Microscan MS-810, User Manual

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Chapter A
A-34
MS-810 Industrial Scanner User’s Manual
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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 buffers.
• If the SCANNER transmits data to the concentrator and the concentrator responds with
an ACK or NAK, but the scanner doesn’t receive the concentrator’s response, the scan-
ner will timeout and transmit 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
scanner receives no response, it ends the transmission with a RES (reset).
Select Sequence
Unlike poll requests, select commands always originate from the host and consist of
serial configuration or operation commands to devices that are configured in Multidrop.
The scanner complies with the command when it is polled during the cycle.
Figure A-13 is an example of a select sequence.
Figure A-11 Select Sequence
A RES (reset) is the first command in the select sequence. The 1F hex is the select address
associated with Scanner 02 address (see Table A-12, “Multidrop Address Charac-
ters” ). It is followed by a REQ (request). The scanner responds with its own select
address, 1F hex, and an ACK (acknowledge). The concentrator then transmits an STX
(start of text), the data (in this case a <T>), an ETX (end of text), and an LRC character.
The scanner replies by transmitting its own address, followed by an ACK, acknowledging
receipt of the command. Upon receipt of an ACK, the concentrator concludes the suc-
cessful exchange with a RES.
In the example above, the scanner only
acknowledges a trigger counter request from
the concentrator. It does not respond to the trigger counter request until a subsequent
poll. For example, if the scanner’s trigger count was 12 at the time the trigger counter
request was received, on a subsequent poll it would transmit 02T/00012. (The 02 at the
beginning of the string is the scanner’s address.)
Select Reset
• If the scanner receives bad data from the concentrator, it transmits a SEL (its select
Concentrator RES 1F REQ STX <T>ETX LRC RES
Scanner 02 1F ACK 1F ACK
Start of Sequence
End of Sequence