Communications Protocols
6-13
pass 4 Reader issues the Done message and switches to the new configuration imme-
diately. The host computer switches its communications parameters immediately after
the transaction is complete.
As noted, the message initiator, such as the Encompass 4 Reader in data mode and the
host computer in command mode, starts a timeout counter at the time a message is
transmitted. If the timeout expires before receiving an acknowledgement message, a
logical NAK condition is declared, and the initiator assumes the message was received
in error. In this instance, the message is retransmitted until an acknowledgement mes
-
sage is received.
The message recipient, such as the host computer in data mode and the Encompass 4
Reader in command mode, starts a timeout counter when a <som> character is
received. If the timeout expires without the receipt of an <eom>, the message acqui
-
sition is aborted (reset), and the receiver waits for the next <som> character.
If the message recipient receives a second <som> character before an <eom> charac-
ter, the message acquisition is aborted (reset), and retransmission of the previous mes-
sage is assumed to be underway.
These strategies allow for recovery during periods when communications are marginal
or completely lost.
Note: It is important that the host computer limit the period during which the Encom-
pass 4 Reader remains in command mode to avoid losing tag IDs. RF is off in com-
mand mode and no tags can be read.
Reader-Addressed Failure Conditions
The Encompass 4 Reader addresses the following failure conditions.
Illegal Sequence Number (not in the range 0–9, A–F)
If the Encompass 4 Reader detects an illegal sequence number in a host command
message, it discards the received message and sends no response. If it receives an ille
-
gal sequence number in an ACK message, it responds as if a NAK had been received
and retransmits the data.
Wrong Sequence Number
If the Encompass 4 Reader receives the wrong sequence number in an ACK message,
it responds as if a
NAK had been received, and it retransmits the data.
Incorrect CRC
If the Encompass 4 Reader detects an incorrect CRC value in a host command mes-
sage, it discards the received message. No response is sent. If it receives an incorrect
CRC value in an ACK message, it responds as if a NAK had been received, and it
retransmits the data.
Illegal Command
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