Super Systems 3L Series - Manual (Page 39)

Series 3L Indicators
39
11. Digital Communications
Digital Communications (or ‘comms’ for short) allows
the indicator to communicate with a PC or a networked
computer system.
This product conforms to MODBUS RTU
 protocol a
full description of which can be found on
www.modbus.org.
Two ports are available both using MODBUS RTU
communication facilities:
1. a configuration port - intended to communicate
with a system to download the instrument
parameters and to perform manufacturing tests
and calibration
2. an optional EIA 485 port on terminals HD, HE and
HF - intended for field communications using, for
example, a PC running a SCADA package.
The two interfaces cannot operate at the same time.
Each parameter has its own unique ModBus address.
A list of these is given at the end of this section.
11.1 Digital Communications Wiring
11.1.1 EIA 485
To use EIA 485, buffer the EIA 232 port of the PC with
a suitable EIA 232/RS485 converter. The use of an
EIA 485 board built into the computer is not
recommended since this board may not be isolated,
which may cause noise problems, and the RX
terminals may not be biased correctly for this
application.
To construct a cable for EIA 485 operation use a
screened cable with one (EIA 485) twisted pair plus a
separate core for common. Although common or
screen connections are not necessary, their use will
significantly improve noise immunity.
The terminals used for EIA 485 digital communications
are listed in the table below.
Standard
Cable
Colour
PC Function
*
Instrument
Terminal
Instrument
Function
White Receive,
RX+ HF (B) or
(B+) Transmit, TX
Red Transmit,
TX+ HE (A) or
(A+) Receive, RX
Green Common HD Common
Screen Ground
•
These are the functions normally assigned to
socket pins. Please check your PC manual to
confirm.
See section 1.11 for wiring diagrams.