Chapter One
Introduction
Man_805U Rev 1.8
Page
9
.
1.3
Controlled Mode
“Controlled mode” provides “point-to-
point” communications similar to telephone
modems. In controlled mode, the flow of
data is controlled by the 805U units. Each
805U unit is configured with an address by
the user, and a destination address for the
data to be transmitted to. Data is
transmitted addressed to the destination
module, and only this module will output
the serial data. The source module will add
an error-check (16 bit CRC) to the data
transmitted by radio. The destination module will process the error-check, and if correct, it
will transmit an acknowledgment message (ACK) back to the source module. If the source
module does not receive a ACK, it will re-transmit the data. The source module will attempt
to transmit the data up to five times, until an acknowledgment (ACK) is received. If an
acknowledgment is still not received, then the DCD signal on the RS232 port will be reset
and a alarm message can be sent to the host via the serial port.
An example of an application using controlled mode would be a radio modem link between
an intelligent gas analyser and a monitoring computer system. Intelligent transducers do not
normally provide addressing or error checking functions - these would be provided by the
805U modules.
In controlled mode, the destination address may be pre-set, or set on-line by the host device
using “Hayes” commands. Hayes commands are a standard set of commands used with
conventional telephone modems. An example of an application that would use Hayes
command to set destination addresses would be a central computer polling data loggers for
periodic information.
PLC
805U
805U
RS485
PLC
PLC
805U
PLC
HOST
805U
DATA
HOST
805U
NO DATA
HOST
805U
DATA
#1
#2
#3
