Alstom iSTAT M355, Manual

Page: 122 / 154

M3x5/EN M/B User Manual
Page 120
iSTAT M3x5
18. APPENDIX B3: MODBUS ERROR RESPONSES
When a slave detects an error other than a CRC error, a response will be sent to the master.
The most significant bit of the function code byte will be set to 1 (i.e. the function code sent
from the slave will be equal to the function code sent from the master plus 128). The
following byte will be an exception code indicating the type of error that occurred.
The slave will ignore transmissions received from the master with CRC errors.
An example of an illegal request and the corresponding exception response is shown below.
The request in this example is to read registers 0201H to 0209H. If these addresses are not
supported in the slave then the following occurs:
Request Message
Starting Register Register Count
Address Function Code HI LO HI LO CRC
01 01 02 01 00 08 6D B4
Exception Response Message
Address Function Code Exception Code CRC
01 81 02 C1 91
18.1 Exception codes
Code Name Meaning
01 ILLEGAL FUNCTION The function code transmitted is not
one of the functions supported by the
slave.
02 ILLEGAL DATA ADDRESSES The data address received in the
request is not an allowable value for
the slave.
Write to password protected registers.
03 ILLEGAL DATA VALUE The value referenced in the data field
transmitted by the master is not within
family for the selected data address.
The register count is greater than 28
(functions 03 and 04).
06 SLAVE DEVICE BUSY The slave is engaged in processing a
long duration program command. The
master should re-transmit the
message later when the slave is free.