The Message Format 6-3
Read Multiple Registers - 03h
The multiple register read function (03h) allows the master to request information from the slave.
The command message of a multiple register read is structured as shown below.
Command Message
Each GPD 515ís slave address is set in advance by the drive parameter H5-01. Valid slave
addresses must be in the range of 1 to 31 decimal (1 to 1F hex). No two slaves may have the
same address. The master addresses the slave by placing the slave address in the address field
of the message. In the command message above, the slave is addressed at 2.
The function code of this message is 03h (read multiple registers).
The starting number is the first register to be read. In the command message above the starting
register is 20h, indicating that the first register is the Frequency Reference. A listing of the GPD
515ís registers is shown in Chapter 7, Registers.
The quantity indicates how many consecutive registers are to be read. The quantity may range
from 1 to 16 registers. If the quantity is greater than 16, an error code of ë3í is returned in the
fault response message. In this command message there is four consecutive registers to be
read: 20h-Frequency Reference, 21h-Output Frequency, 22h-Output Current, and 23h-Control
Method.
A CRC-16 value is generated from a calculation using the values of the address, function code,
and data sections of the message. The procedure for calculating a CRC-16 is described at the
end of this chapter. When the slave receives the command message it calculates a CRC-16
value and compares it to the one in 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 slave address, function code, starting register, and/or quantity the slave will respond with a
fault response message.
QTY.
02h
SLAVE ADDRESS
FUNCTION CODE
03h
00h
20h
00h
04h
45h
F0h
START-
ING
REGISTER
NO.
CRC-16
UPPER
LOWER
UPPER
LOWER
LOWER
UPPER
Summary of Contents for GPD 515/G5 Modbus RTU
Page 1: ...GPD 515 G5 Modbus RTU Technical Manual ...
Page 24: ...6 4 The Message Format ...
Page 30: ...6 10 The Message Format ...
