Figure A-7 Polling Sequence - Microscan MS-3000 User Manual

Appendices
Polling Sequence
Data that is transmitted to the host (bar code data, noread messages, counters,
etc.) via concentrators is solicited by poll requests from the host.
Concentrator
Decoder 02
The polling sequence example in figure
the concentrator followed by poll address 1E (ASCII hex value for Decoder
02) and a REQ (request). The decoder 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 decoder and is
able to validate it with an LRC calculation, it responds with an ACK
(acknowledgment). If the decoder in turn receives the ACK, the decoder
ends this successful exchange with a RES (reset).
Polling Reset
• If the decoder has no information, it responds to a poll request by trans-
mitting a RES (reset).
• If the decoder receives a NAK instead of the ACK after transmitting its
data string, it will attempt to transmit the data string up to three times. If
the decoder still does not receive an ACK, it will transmit a RES (reset)
and discard the data in its buffers.
• If the DECODER transmits data to the concentrator and the concentrator
responds with an ACK or NAK, but the decoder doesn't receive the concen-
trator's response, the decoder will timeout and transmit a REQ to the con-
centrator and request another response. If after three retries (the number of
times it transmits a REQ to the concentrator) the decoder receives no
response, it ends the transmission with a RES (reset).
A-18
MS-3000 Single Head Decoder User's Manual
Start of Sequence
RES 1E REQ
1E STX DATA ETX LRC

Figure A-7 Polling Sequence

A-7 begins with a RES (reset) from
End of Sequence
ACK
RES