ICS ELECTRONICS 4863 Manual page 149

characters. Each asynchronous character has a low going start bit, a
number of data bits, an optional parity bit and 1 or 2 high stop bits. The
transmitter automatically extends the stop bit when it has no more
characters to transmit. The receiver uses the start bit to resynchronize
its clock with the data at the start of each character as shown in Figure
A-7.
ASYNCHRONOUS
DATA CHAR
TYPICAL WAVE-
FORM FOR AN
ASCII "1"
RECEIVING CLOCK
DATA BITS
Figure A-7
Synchronous character do not have start/stop bits and are sent without
spaces between characters. Voids between data characters are filled
by predetermined sync characters which are discarded by the receiver.
The data portion of the serial character usually contains 5 to 8 bits and
is transmitted least significant bit first. Today most of the computers
and terminals use the 7 bit ASCII code to represent numbers and
characters. Figure A-7 shows how the ASCII "1" is transmitted.
Compare the binary code in Figure A-7 against the hex code for an
A2
ASCII '1' (HEX 31) and they will be the same. Binary data is usually
sent in binary form as single 8 bit characters or in hex form as a pair
of the ASCII characters, 0 through 9 and A through F. Each Hex
character represents 4 binary bits so two Hex characters are needed for
each 8 bit binary byte.
Parity bits are added after the data field if the user wants to detect
transmission errors. When parity bits are used, the transmitter counts
the number of high bits in the data field and makes the parity bit a 1
or 0 so the final count will be either even or odd. The receiver then
validates the received characters by counting 1's in the data and parity
bit fields. The 2303 detects parity errors along with data overrun and
Start
Data Bits
Bit
1 0 0
Start
Asynchronous Data Character Waveforms
A-18
Parity
0 1 1 0 Even (Stop)
Stop
Bit
Bit