Appendices
A-20
MS-850 Raster Scanner User’s Manual
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• 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 scanner 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 Multi-
drop. The scanner complies with the command when it is polled during the cycle.
A RES (reset) is the first command in the select sequence. The 1F hex is the select
address associated with Scanner 02 address. 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, acknowl-
edging receipt of the command. Upon receipt of an ACK, the concentrator concludes
the successful 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
address) and a NAK to the concentrator. The concentrator re-transmits the data up
to three times. The concentrator will end the sequence with a RES (reset) if no ACK
is received.
Concentrator RES 1F REQ
STX <T>ETX LRC
RES
Scanner 02
1F ACK
1F ACK
Start of Sequence
End of Sequence
