TransCore MPRX, System Manual

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System Guide
TransCore Proprietary 6–10
where
Τ
char
(ms)
1000 x [ B
c
/ R
b
]
B
c
Bits per character, typically 10
R
b
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 * Τ
char
+ Τ
errchk
where
Τ
errckh
(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 acknowledgment message, a logical NAK condition is
declared, and the initiator assumes the message was received in error. In this instance, the
message is retransmitted until an acknowledgment message is received.
The message recipient, such as the host device in data mode and the MPRX in command
mode, starts a timeout counter when a
<som>
character is received. If the timeout expires
without the receipt of an <eom>, the message acquisition is aborted (reset), and the
receiver waits for the next <som> character.
If the message recipient receives a second <som> character before an <eom> character,
the message acquisition is aborted (reset), and retransmission of the previous message is
assumed to be underway.
These strategies allow for recovery during periods when communications are marginal or
lost completely.