Microscan MS-890, Руководство пользователя

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MS-890 Industrial Automation Scanner User Manual
Multidrop Communications
Polling Sequence
Data that is transmitted to the host (symbol data, No Read messages, counters, etc.) via
concentrators is solicited by poll requests from the host.
The polling sequence example by poll address 1E (ASCII hex value for Scanner 02) and a
REQ (request). The scanner responds by first transmitting its own address, 1E, followed
by a STX (start of text) character, and then the data. Next it transmits an ETX (end of text)
character and an LRC (longitudinal redundancy check) character.
If the concentrator (or controller) receives the data from the scanner and is able to validate
it with an LRC calculation, it responds with an ACK (acknowledgment). If the scanner in
turn receives the ACK, the scanner ends this successful exchange with a RES (reset).
Polling Reset
• If the scanner has no information, it responds to a poll request by transmitting a RES
(reset).
• If the scanner receives a NAK instead of the ACK after transmitting its data string, it will
re-attempt to send the data string up to three times. If the scanner still does not receive
an ACK, it will send a RES (reset) and discard the data in its buffers.
• If the scanner transmits data to the concentrator and the concentrator responds with an
ACK or NAK, but the scanner doesn’t receive the concentrator’s response, the scanner
will timeout and send a REQ to the concentrator and request another response. If after
three retries (the number of times it transmits a REQ to the concentrator) the scanner
receives no response, it ends the transmission with a RES (reset).
Concentrator RES 1E REQ ACK
Scanner 02 1E STX DATA ETX LRC RES
Start of Sequence
End of Sequence
Polling Sequence
End of Sequence