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B.1 MODBUS INTRODUCTION
When a system host command received by an ALPS cannot be performed, it replies with an error code. This is different
from detecting communication related errors, such as a CRC error, for which the ALPS ignores the command.
The format of an error reply is to return the received address and function back to the system host with the most significant
bit of the function code set. Also, a one-byte error code is added to the reply packet to identify the problem. The error codes
supported by the ALPS are:
B
01: Illegal Function: The function code received in the query is not an allowable action for the slave.
02: Illegal Address: The data address received in the query is not an allowable address for the slave. This exception
is returned if the query contains a reserved address or an address that is not supported by the ALPS model.
03: Illegal Data Value: A value contained in the query data is not an allowable value for the slave.
04: Slave Device Failure: An unrecoverable error occurred while the slave was attempting to perform the requested
action. Note that this exception is returned if an EEPROM CRC or data contents error is detected during startup.
05: Acknowledge: The slave has accepted a request for a long duration command.
06: Slave Device Busy: The slave is busy servicing another long duration command. The master should retransmit
this message at a later time. This exception is reported if the query target resource is being accessed by another
interface.
07: Negative Acknowledge: The slave cannot perform the function received in the query. This exception is reported
if the query requests data that is not available, such as a non-existent fault report.
09: Invalid Index.
The ALPS is a digital protection, control, metering and monitoring system. Most of the measurement values are stored in
word wide integer variables. Fixed point format values are implemented as integers as well; for example, value ranges from
0.01 to 50.00 may be stored as 1 to 5000. LSB will be stored in the lower byte and MSB in Upper byte of 16 bit registers.
Even while transmission LSB will be the first bit to be transmitted.
B-2
ALPS Advanced Line Protection System
APPENDIX B
B.1.4 EXCEPTION OR ERROR REPLIES
B.1.5 DATA FORMATS
GE Power Management
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