MagTek MCP - Manual (Page 16)

MCP Serial Transport Protocol
10
Receive Sequence Number (N(R))
The receive sequence number indicates what the next expected sequence number is, confirming
all received I-frames with lower sequence numbers (in this case, no more than one). This is used
to acknowledge received I-frames. This is the same as the receive sequence number used in I-
frames.
Data Field
The data field is not used for R-frames and should remain empty.