TransCore encompass 4 - Error Correcting Protocol (ECP)

Encompass 4 Reader System Guide
6-4
Basic Protocol
With basic protocol, messages sent to and from the Encompass 4 Reader and the host
are transmitted without error checking. For each host transmission, the Encompass 4
Reader returns a Done
or Error message to the host.
When the host computer is physically close to the Encompass 4 Reader 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 proto-
col the Encompass 4 Reader does not wait for the host to acknowledge a message
before transmitting the next message. If necessary, the host may halt reader transmis
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sions by using software or hardware flow control. Refer to Chapter 8 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 Encompass 4
Reader and the host.
Note: TransCore recommends that basic protocol (not ECP) be used when commands
are entered manually at the keyboard.
Wiegand output is automatically disabled when the reader is put into ECP mode.
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 num-
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.
Parity is required to achieve the most reliable communications. If parity is enabled,
both the Encompass 4 Reader and the host must issue a message if any received char
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acter has a parity error. However, the message must not be transmitted before receipt
of the eom character. In this case, the Encompass 4 Reader will issue an Error mes
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sage, and the host computer will issue a negative acknowledgment message.