Multidrop Communications
MS-850 Raster Scanner User’s Manual
A-19
Appe
ndices
Appendix J — Multidrop Communications
This appendix describes the rules for setting up a concentrator or controller to com-
municate with a scanner in standard Multidrop protocol.
The diagram to the right shows a typical
Multidrop network in which 1 to 50 scan-
ners can communicate with a host via an
intermediary device, a concentrator or a
controller.
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.
The polling sequence example 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
validate it with an LRC calculation, it responds with an ACK (acknowledgment). If
the scanner in turn receives the ACK, the scanner ends this successful exchange
with a RES (reset).
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 transmit the data string up to three times. If the scanner still
does not receive an ACK, it will transmit a RES (reset) and discard the data in its
Scanner 01
Scanner 02
Scanner 03
Host
Concentrator
Multidrop ONLY
RS-485 Interface
Polled Protocol
Concentrator
RES 1E REQ
ACK
Scanner 02
1E STX DATA ETX LRC
RES
Start of Sequence
End of
