User`s guide

DDCMP Communication Protocol
DDCMP is a rigorous protocol that automatically handles the detection of errors and the
retransmission of messages when an error occurs. (The name stands for Digital Data
Communications Message Protocol.) Originally, this protocol was used in Digital Equipment
Corporation's computer network DECnet.
The Adept implementation of DDCMP does not support maintenance messages or multidrop
lines. In all other respects it is a full implementation of the protocol.
This section is not intended to be a thorough description of DDCMP. For additional details on
protocol operation and implementation, refer to DDCMP documentation which is available on
the Internet.
1
General Operation
All messages transmitted by DDCMP are embedded in a packet that includes sequence
information and check codes. Upon receipt, a message packet is checked to verify that it is
received in sequence and without transmission errors.
To initiate communications, a system sends special start-up messages until the proper
acknowledgment is received from the remote system. This handshaking guarantees that
both sides are active and ready to exchange data packets. If a start request is received after
the protocol is active, it means that a system has stopped and restarted its end of the
protocol, and an error is signaled to the local system.
Once the protocol is active, each transmitted message is acknowledged by the remote
system, indicating that it was received correctly or requesting retransmission. If a message
is not acknowledged after a certain time, the remote system is signaled and a retry sequence
is initiated. If a message is not sent correctly after a number of retries, DDCMP stops the
protocol and signals an error to the local system.
The following table shows the standard DDCMP NAK reason codes generated by the Adept
implementation of DDCMP.
Code Description
1 Check code error in data
header or control message
2 Check code error in data field
3 REP response with NUM in
REP <> R
8 Buffer temporarily
unavailable for incoming data
Standard DDCMP NAK Reason Codes
DDCMP Communication Protocol
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