TransCore MPRX, System Manual

Page: 73 / 208

MPRX
TransCore Proprietary 6–3
Basic Protocol
With basic protocol, messages sent to and from the MPRX and the host are transmitted
without error checking. For each host transmission, the MPRX returns a
Done
or
Error
message to the host.
When the host device is physically close to the MPRX and no sources of interference
exist, the basic protocol provides reliable communications.
The host must be ready to receive reader-transmitted messages because in basic
protocol the MPRX does not wait for the host to acknowledge a message before
transmitting the next message. If necessary, the host may halt reader transmissions by
using software flow control. Refer to
“Chapter 8 Configuring the MPRX” on page 8–2
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 MPRX and the
host.
Note: TransCore recommends that basic protocol (not ECP) be used when commands
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.
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.
Message sequence numbers are also included when using ECP. These sequence numbers
are checked to determine if the message received has the correct sequence number; if
not, the recipient rejects the message.
Because 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.
Parity is required to achieve the most reliable communications. If parity is enabled, both
the MPRX and the host must issue a message if any received character has a parity error.
However, the message must not be transmitted before receipt of the
eom
character. If the
message is transmitted prematurely, the MPRX will issue an
Error
message, and the
host device will issue a negative acknowledgment message.