Read Multiple Registers - Siemens SITRANS F Operating Instructions Manual

Functions codes

6.3 Read multiple registers

6.3 Read multiple registers
Command messages
The multiple register read functions (03hex) allows the master to request information from
the slave. The command message of a multiple register read is structured as shown below.
Each row in the table compares to byte in the message – top byte (Slave address) is
transmitted first.
The initial slave address is 1. Address 0 (broadcast) and other values between 1 and 255
can be used.
The function code of this message is 03hex (read multiple registers)
The starting register is the first register to be read.
The quantity indicates how many consecutive 16-bit registers are to be read. The quantity
may range from 1 to 26 registers. If the quantity is greater than 26 an error code of 03 hex is
returned in the exception response message.
A CRC value is generated from a calculation using the values of the slave address, function
code, and data sections of the message. When the slave receives the command message it
calculates a CRC-16 value and compares it to the one in the CRC-16 field of the command
message. If these two CRC-16 values are the same the slave has received the proper
command message. If the two CRC-16 values are not the same the slave will not respond.
If the command message has a valid slave address, function code, starting register and
quantity value the slave will respond with a normal response message. If the command
message has an invalid function code, starting register and/or quantity the slave will respond
with an exception response message.
Table 6- 5
Message byte
Slave address
Function code
Starting register
Quantity
CRC-16
Normal response
The normal response message contains the same slave address and function code as the
command message.
The Byte count is the number of data bytes returned in the response message. The number
is actually the quantity (in the command message) times 2, since there are two bytes of data
in each register.
The data section of the response message contains 8 upper and 8 lower bits of data for each
register that has been read from the device.
26
Read multiple registers command messages
Upper
Lower
Upper
Lower
Lower
Upper
Example
xxhex
03hex
00hex
20hex
00hex
04hex
xxhex
xxhex
Operating Instructions, 06/2010, A5E00753974-04
Modbus RTU RS485