ICS ELECTRONICS 4809A, Instruction Manual

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A-16
A2
Start
Bit
Parity
Bit
Stop
Bit
Data Bits
Start 1 0 0 0 1 1 0
Even
Parity (Stop)
ASYNCHRONOUS
DATA CHARACTER
DATA BIT VALUES
WAVEFORM FOR
AN ASCII "1"
RECEIVING CLOCK
Figure A-7 Asynchronous Data Character Waveforms
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 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 transmis-
sion 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 framing errors, generates a Bus
SRQ message for each data error and indicates the error by setting the bit
3 in the Standard Event Status Register.
Although serial data can be transmitted over any medium, most of today's
computer systems use metallic cable. To ensure compatibility, the manu-
facturers have adopted interface standards so that they are electrically
compatible. The more popular standards are: