Datasheet

Thursday, 17 July 2003 MiniProject: Design Aspects Colin K McCord

EEE512J2 – Electronic Product Design Page 9 Chapter 2: Fundamentals

2.8. RS232 Serial Interface

RS232 is simple, universal, well understood and supported, but it has some serious shortcomings as a data

interface. Its origins predate modern computers and it contains many features that are not relevant to the

modern user. It can control very old primitive modems and has many control signals to do this in hardware,

but often it is used without these old control and status lines.

Its major feature is that it does not require the transmission of a clock, the reception of a ‘start bit’ is enough

to cause the receiver to time all its actions from this one edge. This is called asynchronous transmission.

RS232 allows a 5% difference in transmitted timings and receiver chip timings. This is important if using a

PIC as the datasheet specifies the % error of the baud rate generator at certain baud rates (the higher the

baud rate, the higher the % error), as long as this error is less than 5% the RS232 standard is capable of

coping.

Electronic data communications between elements will generally fall into two broad categories: single-ended

and differential. RS232 (single-ended) was introduced in 1962, and despite rumours for its early demise, has

remained widely used.

±12V

Figure 2.8a. Illustration of RS232, 1 driver and 1 receiver

“Both RS232 and RS423 are unbalanced (or single-ended) standards, where the receiver measures the

potential between signal line and ground reference. Even though the transmitter and receiver grounds are

usually connected though the transmission line return, the impedance over a long distance may support a

significant difference in the two ground potentials, which will degrade noise immunity. Furthermore, any

noise induced from the outside will affect signal lines differently from the ground return due to their dissimilar

electrical characteristics – hence the name unbalanced.” [B3]

RS232 data is bi-polar, e.g. a +3 to +12 volt indicates an SPACE (ON) while a -3 to -12 volt indicates an

MARK (OFF). Modern computer equipment ignores the negative level and accepts a zero voltage level as

the MARK (OFF) state. This means circuits powered by 5 VDC are capable of driving RS232 circuits directly;

however, the overall range that the RS232 signal may be transmitted/received is dramatically reduced.

The output signal level usually swings between +12V and -12V. The ‘dead area’ between +3v and -3v is

designed to absorb line noise. This dead area can vary for various RS232 like definitions, for example the

definition for V.10 has a noise margin from +0.3V to -0.3V. Many receivers designed for RS232 are sensitive

to differentials of 1v or less.

Pin Signal Pin Signal

1 Data Carrier Detect 6 Data Set Ready

2 Receive Data 7 Request to Send

3 Transmit Data 8 Clear to Send

4 Data Terminal Ready 9 Ring Indicator

5 Signal Ground

Figure 2.8b. 9-pin RS232 D-connector, pin signal description

Typical line drivers / receivers chips for RS232 are the Maxim MAX232 or MAX233 chips (see

http://www.maxim-ic.com

) the original specification states that RS232 should drive 50 feet, but modern line

driver/receivers can manage much better than this.