Protocols
An important addition to the physical standards is a protocol. A protocol is essentially a common set of rules governing the exchange of data between transmitter and receiver on a data communications link. It is a way of packaging the data transmitted. A typical example of a protocol is given in Figure 5.
Figure 5. Format of a Read
Command and its Response
The following fields are used:
ADD - The
address field. It is the address of the slave device on the data communications
link.
BCC - The Block Check Character, a 'unique fingerprint' which the
receiver checks against the message to detect any
errors in transmission.
PAR - The address of the parameter requested and can be in the range of 000 to 999.
The write request frame is sent to a slave device from a master computer terminal to change, for example, a set point of a variable speed drive. The ACK response is returned by the slave device to indicate that the setpoint has actually been changed.