RS-232 signal and RS232 voltage levels
- an overview RS232 signals and signal levels used for communications using
this serial bus. RS232 voltage levels are also described.
The voltage levels are one of the main items in the
specification. For RS232 data signals a voltage of between -3V and -25V
represents a logic 1. The logic 0 is represented by a voltage of between +3V and
+25V. Control signals are in the "ON" state if their voltage is between +3V and
+25V and "OFF" if they are negative, i.e. between -3V and -25V.
The data is sent serially on RS232, each bit is sent one
after the next because there is only one data line in each direction. This mode
of data transmission also requires that the receiver knows when the actual data
bits are arriving so that it can synchronise itself to the incoming data. To
achieve this a logic 0 is sent as a start bit for the synchronisation. This is
followed by the data itself and there are normally seven or eight bits. The
receiver obviously has to know how many data bits to expect, and there are often
small dual in line switches either on the back of the equipment or inside it to
set this information.
Data on RS232 is normally sent using ASCII (American Standard
Code for Information Interchange). However other codes including the Murray Code
or EBCDIC (Extended Binary Coded Decimal Interchange Code) can be used equally
well.
After the data itself a parity bit is sent. Again this
requires setting because it is optional and it can be even or odd parity. This
is used to check the correctness of the received data and it can indicate
whether the data has an odd or even number of logic ones. Unlike many systems
these days there is no facility for error correction.
Finally a stop bit is sent. This is normally one bit long and
is used to signify the end of a particular byte. Sometimes two stop bits are
required and again this is an option that can often be set on the equipment.
RS232 data transmission is normally asynchronous. However
transmit and receive speeds must obviously be the same. A certain degree of
tolerance is allowed. Once the start bit is sent the receiver will sample the
centre of each bit to see the level. Within each data word the synchronisation
must not differ by more than half a bit length otherwise the incorrect data will
be seen. Fortunately this is very easy to achieve with today's accurate bit or
baud rate generators.
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