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Multiprotocol Rail Reader System Guide
Field containing four ASCII digits that represent the 16-bit CRC
value calculated on the message. The CRC value is calculated on
bytes between the som character and the first <crc> byte.
<crc>
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.
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 MPRR is in the process of
retransmitting the previous message.
ECP Host ACK/NAK Response
With ECP, the host device responds to all data message transmissions from the MPRR
using the following acknowledgment or negative acknowledgment response format.
<som><seq><ack/nak><crc><eom>
where
<som>
Start-of-message (ASCII # character)
<seq>
Echo of the sequence number received from the MPRR. The
sequence number should correspond to the data message that is
being positively or negatively acknowledged by the host. If the
MPRR receives an ACK message with the incorrect sequence
number, the data message is retransmitted.
The host device resets the anticipated data message sequence
number to that of the MPRR before communications can resume
without error.
6-6
<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)