TransCore encompass 4, Руководство пользователя

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When the host receives a properly framed message, it can calculate a 16-bit CRC value.
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 transmitted CRC value can then be compared with the binary equivalent of
the received <
crc
> characters. If the transmitted and received CRC values do not
match, the recipient assumes the message was received in error, and transmits a
NAK
message response.
<eom>
End-of-message characters (ASCII CR and LF). The system includes both a carriage
return (CR) and line feed (LF) 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 is aborted. The assumption is that an <eom> was lost and the Encompass 4
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 Encompass 4 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 Encompass 4 Reader. The
sequence number should correspond to the data message that is being positively or
negatively acknowledged by the host. If the Encompass 4 Reader receives an ACK
message with the incorrect sequence number, the data message is retransmitted.
The host computer resets the anticipated data message sequence number to that of
the Encompass 4 Reader before communications can resume without error.
<ack/nak>
ASCII @ character for
ACK
response; ASCII ? character for
NAK
response
<crc>
CRC value for the message
<eom> End-of-message character (ASCII CR)
The Encompass 4 Reader sets a user-programmable timeout delay at the time each
message is transmitted based on command #612NN Set Error Correcting
Protocol Timeout , where NN = timeout delay. To disable the timeout delay for
diagnostic purposes, issue the command #612FF Disable Error Correcting
Protocol Timeout .
If the timeout delay expires before the Encompass 4 Reader receives an
ACK
or
NAK
message from the host, a logical
NAK
condition will be declared. If the Encompass 4
Reader receives a
NAK
or timeout, the reader retransmits the data message.
When the Encompass 4 Reader receives an
ACK
message, the system software treats
the message as having been properly received by the host. The software increments
the sequence number, and advances pointers to the next message in the Encompass 4
Reader’s message queue to prepare for sending the next message.
Encompass 4 Reader System Guide
TransCore Proprietary.
6–74