TransCore SmartPass AI1611, System Manual

Page: 112 / 179

Communication Protocols
The host computer is responsible for resetting its anticipated
data message sequence number to that of the reader before
communications can resume without error.
<ack/nak>
ASCII @ character for ACK response; ASCII ? character for
NAK response
<crc>
CRC for the message
<eom>
End-of-message character (ASCII CR)
The reader sets a user-programmable timeout delay at the time each message is
transmitted (based on command 612NN, where NN = timeout delay). The timeout
delay can be disabled for diagnostic purposes by setting NN = FF.
If the timeout delay expires before the reader receives an
ACK
or
NAK
message
from the host, a logical
NAK
condition will be declared. In the event the reader
receives a
NAK
or timeout, the data message is retransmitted.
When the reader receives an
ACK
message, the message is treated as having been
properly received by the host. The sequence number is then incremented and pointers
are advanced to the next message in the reader’s message queue to prepare for
sending the next message.
Switch to Command Mode Request
The host may issue a switch to command mode command (command 01) while in
data mode.
The basic protocol format is:
<som><cmd><eom>
The ECP format is:
<som><seq><cmd><crc><eom>
where
<som>
Start-of-message (ASCII # character)
<seq>
The sequence number is generated by the host computer
separately from that appearing in data messages transmitted
by the reader.
<cmd>
Switch to command mode command code (ASCII characters
01)
<crc>
CRC for the message
<eom>
End-of-message character (ASCII CR)
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