TransCore SmartPass AI1611, Системное руководство

Страница: 111 / 179

SmartPass® Integrated Reader 2450 MHz (High Voltage) System Guide
<data>
An ASCII string up to 72 characters long. This string may
contain tag data, a presence without tag report, an input
status change report, an Error0x message (buffer level
report), or a sign-on message. Time, date, and auxiliary data
may also be included.
<crc>
Cyclic redundancy check (CRC). This field contains 4
ASCII digits that represent the 16-bit CRC calculated on the
message. The CRC is calculated on bytes between the som
character and the first CRC byte.
Once the host receives a properly framed message, it can
calculate a 16-bit CRC. The calculation is applied to the
character string that immediately follows the <som> and
that ends with the character immediately preceding the first
<crc> character.
The calculated CRC can then be compared with the binary
equivalent of the received <crc> characters. If a difference
results, the message is assumed to have been received in
error and a NAK message response should be transmitted.
<eom>
End-of-message characters (ASCII CR and LF). Both a CR
(carriage return) and LF (line feed) are included to facilitate
the use of terminals and printers.
If the host receives a <som> character in the middle of a
data message, the message in progress must be aborted. The
assumption is that an <eom> was lost and the reader is in
the process of retransmitting the previous message.
ECP Host ACK/NAK Response
With ECP, the host computer responds to all data message transmissions from the
reader using the following acknowledgment or negative acknowledgment response
format.
<som><seq><ack/nak><crc><eom>
where
<som>
Start-of-message (ASCII # character)
<seq>
An echo of the sequence number received from the reader.
The sequence number should correspond to the data
message that is being positively or negatively acknowledged
by the host. If the reader receives an ACK message with the
incorrect sequence number, the data message is
retransmitted.
8-6