Asynchronous Message/Command Message Collision
If the MPRX transmits asynchronous data at the same time that the host sends a command, the MPRX
gives priority to receiving the command. It processes the command and sends a message before it
retransmits the asynchronous data.
Host-Addressed Failure Conditions
The host device addresses the following failure conditions.
Illegal or Wrong Sequence Number
If the host detects an illegal or wrong sequence number in a reader response, it retransmits the command
with the same sequence number. If the host detects an illegal sequence number in an asynchronous
reader transmission, it sends a
NAK
message.
Incorrect CRC
If the host detects an incorrect CRC value in a reader message, it retransmits the command with the same
sequence number. If the host detects an incorrect CRC value in an asynchronous reader transmission, it
transmits a
NAK
message.
Transmission Timeout
If the MPRX does not respond to a host command within a specified interval, the host software retransmits
the command with the same sequence number.
Receive Timeout
If the host receives a
<som>
but does not receive a matching
<eom>
within a specified timeout interval, it
discards the incomplete message and resets its receiver.
Asynchronous Message/Command Message Collision
If the host receives an asynchronous reader transmission at the same time it transmits a command,
it ignores the asynchronous message and waits for the MPRX’s response. The MPRX retransmits
asynchronous data after it transmits the command message.
ECP Reliability
An undetected error is defined as a message having incorrect data or status but no parity or CRC errors.
An error transaction is defined as a message having either a parity or CRC error. Laboratory testing
indicates an undetected error rate of less than one undetected error per 1,000,000 error transactions with
parity enabled.
To ensure this error rate is not exceeded, the host must enable parity and adhere closely to the timing
specifications discussed previously in
“Timing and Synchronization” on page 4–49
.
CRC Calculation
The CRC used by the ECP is based on a 16-bit algorithm. The algorithm, as implemented, operates on
eight-bit characters, for example, a seven-bit ASCII character plus one optional parity bit. The 16-bit result
is converted to four ASCII hex characters and is appended to messages transmitted by the MPRX.
Chapter 4 Communications Protocols
TransCore Proprietary
4–51
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