TransCore encompass 4 - ECP Host ACK;NAK Response

Encompass 4 Reader System Guide
6-6
message; or a sign-on message. Auxiliary data may also be
included.
<crc> 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.
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 acknowledg
-
ment 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.