Allen-Bradley 1779-KFMR User Manual page 146

Command Format:
In the reply, if the STS field has a value of 00, that indicates a successful-
transaction reply which has a data field. The data field is the specified
immediate-access block. The size of the block is determined by the
programmable controller that generates it. The maximum block size is
42 bytes.
Success Reply Format:
In the reply, if the STS field has a value other than 00, that indicates an
error reply which has no data field.
Error Reply Format:
Chapter 9