100/180 mm PAPERLESS GRAPHIC RECORDER: USER GUIDE
User Guide
Page 149
HA028910
Issue 13 Sep 16
4.3.15 Connections
INTRODUCTION
ASCII (INPUT)
Note: See
Section 12
for details of the ASCII Printer Output option.
This mode allows the recorder to receive simple ASCII messages from, for example, barcode readers,
Programmable Logic Controllers (PLCs), Global Positioning Systems (GPSs) (NMEA-0183 protocol) etc.
Messages are sent to as many groups as are set up to receive them, and become a part of these groups’
histories, and appear on vertical and horizontal trend displays in the following format:
Date and time
Message
Source
23/08/05 10:25:06 (Serial), Message of up to 85 characters
MODBUS RTU
This allows the recorder’s serial communications ‘port’ to be used as Modbus master or Modbus slave.
When acting as a master, the unit can communicate with up to 16 slaves. Use of the Serial port is comple-
mentary to the use of Ethernet connections.
Note: When confi guring a unit as a Modbus slave, it is recommended that the Instrument and
Point Confi gurations give easily identifi able instrument and point names (descriptors).
INSTALLATION
If the option is fi tted, two 9-way D-type connectors (plugs) are fi tted at the rear of the recorder, as shown
in
section 2.2
.
TERMINATION AND BIASSING (Not EIA232)
If the communications line is left open-ended, the end of the cable acts as a refl ector, returning what
can appear to be ‘true’ data signals back down the line. A receiver cannot distinguish between ‘true’ and
refl ected data, with the result that the ‘true’ data is corrupted.
In order to avoid this, a termination resistor is fi tted across the line at the fi nal instrument. If the value of
this resistor is equal to the characteristic impedance of the cable (e.g. 120 Ohms), then the line appears
to be of infi nite length and no refl ections occur. Such a value however, does not always give the best
signal-to-noise ratio, so a compromise value (e.g. 220 Ohms) is normally selected to give the optimum
performance in reducing unwanted refl ections and in improving the signal-to-noise ratio. The recorder
communications port is terminated as shown in fi gure 4.3.15a, below, for a single point-to-point applica-
tion.
Figure 4.3.15a Recorder termination (single-drop EIA485 application)
RxB
Recorder
Transmit lines
from sender
TxB
TxA
RxA
0 Volts
5 Volts
100k
Ω
100k
Ω
Receiver
device
220
Ω
