DCP200 Profile Controller & Recorder -Product Manual
51-52-25-150, Issue 1 –April 2009 Serial Communications Page 79
Link Layer
A Query (or command) is transmitted from the Modbus Master to the Modbus Slave. The
slave instrument assembles the reply to the master. All of the instruments covered by this
manual are slave devices, and cannot act as a Modbus Master.
Figure 48. Modbus Link Layer
A message for either a QUERY or RESPONSE is made up of an inter-message gap followed
by a sequence of data characters. The inter-message gap is at least 3.5 data character times
- the transmitter must not start transmission until 3 character times have elapsed since
reception of the last character in a message, and must release the transmission line within 3
character times of the last character in a message.
Note:
Three character times is approximately 0.25ms at 115200 bps, 0.51ms at 57600 bps,
0.75ms at 38400 bps, 1.5ms at 19200 bps, 3ms at 9600 bps and 6ms at 4800bps.
Data is encoded for each character as binary data, transmitted LSB first.
For a QUERY the address field contains the address of the slave destination. The slave
address is given together with the Function and Data fields by the Application layer. The
CRC is generated from the given address, function and data characters.
For a RESPONSE the address field contains the address of the responding slave. The
Function and Data fields are generated by the slave application. The CRC is generated from
the address, function and data characters.
The standard MODBUS RTU CRC-16 calculation employing the polynomial 216+215+22+1 is
used.
Inter-message
gap
Address
1 character
Function
1 character
Data
ncharacters
CRC Check
2 characters
MODBUS
MASTER
SLAVE
INSTRUMENT
QUERY
RESPONSE