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A-34 Quadrus EZ Reader User’s Manual

Multidrop Communications

Appendix P — Multidrop Communications

This appendix describes the rules for setting up a concentrator or controller to communi-

cate with a reader in standard Multidrop protocol.

The diagram to the right shows a typical Multi-

drop network in which 1 to 50 readers can com-

municate with a host via an intermediary device,

a concentrator or a controller.

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 by poll address

1E (ASCII hex value for Reader 02) and a REQ

(request). The reader responds by first transmit-

ting 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) char-

acter.

If the concentrator (or controller) 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 reader ends this suc-

cessful exchange with a RES (reset).

Polling Reset

• If the reader has no information, it responds to a poll request by transmitting a RES

(reset).

• If the reader 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 reader still does not receive an

ACK, it will send a RES (reset) and discard the data in its buffers.

• If the reader transmits data to the concentrator and the concentrator responds with an

ACK or NAK, but the reader doesn’t receive the concentrator’s response, the reader 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 reader

receives no response, it ends the transmission with a RES (reset).

Reader 01

Reader 02

Reader 03

Host

Concentrator

Multidrop ONLY

RS-485 Interface

Polled Protocol

Multidrop

Concentrator RES 1E REQ ACK

Reader 02 1E STX DATA ETX LRC RES

Start of Sequence End of Sequence

Polling Sequence

1 2 ... 333 334 335 336 337 338 339 340 341 342 343 ... 365 366

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