Rockwell-automation 2755 AtomScan Bar Code Reader Manual de usuario descargar pdf (Pagina 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

1 2 ... 110 111 112 113 114 115 116 117 118 119 120 ... 140 141

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Rockwell-automation 2755 AtomScan Bar Code Reader Manual de usuario Pagina 115