Select Sequence; Select Reset - Allen-Bradley 2755-LS7-SB User Manual

D–4 Multidrop Communications

Select Sequence

Publication 2755-6.9
Unlike poll requests, select commands always originate from the
host device and consist of serial configuration or operation
commands to devices that are configured in Multidrop. The reader
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 02 address. It is followed
by a REQ (request). The reader responds with its own select
address, 1F hex, and an ACK (acknowledge). The controller then
transmits an STX (start of text), the data (in this case a <T>), an
ETX (end of text), and an LRC character.
The reader replies by transmitting its own address, followed by an
ACK, acknowledging receipt of the command. Upon receipt of an
ACK, the controller concludes the successful exchange with a RES.
In the example below, the reader only acknowledges a trigger
counter request from the controller. The reader does not respond to
the trigger counter request until a subsequent poll. For example, if
the reader trigger count was 12 at the time the trigger counter request
was received, on a subsequent poll the reader would transmit
02T/00012. (The 02 at the beginning of the string is the reader
address.)
Start of Sequence
Concentrator RES 1F REQ
02

Select Reset

If the reader receives bad data from the controller, it transmits a SEL
(its select address) and a NAK to the controller. The controller
re-transmits the data up to three times. The controller will end the
sequence with a RES (reset) if no ACK is received.
STX <T> ETX LRC
1F ACK
End of Sequence
RES
1F ACK