TransCore SmartPass AI1611, System Manual

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SmartPass® Integrated Reader 2450 MHz (High Voltage) System Guide
As noted, the message initiator (i.e., the reader in data mode, and the host computer
in command mode) starts a timeout counter at the time a message is transmitted. In
the event the timeout expires before receiving a message, a logical
NAK
condition is
declared and the message is assumed to have been received in error. In this instance,
the message is retransmitted until a message is received.
The message recipient (i.e., the host computer in data mode, and the reader in
command mode) starts a timeout counter when a
<som>
character is received. In
the event the timeout expires without the receiver’s having received an
<eom>,
the
message acquisition is aborted (reset) and the receiver waits for the next
<som>
character.
In the event a second
<som>
character is received before an
<eom>
character, the
message acquisition is aborted (reset), and retransmission of the previous message is
assumed to be underway.
These strategies are designed to allow for graceful recovery during periods when
communications are marginal or completely lost.
It should be noted that while the reader is in command mode, all acquired tag IDs are
buffered but not transmitted.
Note: It is important that the host limit the period during which the reader remains
in command mode in order to avoid overflowing the reader’s tag ID buffer
and subsequently losing tag IDs.
Reader Addressed Failure Conditions
The reader addresses the following failure conditions.
Illegal Sequence Number (Not in the Range 0–9, A–F)
If the reader detects an illegal sequence number in a host command message, it will
discard the received message, and it will not send a response. If it receives an illegal
sequence number in an
ACK
message, it will respond as if a
NAK
had been received,
and it will retransmit the data.
Wrong Sequence Number
If the reader receives the wrong sequence number in an ACK message, it will respond
as if a NAK had been received (it will retransmit the data).
Bad CRC
If the reader detects a bad CRC in a host command message, it will discard the
received message (no response is sent). If it receives a bad CRC in an
ACK
message,
it will respond as if a
NAK
had been received (it will retransmit the data).
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