Receive Timeout
If the Encompass 4 Reader receives a <som> but does not receive a matching <eom> before the ECP
timeout occurs, it discards the incomplete message and resets its receiver.
Asynchronous Message/Command Message Collision
If the Encompass 4 Reader transmits asynchronous data at the same time that the host sends a command,
the Encompass 4 Reader gives priority to receiving the command. It processes the command and sends a
message before it retransmits the asynchronous data.
Host-Addressed Failure Conditions
The host computer addresses the following failure conditions.
Illegal or Wrong Sequence Number
If the host detects an illegal or wrong sequence number in a reader response, it retransmits the command
with the same sequence number. If the host detects an illegal sequence number in an asynchronous
reader transmission, it sends a NAK message.
Incorrect CRC
If the host detects an incorrect CRC value in a reader message, it retransmits the command with the same
sequence number. If the host detects an incorrect CRC value in an asynchronous reader transmission, it
transmits a NAK message.
Transmission Timeout
If the Encompass 4 Reader does not respond to a host command within a specified interval, the host
software retransmits the command with the same sequence number.
Receive Timeout
If the host receives a <som> but does not receive a matching <eom> within a specified timeout interval, it
discards the incomplete message and resets its receiver.
Asynchronous Message/Command Message Collision
If the host receives an asynchronous reader transmission at the same time it transmits a command, it
ignores the asynchronous message and waits for the Encompass 4 Reader’s response. The Encompass 4
Reader retransmits asynchronous data after it transmits the command message.
ECP Reliability
An undetected error is defined as a message having incorrect data or status but no parity or CRC errors.
An error transaction is defined as a message having either a parity or CRC error. Laboratory testing
indicates an undetected error rate of less than one undetected error per 1,000,000 error transactions with
parity enabled.
Encompass 4 Reader System Guide
TransCore Proprietary.
6–80
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