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These 10 free FPABS questions are organized by exam domain, so you can see how each part of the Fundamentals of PLCs - Allen Bradley/Siemens blueprint is tested. Reveal the answer and explanation under each question.
An ordinary 24 VDC control relay causes a voltage spike when its coil is switched off. The equipment manufacturer approves a flyback diode for this circuit. With power isolated, the technician must install it so that it does not conduct while the coil is energized. Where does it belong?
Correct answer: A - Across the coil, with the cathode at the coil's positive terminal.
A remote I/O adapter has been replaced. Its Ethernet link is up, and it answers a ping, but cyclic I/O data never reaches the controller. The controller's configured I/O connection uses EtherNet/IP; the replacement adapter supports PROFINET only. Keeping that controller configuration, which remedy addresses the incompatibility?
Correct answer: B - Fit an EtherNet/IP adapter compatible with the configured I/O connection.
On an Allen Bradley controller, a normally closed Stop pushbutton supplies the input tag Stop_OK: the tag is 1 when the button is released and 0 when pressed. The motor rung contains parallel Start and seal-in branches. To make the Stop button interrupt both starting and holding, how should Stop_OK be used?
Correct answer: B - As an XIC instruction in series with the Start/seal-in branches.
A pump's HMI symbol stays green after an overload trip because its animation uses the PLC's Run_Command bit. The operator needs that symbol to mean 'pump shaft confirmed rotating,' not merely 'start requested.' Which available signal should drive the running indication?
Correct answer: C - The shaft-speed sensor's validated rotation-detected signal.
A production line is operating normally. A technician connects a service laptop for authorized read-only monitoring, but it cannot reach the PLC. Both devices connect to the same isolated Ethernet switch; no router is present. The PLC is 192.168.40.20 and the laptop is 192.168.41.50, both with mask 255.255.255.0. Address 192.168.40.50 is reserved for this laptop and is unused. What restores access without changing the running machine's network configuration?
Correct answer: A - Set the laptop to 192.168.40.50 and retain its subnet mask.
During a Siemens S7-1200 maintenance observation, the engineering laptop disconnects. It was only displaying live values; the machine does not obtain commands or process data from it. The CPU remains in RUN, the dedicated HMI and I/O remain connected, and operation is normal. The supervisor asks whether the laptop must be reconnected before production continues. The technically sound response is:
Correct answer: D - No; the CPU keeps scanning its resident program without the monitoring laptop.
A solenoid will not energize. During authorized electrical diagnosis, the technician records: the initiating input tag is 1; all rung conditions are true; the output command is 1; and the channel's command-status LED is lit. The CPU supply measures 24 VDC, but the output group's separate field-supply terminals measure 0 V. Which part of the input-logic-output chain has a demonstrated problem?
Correct answer: C - The output section lacks its required external field supply.
A tank-level transmitter produces 4-20 mA for 0-200 cm, with a linear relationship. The PLC receives 12.8 mA. Its non-latching high-level alarm is active only when the scaled level exceeds 120 cm. Select the correct level and alarm state.
Correct answer: D - 110 cm; high-level alarm inactive.
After a power interruption, a Siemens S7-1200 still contains the correct program, but an operator-adjusted batch setpoint returns to its start value. The setpoint is in an optimized global data block and is marked non-retentive; no startup instruction overwrites it. Which change preserves the operator's latest value through a future power cycle?
Correct answer: C - Enable the Retain property for that setpoint in the data block.
An Allen Bradley Logix routine checks the same input tag in several rungs. The I/O module can update that tag asynchronously, so two checks in one routine execution can see different states. An inspection decision must use one consistent snapshot for the entire execution. Which design provides it?
Correct answer: A - Copy the input once on entry; use that unchanged internal copy throughout.
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