AMETEK 952 BlueOx LDT - Manual (Page 17)

17 1080 N. Crooks Road • Clawson, MI 48017 • 800.635.0289 • 248.435.0700 • Fax 248.435.8120 • www.AMETEKAPT.com
It is important that the customer drive the signal
levels much greater or lower than the threshold
voltages. Asserting a signal with a voltage level
close to the threshold voltage could induce multiple
activations of that input (or none at all) and therefore
produce unexpected results or probe readings.
Outputs
The quadrature probe has three outputs, the A, B
and Z outputs. These outputs are differential (also
known as balanced). That is, the connection for
each output consists of two signal wires. These
are typically described as the + and - signals. For
example, the A channel consists of A+ and A-. The
same applies to the B and Z channels. For these
(differential) outputs, the signal is measured with
reference to the other signal (i.e. the difference or
differential). For example; if the A+ signal voltage
is greater than the A- signal, channel A is a logic 1.
Conversely, if the A+ signal voltage is lower than the
A- signal, channel A is a logic 0. Again, this applies
to the B and Z channels as well. Differential type
signals are much less prone to interference caused
by electrical noise or ground loops more often found
in single ended signal connections.
The differential outputs of the A, B, and Z channels
are at RS-422 signal levels. RS-422 is a well known
TIA/EIA standard and common interface type for
incremental encoders. The RS-422 receiver channel
(on the PLC or controller side of the connection)
typically has what is referred to as a termination
resistor connected across the + and - signal pins.
The value of the termination resistor is (by RS-422
specifications) typically 100 ohms. However, some
receivers will work with greater resistance values
and some with no termination resistor at all. For
proper signal integrity, especially at higher data
rates (i.e. quadrature pulse frequency) a termination
resistor of no greater than 1Kohm is recommended.
Driving Single Ended Inputs
A differential output (i.e. our RS-422 drivers) can
also, for most but not all cases, be used to drive
single ended inputs. Special consideration must be
given to these types of applications. It should be
noted the main signal requirements for an RS-422
signal is the differential voltage of the + relative to
the - signals and not necessarily the voltage level
of any one of these signals with respect to ground
(or common). To meet the RS-422 specification,
this differential voltage only needs to be +0.2 volts.
However, an RS-422 driver will typically drive either
the + or - signal to around 3.8 volts with respect to
ground. This voltage is more than sufficient to drive
TTL level inputs as well as other low level inputs.
The input voltage level specifications of the PLC or
controller being used should be consulted for the
actual level required.
When using PLS’s or controllers that are not TTL
compatible output driver option L should be used.
Option L uses a 0L7272 line driver I.C. The output
from this driver will be 1 volt less than the LDT’s
input power.
When physically connecting a differential output to
a single ended input, only use the + signal, leaving
the - signal unconnected (do NOT connect the
- signals to ground). The A+, B+, and Z+ signals
should be connected to their corresponding inputs.
Insulate and tie back the - signals. See Figure 4-2,
Single Ended Interface.