Echelon IzoT U60 DIN, User Manual

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IzoT U60 DIN Network Interface User’s Guide
25
Acknowledgement Rules
The Ack bit is used to send a success response to a previous transfer, specifically
transfers that send data that is above the LLP (non-LLP data). These are transfers
of messages and transfers of local network interface commands.
To reduce traffic, not all data transfers require acknowledgement. The following are
the acknowledgement rules:
•
If a transfer requires an acknowledgment and there is no other data that needs
to be sent, a CpNull packet is sent with the Pass/Ack bit set. Otherwise, if
there is outgoing traffic to be sent the Ack bit will be included in the following
transfer. This is true for both uplink and downlink data messages.
• Code packets that do not require an acknowledgment via the Ack bit are
packets that result in an immediate response anyway. The response implies
acknowledgment. The following table lists these types of code packets:
Code Packet Description
CpMsgReq
The U60 will respond with either the CpMsgAck ,
or a criss-cross could occur and an unrelated
code packet will be sent by the U60.
CpMsgAck
The host does not need to acknowledge this
message. Instead it provides an implied
acknowledgement by sending the
CpMsg code
packet followed by a message packet
•
Data that is sent and requires acknowledgement will persist in the source until
the acknowledgment is received. While the persistent data is waiting for
acknowledgement, the Acknowledge Wait timeout, which is 300ms for the
U60 and the driver, causes the persistent data to be re-sent.
Sequence Number Cycling and Duplicate Detection
The sequence number is used to reject duplicate non-LLP data. Duplicate LLP data
does not affect the LLP; therefore, the sequence number is only cycled for each
transfer of non-LLP data. For example, two consecutive CpMsgReq packets have no
effect—the second CpMsgReq packet reinstates the ready-receive state and the
CpMsgAck is re-sent.
U60 Command Set
Uplink local network interface commands that must also convey additional data (for
example, the niL2MODE response command) will always result in a data message
that is at least 3 bytes in length. This is because the host driver expects at least 4
bytes of data (3 bytes plus checksum) to appear following a code packet.