AMETEK 955DQ Brik LDT - Manual (Page 8)

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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 on option D (output drivers)
units. 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 1K ohm is recommended.
Driving Single Ended Inputs
A differential output can also 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.
The internal resolution of the 955DQ Gemco LDT is
0.001”. This would be represented to the encoder
input device by specifying an output resolution of 1,000
cycles per inch for the transducer. Although the typical
resolution is 1,000 cycles per inch (CPI), the transducer
can be ordered with virtually any CPI setting.
For a typical rotary type shaft encoder with incremental
quadrature output, the output frequency of the pulses is
governed by the resolution of the encoder (pulses per
turn) and the rotational speed (RPM) of the encoder. The
output pulses rate from the transducer is stretched out
over the LDT internal update time. The output frequency
must be specified so that it does not exceed the
maximum pulse rate of the encoder input card the sensor
is connected to. The output pulse frequency range can
be ordered from 1KHz to 1MHz.
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 3-4 or
3-5, Single Ended Interface.
3.3 Quadrature Output
Resolution & Speed