TransCore 05716 Manuel D’Utilisation
Multiprotocol Rail Reader System Guide
6-4
Basic Protocol
With basic protocol, messages sent to and from the MPRR and the host are transmitted
without error checking. For each host transmission, the MPRR returns a Done
without error checking. For each host transmission, the MPRR returns a Done
or
Error
message to the host.
When the host computer is physically close to the MPRR and no sources of interfer-
ence exist, the basic protocol provides reliable communications.
ence exist, the basic protocol provides reliable communications.
The host must be ready to receive reader-transmitted messages because in basic proto-
col the MPRR does not wait for the host to acknowledge a message before transmit-
ting the next message. If necessary, the host may halt reader transmissions by using
software or hardware flow control. Refer to Chapter
col the MPRR does not wait for the host to acknowledge a message before transmit-
ting the next message. If necessary, the host may halt reader transmissions by using
software or hardware flow control. Refer to Chapter
for flow control information.
Error Correcting Protocol
When the quality of data communications is imperative or may be suspect, you can
invoke ECP to ensure the integrity of data transmitted between the MPRR and the
host.
invoke ECP to ensure the integrity of data transmitted between the MPRR and the
host.
Note: TransCore recommends that basic protocol (not ECP) be used when commands
are entered manually at the keyboard.
are entered manually at the keyboard.
Error correction is accomplished with the use of a cyclic redundancy check (CRC)
value that is based on the message data. The originator (reader or host) calculates the
CRC value of a message and includes it in the transmitted message.
value that is based on the message data. The originator (reader or host) calculates the
CRC value of a message and includes it in the transmitted message.
The recipient (reader or host) also calculates a CRC value for the received message. If
the transmitted message data is correct, the CRC value calculated by the recipient will
agree with the CRC value calculated by the originator. If the CRC values do not agree,
the recipient rejects the message.
the transmitted message data is correct, the CRC value calculated by the recipient will
agree with the CRC value calculated by the originator. If the CRC values do not agree,
the recipient rejects the message.
Message sequence numbers are also included when using ECP. These sequence num-
bers are checked to determine if the message received has the correct sequence num-
ber; if not, the recipient rejects the message.
bers are checked to determine if the message received has the correct sequence num-
ber; if not, the recipient rejects the message.
Since the seven-bit ASCII code is used and there are eight data bits per character, the
eighth bit can optionally be used to support parity. Where parity is selected, the CRC
value calculation includes the parity of each character in the calculation of the CRC
value.
eighth bit can optionally be used to support parity. Where parity is selected, the CRC
value calculation includes the parity of each character in the calculation of the CRC
value.
Parity is required to achieve the most reliable communications. If parity is enabled,
both the MPRR and the host must issue a message if any received character has a par-
ity error. However, the message must not be transmitted before receipt of the eom
character. In this case, the MPRR will issue an Error message, and the host com-
puter will issue a negative acknowledgment message.
both the MPRR and the host must issue a message if any received character has a par-
ity error. However, the message must not be transmitted before receipt of the eom
character. In this case, the MPRR will issue an Error message, and the host com-
puter will issue a negative acknowledgment message.