PLC Communication II

START/STOP Bits

start bit. This is a synchronizing bit added just before each character we are sending. This is considered a SPACE or negative voltage or a 0.
stop bit. This bit tells us that the last character was just sent.
This is considered a MARK or positive voltage or a 1.

Parity bit
Parity bit is added to check whether corruption has occurred. Common forms of parity are: None, Even, and Odd. During transmission, the sender calculates the parity bit and sends it. The receiver calculates parity for the character and compares the result to the parity bit received. If the calculated and real parity bits don’t match, an error occurred and we act appropriately.

Baud rate
it is the number of bits per second that are being transmitted or received. Common values (speeds) are 1200, 2400, 4800, 9600, 19200, and 38400.

RS232 data format
RS232 data format (baud rate-data bitsparity-stop bits). 9600-8-N-1 means a baud rate of 9600, 8 data bits, parity of None, and 1 stop bit.

Software handshaking
Software handshaking (flow control) is used to make sure both devices are ready to send/receive data. The most popular “character flow control” is called XON/XOFF. The receiver sends the XOFF character when
it wants the transmitter to pause sending data. When it’s ready to receive data again, it sends the transmitter the XON character.

STX & ETX
Sometimes an STX and ETX pair is used for transmission/reception as well. STX is “start of text” and ETX is “end of text”. The STX is sent before the data and tells the external device that data is
coming. After all the data has been sent, an ETX character is sent.

ACK / NAK Pair
The transmitter sends its data. If the receiver gets it without error, it sends back an ACK character. If there was an error, the receiver sends back a NAK character and the transmitter resends the data.