Reader response
#7Done<crc><eom> or #7Error<eom>
For some commands, the MPRX responds with data that relates to the command, such
as T0F 0, to indicate the mode enabled for a #570 Display Operating Mode Status
command.
#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 MPRX 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:
Τ
rec (ms) = L x [Τchar + Τidle]
where
Τ
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
Τ
idle Maximum idle period between characters (ms)
Note: The MPRX 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 processing allowance for the receiver to process the message
and check for error conditions.
Thus, the sending timeout delay can be expressed as follows:
Τ
send (ms) = 9 * Τchar + Τerrchk
where
Τ
errchk(ms) Processing period to perform error checking by receiver
The host device can remotely set the MPRX’s communications parameters while in the command mode,
but TransCore does not recommend this action if communications conditions are marginal.
After the MPRX receives new communications parameters, the MPRX issues the #Done message and
switches to the new configuration immediately. The host device switches its communications parameters
immediately after the transaction is complete.
As noted, the message initiator, such as the MPRX in data mode and the host device in command mode,
starts a timeout counter at the time a message is transmitted. If the timeout expires before receiving an
Chapter 4 Communications Protocols
TransCore Proprietary
4–49
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