TransCore 05531 User Manual

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 4/4800 
Multiprotocol Reader  is in the process of retransmitting the 
previous message.

With ECP, the host computer responds to all data message transmissions from the 
Encompass 4/4800 Multiprotocol 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/
4800 Multiprotocol Reader. The sequence number should 
correspond to the data message that is being positively or 
negatively acknowledged by the host. If the Encompass 4/4800 
Multiprotocol 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 4/4800 Multiprotocol 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)