Communications Protocols
6-15
Transmission Timeout
If the Encompass 4 Reader transmits an asynchronous message and the host does not
send an ACK
before the ECP timeout occurs, the Encompass 4 Reader retransmits the
message.
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 retrans-
mits 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 com-
mand 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.
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