Encompass 4 Reader System Guide
6-12
#7Error<eom> will be returned if host transmission is not a legal command with
legal data.
Timing and Synchronization
The ECP is largely independent of baud rate. The timeout delays previously described
are a function of baud rate.
The Encompass 4 Reader supports an ECP timeout, which applies equally to both
transmit and receive.
The receiver’s minimum timeout delay equals the time to transmit/receive the longest
anticipated message at the current baud rate setting. Additional margin should be
included for idle periods between characters; for example, processing overhead, if
any. The timeout delay period can be expressed as follows:
rec (ms) = L x [char + idle]
where
char (ms) 1000 x [ B
c
/ R
b
]
Bc Bits per character, typically 10
Rb Baud rate, 1200–38.4 K
L Length of message in characters
idle Maximum idle period between characters (ms)
Note: The Encompass 4 Reader supports baud rates between 1200 and 38.4 K.
Likewise, the sender must set a timeout delay equal to the delay of nine characters at
the current baud rate setting. For example, 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 pro
-
cessing allowance for the receiver to process the message and check for error condi-
tions.
Thus, the sending timeout delay can be expressed as follows:
send (ms) = 9 * char + errchk
where
errckh (ms) Processing period to perform error checking by receiver
The host computer can remotely set the Encompass 4 Reader’s communications
parameters while in the command mode, but TransCore does not recommend this
action if communications conditions are marginal.
After the Encompass 4 Reader receives new communications parameters, the Encom-
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