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 506 slave address is set in advance by the drive parameter n104. 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 Status Signal. A listing of the GPD 506
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 8 registers. If the quantity is greater than 8, an error code of ‘3’ is returned in the fault
response message. In this command message, there are four consecutive registers to be read:
20h-Staus Signal, 21h-Drive Fault Contents, 22h-Communication Data Link Status, and 23h-
Frequency Reference.
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 presented 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
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