Chapter One
Introduction
Page
9
PLC
WI-MOD
WI-MOD
RS485
PLC
PLC
WI-MOD
PLC
Transpar
ent mode is “point-to-multipoint” communications, suitable for a host device which is able to
communicate on a multi-drop “bus” type network. An example of an application is the use of
radio modems to extend a PLC RS485 network. The serial messages from the PLC’s already
include PLC addressing and error detection/correction to control the flow of data.
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 WI-MOD-9-D
units. Each WI-MOD-9-D 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 analyzer and a monitoring computer system. Intelligent transducers do not
normally provide addressing or error checking functions - these would be provided by the WI-
MOD-9-D 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
HOST
WI-MOD-9-D
DATA
HOST
WI-MOD-9-D
NO DATA
HOST
WI-MOD-9-D
DATA
#1
#2
#3