MagTek MCP - Manual (Page 36)

MCP Serial Transport Protocol
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Rule 4.7
A valid response to an I-frame is sending an R-frame with node’s current receive sequence
number in the frame’s N(R) field.
Rule 4.8
When a node that is awaiting response to its I-frame receives a valid response that contains N(R)
field that is equal to the node’s send sequence number plus one, the node shall increment its send
sequence number and consider the user data successfully transferred.
Rule 4.9
An I-frame sent by the node is considered outstanding until (a) the node receives a valid response
according to Rule 4.8, (b) block wait timeout expires, or (c) the node sends RESYNC request or
response.
Error Recovery
Rule 5.1
When a node sends an I-frame and does not receive the response defined in Rule 4.8 within the
block wait timeout, the node shall initiate error recovery.
Rule 5.2
A node can initiate error recovery by sending an R-frame with poll indicator set. The R-frame
shall contain the node’s current receive sequence number in the N(R) field of the frame header.
Rule 5.3
A node can initiate error recovery by re-sending the I-frame for which it is expecting response.
The I-frame shall be sent with the same N(S) as the original I-frame and with N(R) equal to the
node’s current receive sequence number, which may have changed since the original I-frame was
sent.
Rule 5.4
A node that receives R-frame with poll indicator set shall, in addition to its normal processing of
the R-frame, respond either (a) with R-frame as per
Rule 4.7 , or (b) with I-frame as per
Rule 4.6.