TransCore SmartPass AI1611, System Manual

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SmartPass® Integrated Reader 2450 MHz (High Voltage) System Guide
Asynchronous Message/Command Message Collision
If the host receives an asynchronous reader transmission at the same time it transmits
a command, it should ignore the asynchronous message and wait for the reader's
response. The reader will retransmit asynchronous data after it transmits the
command message.
Error Correcting Protocol 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
1 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 specification, including the timing specifications discussed in the
“Timing and Synchronization” section previously in this chapter.
CRC Calculation
The CRC used by the ECP is based on a 16-bit algorithm. The algorithm, as
implemented, operates on 8-bit characters (i.e., 7-bit ASCII character plus 1 optional
parity bit). The 16-bit result is converted to 4 ASCII hex characters and is appended
to messages transmitted by the reader.
The reader accepts 4 ASCII <`> characters (60 hex) as a wild card CRC in lieu of a
valid 4-character CRC to facilitate testing and diagnostic checkout.
The reader implements the algorithm with a 512-byte lookup table to reduce the
processing overhead requirements. However, this method requires additional code
memory.
To simplify the implementation of the CRC algorithm by host software developers,
several examples of the calculation are provided in C source code in Tables 8-1
through 8-4. The calculation may be performed with or without a lookup table,
depending upon the tradeoff between code memory and processing overhead.
An example of a function (CALCCRC) that calculates the CRC through a call to a
separate function (UPDCRC) is given in Table 8-1. An example of UPDCRC that
does not require a lookup table is given in Table 8-2. An example of UPDCRC that
does require a lookup table is given in Table 8-3. Finally, a function that creates the
lookup table is given in Table 8-4.
8-14