AMETEK 7250 Digital Stick - Manual (Page 2)

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7250 Series Manual
Overview
This document explains the basic hardware
interface requirements, operating characteristics
and describes the software protocols for the 7250
series HR Digital Stik. In addition, the dimension
and installation drawings, part number sequence
and agency approvals have been included in the
back of this document. It is important to note
that the standard product is intrinsically safe and
any device, controller, or radio connected to this
product must have a barrier designed to meet
the entity parameters and special requirements
indicated in the installation drawing E0241200
(page 6).
Electrical Interface
Signal Descriptions
The 7250 requires only three wires for its power
and interface connections, Power, Data, and
Ground. On the Stainless Steel housing models,
the cable shield or “Shield” pin in the 4 pin
connector, is connected to the steel housing and
must be connected to earth ground.
Power Supply
The supply voltage to the probe “Power” is
typically +5VDC but can range from 3.7VDC to
7.93VDC. The main power supply in the 7250 is
not a “switching” design, so running it at a higher
voltage does not reduce the input current draw.
Data Signal
The “Data” signal is an “open-drain” type signal
and is used for the bidirectional half duplex
asynchronous serial communications. Any device
connected to the probe must be of an “open-
drain” type signal and must not be driven to a
high logic level. Because this data signal may be
driven by either the master or any slave device, a
single pull up resister of typically 1kΩ should be
the only element that establishes the high logic
level voltage. Also, because of this scheme,
there could be multiple master or slave devices
connected together.
The logic level voltage thresholds are similar to
TTL levels and a pull-up resistor must be included
in the user’s interface circuitry. This signal is
clamped internally by the 7250 with a +5V TVS
device. The inactive or “idle” state is at a “high”
logic level.
Ground
The “Ground” connection is the common return
path for both “power” and “data”.
Intrinsically Safe Connections
The 7250 is an intrinsically safe device and
therefore, when used in an intrinsically safe
installation or application, must be connected to
an apparatus that limits the power, voltage, and
current to the 7250 in accordance to the entity
parameters specified. Reference the installation
drawing E0241200 (page 6) for more details.
Operation
The 7250 runs continuously once power is
applied to it. When connected to a battery
powered device (typically a wireless radio),
power is supplied for a short duration of time,
long enough to collect the required amount of
data, then power is switched off to the probe in
an effort to reduce the power draw and conserve
battery life. If power is applied to the 7250 on a
continuous basis, it will continue to take level and
temperature readings and communicate the data
through the asynchronous serial communications
signal.
Power Consumption
The 7250 draws roughly 10mA of current when
it is not taking temperature measurements
and roughly 12mA of current when it is taking
temperature measurements (with 5 temperature
sensors). Figure 1 shows the current draw of a
model 7252 which takes the temperature readings
in the beginning (approximately 700mS) and also
takes 25 product and 1 interface (water) reading
at 100mS per reading. As seen in the chart, this
is represented by the 700mS high level followed
by the 2 second lower reading. The signal in the
screen capture is a voltage reading taken across
a 10 ohm resistor in series with the power supply
input. The scale for this reading is 1mA per minor
division and 5mA per major division.
Software Protocol
There are several different product numbers
available for the 7250 Series of probes. The ‘x’ in
the part number 725x identifies the data protocol.
The electrical interface remains the same,
regardless of the software protocol specified by
the model number.