bit.
At the receiver the parity bit is used to tell if an error occurred during transmission. You can use this in the receiver software by reading a flag in the UART module.
The problem with error detection using the parity bit is that if two bits are in error then the parity check fails. This is because each error cancels the effect of the other (in terms of the parity calculation). Any even number of errors causes a failure in error detection.
It won't be a problem on a bench top based system (that has no critical data transfer). Over a short cable e.g. 6ft you probably won't see any errors anyway. Normally I use no parity and there is no problem at all.
For systems running over a long distance or in a noisy environment a better system should be used e.g. Adding a cyclic redundancy check to the data stream before and after it is sent over the RS232. CRCs let you check for and correct quite a few errors without re transmitting the data.
The stop bit
merely gives a period of time before the next start bit can be transmitted. It
is the opposite sense to the start bit and because of this allows the start bit
to be seen.
If there was no stop bit then the last bit in the data stream would be the parity bit (or data bit if parity is not active). This would change depending on the data sent so if it had the same sense as the start bit then the start bit could not be seen!
The stop bit can be set choosing from 1, 1.5, or 2 bit periods.
for use on the desktop e.g. between a microcontroller and hyperterminal:
Hardware Connections 3 (Rx,Tx,GND) - Rx and Tx crossed over.
At some point
you may want to make a software UART perhaps to save code space in your current
design (maybe you don't need the receive part - just outputting variables) or
to use a spare pin or perhaps your provider's library does not work.
Note: you can find receive and transmit software USART code in the 12F675 Tutorial pages.
To create it you need the actual signal diagrams that you see at the microcontroller pin (strangely these are hard to find on the web).
The following diagram shows how RS232 works by generating 0-5V logic bit stream at the output pin of the microcontroller or UART followed by the translated voltages that are transmitted to the serial cable. These are generated by sending the 0-5V logic levels to a transceiver chip e.g. MAX232. which can use a 5V power supply and boost it to the required 12 volts.
End of page : How RS232 works.
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