102
Communications Operation
5.2.2 EIA RS-422 and EIA RS-485 Standards
Both RS-422 and RS-485 are serial differential interface standards, permitting
reliable communications for distances up to 1200 meters. Because of the differential
mode used, line noise is nullified.
In the RS-422 standard, the interconnection of instruments with a computer is
performed via two pairs of lines, one pair for transmission, and one for reception
(full duplex). In the RS-485 standard, instruments are connected to a computer, via
a pair of lines. The same pair is used both for transmission and reception (half
duplex).
5.3 Configuring The Communications Port
Before connecting the Powermeter to a master computer or a printer, the
Powermeter communications port should be configured to match that of the master
or a printer. Communications parameters alter the way in which the Powermeter
interacts with a master computer, but does not affect the electrical operation of the
serial interface that is only depend on the serial connector pins to which the
communications line is connected.
Table B-3
(see
Appendix B
) lists the communications parameters that should be
defined by the user. These parameters must have the same values as those on either
the computer or on the printer, connected to the Powermeter.
The optional flow control parameters can be used to define functions of the
additional DTR/RTS control line, or to select type of the flow control protocol that
may be necessary to adjust transmission rate of the Powermeter to the needs of a
master or a printer.
The following sections explain communications setup parameters and their
operations.
5.3.1 Communications Mode
Communications mode defines communications protocol the Powermeter will
support and its possible variations. The user can choose between ASCII protocol
and Modicon's Modbus RTU protocol. The protocols' rules are specified in
supplement publications mentioned in
Section 5.1
. When connecting to a master
computer via a modem, you should select the
ASCII
or
tElE
entry. When
tElE
mode
is selected, the Powermeter will work as if ASCII mode was chosen, but at power
up
it will send an initialization string in order to set the modem to auto answer mode.
