Figure A-12 Polling Sequence - Microscan MS-9 User Manual

Multidrop Communications
Polling Sequence
Data that is transmitted to the host (symbol data, noread messages, counters, etc.) via
concentrators is solicited by poll requests from the host.
The polling sequence example is 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 vali-
date it with an LRC calculation, it responds with an ACK (acknowledgment). If the scan-
ner in turn receives the ACK, the scanner ends this exchange with a RES (reset).
Concentrator
Scanner 02
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 con-
centrator) the scanner receives no response, it ends the transmission with a RES
(reset).
MS-9 Scanner User's Manual
A-34
Start of Sequence
RES 1E REQ
1E STX DATA ETX LRC

Figure A-12 Polling Sequence

End of Sequence
ACK
RES