TransCore encompass 4, Руководство пользователя

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before receiving an acknowledgment 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
acknowledgment message 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 acquisition is aborted (reset), and the receiver waits for the
next
<som>
character.
If the message recipient receives a second
<som>
character before an
<eom>
character, the message
acquisition is aborted (reset), and retransmission of the previous message 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 Encompass 4 Reader
remains in command mode to avoid losing tag IDs. RF is off in command mode and no tags can be
read. One Technique to enhance the tag reading and avoid having to go into command mode is to
enable Command #831 for the automatic periodic RF status report. Refer to "831 Enable Automatic
Periodic RF Status Report" on page 7–131 .
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 illegal 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 message, 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
If the Encompass 4 Reader receives an illegal command, it returns its standard Error message.
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.
Chapter 6 Communications Protocols
TransCore Proprietary
6–79