Rockwell Automation 1771-DCM USER MANUAL 1771-DCM - Manual (Page 18)

Chapter
5
5Ć1
Using the Status Word
The 1771-DCM sets status/alarm bits in the upper byte of the first word
transferred to either processor, regardless of whether you select block
transfer or discrete data transfer. In block transfer operation, the status
word is the first word of the read data block. In discrete data transfer, the
status word is found in the equivalent of the first module group of the I/O
chassis simulated by the 1771-DCM.
Important: Reserve the first transferred word for status data inserted by
the 1771-DCM. Be sure this word contains zeros when you initialize the
ladder program for either processor. The upper byte contains status data.
The lower byte is not available for transferring data under any
circumstances.
When you select discrete data transfer and set your rack size for the
number of words required for your application, keep in mind that the
number of words available for your data is one less than the number of
words transferred.
Rack Size
Number of
Transfer Words
Words Available
For Your Data
1/4 2 1
1/2 4 3
3/4 6 5
Full 8 7
Examine bits in the upper byte of the status word using your ladder
program so it can take appropriate action when the 1771-DCM detects an
error or change in status. Upon detecting an error or change in status, the
1771-DCM sets an invalid data bit along with one or more bits that
indicate the nature of the error or change in status. These bits are
non-retentive.
Using the Status Word
Examining Status Bits