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.
Reader-Addressed Failure Conditions
The MPRX addresses the following failure conditions.
Illegal Sequence Number (Not in the Range 0–9, A–F)
If the MPRX detects an illegal sequence number in a host command message, it discards the received
message and sends no response. If it receives an illegal sequence number in an ACK message, it
responds as if a NAK had been received and retransmits the data.
Wrong Sequence Number
If the MPRX receives the wrong sequence number in an ACK message, it responds as if a NAK had been
received, and it retransmits the data.
Incorrect CRC
If the MPRX detects an incorrect CRC value in a host command message, it discards the received
message. No response is sent. If it receives an incorrect CRC value in an ACK message, it responds as if a
NAK had been received, and it retransmits the data.
Illegal Command
If the MPRX receives an illegal command, it returns its standard #Error message.
Transmission Timeout
If the MPRX transmits an asynchronous message and the host does not send an ACK before the ECP
timeout occurs, the MPRX retransmits the message.
Receive Timeout
If the MPRX receives a <som> but does not receive a matching <eom> before the ECP timeout occurs, it
discards the incomplete message and resets its receiver.
MPRX User Guide
TransCore Proprietary
4–50
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