Rockwell Automation 1771-DCM USER MANUAL 1771-DCM - Manual (Page 19)

Using the Status Word
Chapter 5
5Ć2
Bits in the status word read by the supervisory processor differ from those
read by the local processor. Some bits apply only when the 1771-DCM is
communicating with the supervisory processor by block transfer.
When the 1771-DCM detects an error in its communication with the
supervisory processor, it indicates this condition in the status word read by
the local processor as follows:
17 16 15 14 13 12 11 10
Insuf Excess Scanner No Inhibit Invalid
Data Data Comm 0 Data 0 Outputs Data
Error
Bit 17 Ć Insufficient Data Available
Set by the 1771-DCM when data received from the supervisory processor
is less than that requested by the local processor. It appends zeroed data
words to provide the requested number of words.
Bit 16 Ć Excessive Data Available
Set by the 1771-DCM when the data received from the supervisory
processor is more than that requested by the local processor. It truncates
the data and returns only the number of words requested.
Important: In block transfer mode, the 1771-DCM sets Bit 17 or 16
(defined above) when it detects that the length of a read block transfer
request from one processor differs from the number of words previously
written by the other processor. In discrete data transfer, the 1771-DCM
sets Bit 17 or 16 when it detects that the number of words read by one
processor differs from the number of words that the 1771-DCM
temporarily stored. This depends on the equivalent rack size that you set,
such as eight slots for four words.
Bit 15 Ć Scanner Communications Error
Set by the 1771-DCM when it detects a loss in communications with the
scanner of the supervisory processor followed by a request for a read
block transfer from the local processor. The 1771-DCM returns the last
data received from the scanner, or zeroed data words if communications
had not been established. The 1771-DCM turns off the serial
communications LED until communications are restored.
Status Bits Read by the Local
Processor