Appendix D - Recovery after Timeout Flow

Simplify attempts to transmit the message a parameterized number of times before assuming communications are lost.

The message flow is bi-directional. The POS device or Simplify can initiate a message. In order to avoid timeouts and retransmissions between the POS device and Simplify, each endpoint (POS device or Simplify) should read an incoming message, calculate the LRC, and immediately respond with an ACK or NAK.

message resent after timeout

* Recommended configuration for timeout on ACK is at least 3 seconds.

Exception to Timeout/Recovery Rules

After a timeout on a non-critical message, Simplify does not expect an ACK and will not retry the message. Sending an ACK for a non-critical message will not cause any issue. The following messages are considered non-critical, with the default exceptions noted:

Message TypeExceptions
Status messages
(Tran Type 36-51)
Exceptions: Status Identifier = 001, 021
(For information on Status Identifiers, see Field 11 (User Data).)

The list of Status Identifiers for which a Status Message must be ACK’d is configurable. Please consult your Elavon representative for more information.

The message flow without an ACK is shown below:

message flow with ACK suppressed