Rockwell Automation 1770, D17706.5.16 Ref Mnl DF1 Protocol Command - Manual (Page 154)

9–16 Diagnostic Counters
Publication 1770Ć6.5.16 - October 1996
This counter byte Counts the number of
30
Replies lost because they could not be delivered over the DH link.
Undeliverable commands can be signaled to the user, because the
user" is located in PC memory, and can always be reached.
If a reply message cannot be delivered over the link, there is no way
to signal the user (of that message)Ċwho is also over the
highwayĊthat this node cannot signal a reply. The local user is not
concerned with the problems of the remote user and can take no
meaningful action, so there is not much to do but destroy the
message and count it.
Important: An intelligent device can read the memory of the
1771-KC it is connected to by setting the destination
equal to the module address.
1771ĆKC DH Diagnostic Counters
This counter byte Counts the number of
0
Bad CRCs on ACK.
1 Times the sender timed out waiting for an acknowledgement.
This is a common error and is one of the first to respond to
reflections or lowĆlevel noise on the link. It seems to be especially
sensitive to problems with longer cables. It also shows up if the
cable connection are loose.
2 Times contention was detected.
This also shows up quickly on noisy or cables that are too long.
This counter corresponds to error 93.
3 Times the ACK was successfully received but contained a nonĆzero
status code other than memory full.
4 Times the highway driver returns a message to sender with a
nonĆzero status code because a reply was not received from a
remote node.
Each count corresponds to one local error bit set or one reply
message lost.
5 Times the receiving node's memory was full.
Each time this happens, the message is placed on a waiting queue
for 0.5s. Each message is reĆtried five times for memory overflow
before it is returned to sender.
6 Times this node grabbed mastership of the highway because it
timed out while waiting to hear a valid frame.
On a DH link that has just been powered up, there should be only
one node that has this counter incremented.
7 Times that this node has tried to relinquish mastership and the node
that was expected to take over failed to respond.
This happens on a noisy link because the noise is mistaken for a
poll response, and the wrong node is selected as the next master.
When this occurs, the old master resumes polling. It also can
happen on a long link, if the poll response is very attenuated and is
not picked up by the carrier detect circuit. If the new node does
respond but the old master does not hear it, the old master records
a false poll and continues polling, and the new master starts polling
also. This usually leads to the second node detecting contention
and relinquishing.