Parker 6513, Technical Manual, Page: 48 / 54

MODBUS
41
RS485/RS232 Communications Interface
Error Response
The MODBUS protocol defines the response to a number of error conditions. A Slave device is
able to detect a corrupted command or one that contains an incorrect instruction, and will
respond with an error code.
With some errors, the Slave devices on the network are unable to make a response. After a wait
period, the Master will interpret the failure to reply as a communications error. The Master
should then re-transmit the command.
A Slave device that has detected a corrupted command, or a command that contains in incorrect
instruction, will respond with an error message. The error message has the following syntax:
Device Address Function Code Error Response Code CRC
1 byte 1 byte 1 byte MSB LSB
The Function Code byte contains the transmitted function code but with the most significant bit
set to 1. (This is the result of adding 128 to the function code.)
The error response code indicates the type of error detected. The following error response codes
are supported by Parker SSD Drives’ units:
Code Error Description
01 Illegal Function The requested function is not supported by the
slave.
02 Illegal Data Address The address referenced in the data field is not an
allowable address for the Slave
03 Illegal Data Value The value referenced in the data field is not
allowable in the addressed Slave location
06 Host Busy The slave cannot process the request at this time.
Try again later.
07 NAK Rejected for an unspecified reason.
Wait Period
There are several errors for which the Slave devices on the network are unable to make a
response:
•
If the Master attempts to use an invalid address then no Slave device will receive the
message
•
For a message corrupted by interference, the transmitted CRC will not be the same as the
internally calculated CRC. The Slave will reject the command and will not reply to the
Master.
After a wait period, the Master will re-transmit the command.
A wait period is also required after a broadcast communication to device address 0.
IMPORTANT: Failure to observe the wait period after a broadcast will negate the broadcast message.