Rockwell Automation 2755 AtomScan Bar Code Reader - Manual (Page 115)

Multidrop Communications D–3
Publication 2755-6.9
The polling sequence occurs when the host device solicits
information from the readers. The host device sends out poll
requests to the readers.
The polling sequence example below begins with a RES (reset) from
the controller followed by poll address 1E (ASCII hex value for
reader 02) and a REQ (request). The reader 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 controller (or concentrator) receives the data from the reader
and is able to validate it with an LRC calculation, it responds with an
ACK (acknowledgment). If the reader in turn receives the ACK, the
ends this successful exchange with a RES (reset).
Concentrator RES 1E REQ ACK
Reader 1E STX DATA ETXLRC RES
Start of Sequence End of Sequence
Polling Reset
A polling reset is when the polling sequence is interrupted. A
polling reset occurs when:
•
the reader has no information and it responds by transmitting a
RES (reset),
•
the reader receives a NAK instead of the ACK after transmitting
its data string. The reader will re-attempt to transmit the data
string up to three times. If the reader still does not receive an
ACK, it will transmit a RES (reset) and discard the data in its
buffers, or
•
the reader transmits data to the controller and the controller
responds with an ACK or NAK, but the reader doesn’t receive the
controller’s response. The reader will timeout and transmit a
REQ to the controller and request another response. If after three
retries (the number of times it transmits a REQ to the controller)
the reader receives no response, it ends the transmission with a
RES (reset).
Polling Sequence