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C-4 Micro PLC Programmer’s Guide – April 1994
GFK-0804B
Character Format
A message is sent as a series of characters. Each byte in a message is transmitted as a
character. The illustration below shows the character format. A character consists of a
start bit (0), eight data bits, an optional parity bit, and one stop bit (1). Between charac-
ters the line is held in the 1 state.
Stop Start
MSB Data Bits LSB
Parity
(optional)
10 9 8 7 6 5 4 3 2 1 0
Message Termination
Each station monitors the time between characters. When a period of three character
times elapses without the reception of a character, the end of a message is assumed. The
reception of the next character is assumed to be the beginning of a new message.
The end of a frame occurs when the first of the following two events occurs:
The number of characters received for the frame is equal to the calculated length of
the frame.
A length of 3 character times elapses without the reception of a character.
Timeout Usage
Timeouts are used on the serial link for error detection, error recovery, and to prevent
missing the end of messages and message sequences.
After sending a query message, the master should wait approximately 500 milliseconds
before assuming that the slave did not respond to its request.