Practical_modern_SCADA_protocols_-_dnp3,_60870-5_and_Related_Systems

48 Practical Modern SCADA Protocols: DNP3, 60870.5 and Related Systems
Synchronization
In order to achieve reliable communication, the reception of a message must be
synchronized with its transmission. In other words, the receiving device must be able to
identify the start of a new message frame. Under the Modbus RTU protocol, frame
synchronization is established by limiting the idle time between successive characters
within a message frame. If three character times (approximately three milliseconds)
elapse without the receiving device detecting a new character, the pending message will be
flushed. The next byte will then be interpreted as the address field of a new message line.
Memory notation
The memory notation allows for four different data types: coils, discrete inputs, input
registers and holding registers. Register variables consist of two bytes, while coils and
discrete inputs are single bytes.
Each function references only one type of data. This allows message-frame memory
references to be expressed as offsets relative to the lowest possible address for that data
type. For example, holding register 40001 is referenced as 0000.
110V AC Coil
Digital Output
Discrete
Digital Input
5V DC
Master
Node
16 Bit Input
Register
Slave Node
16 Bit Output
Register
ADC
DAC
4-20mA
Analog Input
4-20mA
Analog Output
Figure 2.27
Diagram illustrating Modbus data types
Table 2.2 lists the address range and offsets for these four data types, as well as the function
codes which apply to each. The diagram above also gives an easy reference to the
Modbus data types.
Function codes
Each request frame contains a function code that defines the action expected for the target
controller. The meaning of the request data fields is dependent on the function code
specified.