4. Configuration
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application layer. As the TCP configuration is a reference for every instances configured, the time
will be valid if it is smaller than the configured inside a protocol:
Initial Time-out: indicates how long, after the first message transmission, the message must be
retransmitted, assuming it has not been received by the destiny device. At each retransmission
the time-out is doubled. The number of transmission tries is connected to the communication
time-out configured inside the protocol. It will be the maximum time before it gives up the
message delivering, when the transmission failure is concretized. In addition, it is important to
stress that there’s a maximum quantity of attempts for the Nexto Series CPUs. This number is set
in five attempts before the connection is set up and in three attempts after that. Every time it
reaches the maximum number of retries, the communication attempt process restarted. See
Protocols Configuration section further details regarding time-out parameters utilization as they
can be different depending on the situation. It is important to stress this parameter is only used in
the connection setting up, after that it is used statistics from the last communications to calculate
the new time-out.
Example of the initial time-out and the communication time-out parameters inside the MODBUS
TCP Server, considering a not received acknowledge, for the following values: 300 for initial time-
out (300 ms) and 3000 for communication time-out (3000 ms):
Figure 4-1. Initial Time-out and Communication Time-out
Legend:
14. Message transmission instant.
15. First attempt for message transmitting, after initial time-out.
16. Second attempt for message transmission, after two times the initial time-out.
17. Third try for message transmission, after two times the latter time-out.
18. Quit of message transmission and failure indication, after the communication time-out exceeds
(total time until the given up: 300 + 600 + 1200 + 900 = 3000 ms).
19. It would be the fourth message transmission attempt, after two times the latter time-out but the
communication time-out configured inside the protocol was exceeded and the failure was
indicated.
ACK sending delay: defines the maximum time waited by the interface for the TCP ACK
transmitting. This ACK is responsible for the message receiving conformation, in case of
MODBUS, by the destiny device. The set of this field decreases the amount of messages circling
through the network. This mechanism is explained below:
o All request messages, sent by a client to a server, need to be confirmed by the server through
an ACK message transmitted to the client.
o All response messages, sent by a client to a server, need to be confirmed by the server through
an ACK message transmitted to the client.
o If one of the parts does not receive the ACK message, within the time defined by the TCP
time-out, the message will be retransmitted by the origin address (see TCP number of tries
parameter)
o The ACK message does not need to be exclusive. The ACK needed to be sent by the server to
the client, at the moment it receives a request, can be included in the same answer message,
and the ACK needed to be sent by the client to the server, at the request moment, can be
included in the same message including the next request.
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