Appendix D - Recovery after Timeout Flow

Simplify attempts to transmit the message a parameterized number of times before assuming comm is 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 (see Appendix E - LRC Calculation), and immediately respond with an ACK or NAK.

Message resent after timeout. See text above and just below.

* 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 0011 (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. If a Status Message is configured for no ACK, Simplify is ready for the next message after sending the Status message.

Message flow with ACK suppressed. See preceding paragraph.