Despatch Protocol Plus Modbus Communications - Manual (Page 44)

Chromalox Instruments and Controls
A-51643 Rev. 6 10/06/03 38
Without Parity Checking
Start D0 D1 D2 D3 D4 D5 D6 D7 Stop Stop
6.2.3 Error Checking Field
 16-bit CRC
6.2.4 Message Framing
In RTU mode, messages start with a silent interval of at least 3.5 character times. This is most easily
implemented as multiple idle character times at the baud rate chosen for the network. The first field
transmitted is the device address. Following the last transmitted character, a similar interval of at least 3.5
character times marks the end of the message. A new message can begin after this interval.
The entire message frame must be transmitted as a continuous stream. If a silent interval of more than 1.5
character times occurs before completion of the frame, the receiving device flushes the incomplete
message and assumes that the next byte will be the address field of a new message. Similarly, if a new
message begins earlier than 3.5 character times following a previous message, the receiving device will
consider it a continuation of the previous message. This will set an error, as the value in the final CRC
field will not be valid for the combined messages. A typical message frame is shown below.
START ADDRESS FUNCTION DATA CRC CHECK END
T1-T2-T3-T4 8 BITS 8 bits n * 8 bits 16 bits T1-T2-T3-T4
6.3 Address Field
Valid slave device addresses are in the range of 0-247 ($0-$F7). The individual slave devices are
assigned addresses in the range of 1-247. Address 0 is used for broadcast address, which all slave devices
recognize. No response is made to a broadcast message.
6.4 Function Field
Valid function codes are in the range of 1-127 ($1-$7F). When a message is sent from a master to a slave
device the function code field tells the slave what kind of action to perform. When the slave responds, it
uses the function code field to indicate either a normal (error-free) response or an exception response (an
error occurred). For normal response, the slave simply echoes the original function code. For an
exception response, the slave returns the original function code with the MSB set to logic 1.