TransCore SmartPass AI1611, Системное руководство

Страница: 116 / 179

Communication Protocols
Timing and Synchronization
The ECP is largely independent of baud rate. The timeout delays previously
described are a function of baud rate.
Readers support an ECP timeout on both transmit and receive. In addition, for
readers a protection mechanism has been implemented to prevent the assignment of
mismatched ECP timeout and baud rate values. Readers will not allow the host to
alter the ECP timeout (command 612NN) if the specified timeout is too short for the
current baud rate. Conversely, readers will not allow the host to alter the baud rate
(command 100N) if the specified baud rate would be too fast for the current ECP
timeout.
The receiver’s minimum timeout delay should equal the time to transmit the longest
anticipated message at the current baud rate setting. Additional margin should be
included for idle periods between characters (i.e., processing overhead, if any). The
receiving timeout delay period can be expressed as:
Tau
rec
(ms) = L x [Tau
char
+ Tau
idle
]
where
Tau
char
(ms)
1000 x [ B
c
/ R
b
]
B
c
Bits per character (typically 10)
R
b
Baud rate (110–19.2 K)
L Length of message in character
s
Tau
idle
Maximum idle period between characters (ms)
Likewise, the sender must set a timeout delay equal to the delay of nine characters at
the current baud rate setting (i.e., the time required to shift out the
<eom>
character
plus the time to shift in the
ACK
or
NAK
message to be received) plus a processing
allowance for the receiver to process the message and check for error conditions.
Thus, the sending timeout delay can be expressed as
Tau
send
(ms) = 9 * Tau
char
+ Tau
errchk
where
Tau
errckh
(ms)
Processing period to perform error checking by receiver
The reader supports baud rates between 110 and 19.2 K. The host can remotely set
communications parameters of the reader while in the command mode, but
TransCore does not recommend this action if communications conditions are
marginal.
After the reader receives new communications parameters properly, the reader issues
the
Done
message and switches to the new configuration immediately. It is the
responsibility of the host computer to switch its own communications parameters
immediately after the transaction is complete.
8-11