6-6 The Message Format
Loop-back Test - 08h
The loop-back test function (08h) is used for checking signal transmission between master and
slaves. The command message format is shown below.
Command Message
Each GPD 506’s slave address is set in advance by the drive parameter n106. Valid slave
addresses must be in the range of 1 to 31 decimals (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 1.
The function code of this message is 08h (loop-back test).
The test code must be set to ‘0000’. This function specifies that the data passed in the command
message is to be returned (looped back) in the response message.
The data section contains arbitrary data values. These data values are used to verify that the slave
receives the correct data.
A CRC-16 value is generated from a calculation using the values of the address, function code, test
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 these two CRC-16
values are not the same the slave does not respond.
If the command message has a valid slave address, function code, test code, and data value, the
slave will respond with a normal response message. If the command message has an invalid slave
address, function code, test code, and/or data value the slave will respond with a fault response
message.
DATA
01h
SLAVE ADDRESS
FUNCTION CODE
08h
00h
00h
A5h
37h
DAh
8Dh
TEST
CODE
CRC-16
UPPER
LOWER
UPPER
LOWER
LOWER
UPPER
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