What is RS232?
It is the standard defined for the connection of "Data Terminal Equipment" (DTE) to "Data Communications Equipment" (DCE) where:
-
DTE (Data Terminal Equipment) is a generic term for an item which forms
part of the "information processing" portions of a system. Examples
are: computer, printer, terminal.
-
DCE (Data Communications Equipment) is a device which provides an interface
between a DTE and a communications link. Generally used as another word for
"modem"...
The intended way of connecting these together is:
RS232/V.24 Interface
This is the "classic" interface for DTE/DCE connections. It is based on a 25-pin plug and socket, of which only 8 pins (often less) are normally used:
Most computers used to have at least one RS232-compatible serial I/O port: in fact, the term RS232 port was widely used as a synonym for serial port. Typically these can operate at all "standard" bit rates from 300 bps to 115 kbps (or 230 kbps on Apple Macintoshes).
These days RS232 has largely been replaced by the Universal Serial Bus (USB) which we'll cover in a later lecture...
RS232 - Why is it so difficult?
RS232 interfaces caused many problems. Some typical reasons:
-
Designed for DTE-DCE connections, but commonly used
to connect DTE-DTE - introduces need for a null modem,
and all the awfulness that naturally ensues...
- Non standard use of handshaking signals for flow control, see later
- Non-standard connectors, eg: 9-pin PC format, 9-pin SGI (AT&T) format, mini-DIN 8 pin (Apple, etc)
"Asynchronous" Bit Level Synchronisation
Very common in low volume & low bit rate links or where data is isolated characters at random intervals. Virtually all "RS232 ports" use this technique.
Two synchronisation or "framing" bits are added to each byte of data:
Note that there is an indeterminant period of time between transmission of each byte (or character) in this system.
The receiving UART normally samples the incoming signal in the centre of each bit cell, using a x16 local clock to determine bit timing.
Asynchronous Observations
Asynchronous transmission makes inefficient use of a link: for each byte transmitted there is a fixed overhead of 1 start bit and 1 stop bit: using 8 bits of data + 2 bits sync overhead means efficiency is fixed at 80%.
The break signal is a special "out of band" asynchronous condition consisting of a SPACE (logic 0) level of longer duration than a character, thus:
The receiving Universal Asynchronous Receiver/Transmitter (UART) sees the break signal as an ASCII <NUL> character (char 0) received with a framing error.
Balanced vs. Unbalanced Interfaces
RS232 signals are "ground referenced" to pin 7 - this means that all of signal voltages are +ve or -ve with respect to this zero-voltage reference. This is called an "unbalanced" or "single-ended" interface.
A characteristic of this type of interface is susceptability to noise interference:
In a "balanced" or "differential" interface, two wires are used for each signal and the information is carried in the voltage difference between them. A "noise hit" on the line tends to effect both wires roughly equally, so that the signal is relatively immune to interference:
Copyright © 2006 Phil Rice