status of the Interrupt Identification Register. However I have never tested this.
Part 4 : Interfacing Devices to RS-232 Ports
RS-232 Waveforms
So far we have introduced RS-232 Communications in relation to the PC. RS-232
communication is asynchronous. That is a clock signal is not sent with the data.
Each word is synchronized using it's start bit, and an internal clock on each side,
keeps tabs on the timing.
Figure 4 : TTL/CMOS Serial Logic Waveform
The diagram above, shows the expected waveform from the UART when using the
common 8N1 format. 8N1 signifies 8 Data bits, No Parity and 1 Stop Bit. The RS-
232 line, when idle is in the Mark State (Logic 1). A transmission starts with a start
bit which is (Logic 0). Then each bit is sent down the line, one at a time. The LSB
(Least Significant Bit) is sent first. A Stop Bit (Logic 1) is then appended to the
signal to make up the transmission.
The diagram, shows the next bit after the Stop Bit to be Logic 0. This must mean
another word is following, and this is it's Start Bit. If there is no more data coming
then the receive line will stay in it's idle state(logic 1). We have encountered
something called a "Break" Signal. This is when the data line is held in a Logic 0
state for a time long enough to send an entire word. Therefore if you don't put the
line back into an idle state, then the receiving end will interpret this as a break
signal.
The data sent using this method, is said to be framed. That is the data is framed
between a Start and Stop Bit. Should the Stop Bit be received as a Logic 0, then a
framing error will occur. This is common, when both sides are communicating at
different speeds.
The above diagram is only relevant for the signal immediately at the UART. RS-232
logic levels uses +3 to +25 volts to signify a "Space" (Logic 0) and -3 to -25 volts for
a "Mark" (logic 1). Any voltage in between these regions (ie between +3 and -3
Volts) is undefined. Therefore this signal is put through a "RS-232 Level Converter".
This is the signal present on the RS-232 Port of your computer, shown below.
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The Serial - RS-232 Po...
