Watlow Controls 8LS, User Manual

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20 8LS User’s Guide
Installation
NOTE
When using the RTB, proper polarity of the flat
ribbon cable is necessary for correct pin termina-
tions. Install the red marker indicating pin #1 to
the left when terminal #1 of the RTB is to the left.
The flat ribbon cable should have the red marker
down when installing it on the back of the 8LS. To
confirm proper polarity, check that pin #1 is +5vdc
with respect to pin #3.
Analog Inputs
Connecting analog signals to the 8LS is normally straightforward. Most
thermocouples can be directly connected and mixed in any order. Other
types of analog signals such as RTD's or mAdc or Vdc require scaling
resistors installed on the 8LS inputs. Some problems may occur that
could reduce accuracy and possibly damage the unit. Below are some of
the potential areas for concern.
Common Mode Voltage
Common mode voltage is the voltage between the ground at the sensor
and the ground at the 8LS. It can be an AC or DC voltage and appears
equally at the high and low input terminals. Frequently it is caused by
large currents flowing in the ground path between the 8LS and the
sensors. Use isolated sensors or ungrounded thermocouples and locate
the 8LS as close as possible to the sensors in order to minimize the
effects. Do not exceed the maximum common mode voltage of 175 Vac.
Normal Mode Voltage
Normal mode voltage appears across the terminals of the input and is
the signal from the sensor plus any undesirable noise. The major cause
of this noise is AC power line pick-up. The effects are reduced by the
8LS capacity to integrate the signal over a multiple of the power line
frequency. Further reduction can be achieved by locating the 8LS near
the sensors and by using twisted and shielded sensor wires.
To ensure accurate readings, the maximum of normal mode plus signal
should not exceed -10mv to +60mv.
Grounding
For best accuracy, observe the grounding recommendations for
connecting each input and output signal. The analog signal grounds
should be connected to the analog ground terminals. The
communication and control outputs should be connected to their
respective grounds. Do not mix the grounds or connect them together. If
possible, route the analog signal cables separately from the
communication, control and power cables.