Specifications

Page 19–2
FOCUS System Manual
the remote device; with internal addressing, the
master unit uses the address of the PLD module to
which
the target device is attached. Following are
descriptions and examples of both types of applica-
tions.
19.2.1 Applications Using External
Addressing
The most typical application for the Party Line
Data (PLD) Module is a network of polled RTUs.
Figure 19-1 shows a typical application using
external addressing. As noted earlier, external
addressing means that the master unit uses the
address of the target RTU, rather than the PLD
module. Thus, you do not assign an address to the
PLD modules in the network. Because only one
RTU at a time is communicating with the master
unit, only one DS0 time slot is required. Thus, you
can use a single channel to poll a larger number of
RTUs from a single master unit.
You set up this type of application with one
FOCUS terminal connected to the master
computer. The connection between the PLD
module’s interface and the master computer
requires a 9-pin sub “D” connector wired to the
“TXDATA,’ “RXDATA,” and “SIG GND” of the
master computer (see “Hardware Installation and
Connections” later in this chapter for specific
wiring examples).
The PLD module connected to the master
computer is configured as the “Head” module. To
configure the “Head” for external addressing, the
module must first be mapped to a DS0 channel.
During the mapping process, the user will be
prompted “Do you wish this module to be a Head
or End module?”. Selecting “Yes” displays
another dialog box where the choice of “Head” is
made. The “Head” designation means the module
is the beginning of the PLD “chain” or system. It’s
DS0 channel is mapped in the direction of the first
“Remote” or the “End” module. See “Software
Settings” later in this chapter for programming
details.
Next configure all other PLD modules as
“Remote” modules except the last PLD in the
string which will be configured as an “End”
module.
Once the master unit has been connected to the
“Head” PLD, the remote devices have been
connected to either “Remote” or “End” PLDs and
all PLDs have been mapped and configured, the
system is set to collect data. The data collection
operation begins when the master computer sends
out a message to have a particular RTU send its
current status. This message contains a command
and the target RTU’s address. All the RTUs in the
network receive this message, but only the one
that is addressed responds. The addressed RTU’s
first response is to assert its RTS (Request To
Send) output line. This is detected by the attached
PLD module, which passes the request on to the
FOCUS Maintenance Module. The Maintenance
Module responds as soon as the return path is
connected. That is, when the PLD connection has
been switched from a “Drop” to an “Insert”
connection. This switch occurs within 10 millisec-
onds. The PLD module then asserts the CTS
(Clear To Send) line and allows the RTU to send
its status to the master. Some RTUs do not require
the receipt of CTS to send it’s data but, simply
sends the data after a time delay. In this case the
time delay must be set to a minimum of 10ms.
After completing its transmission, the RTU drops
its RTS line. At this point, the PLD module signals
the Maintenance Module to terminate the “Insert”
connection and return to the “Drop” connection or
remote mode (This also happens within 10
milliseconds.). The cycle can now be repeated
again at this or any other RTU in the network.