Industrial Automation & PLC test (Intermediate)

Sample questions

5 sample questions from the Industrial Automation & PLC test, with the answers. Samples are taken out of the live test, so you will not meet these when you sit it.

1. Which Modbus exception response code indicates an illegal function?

   1. 0x81
   2. 0x84
   3. 0x83
   4. 0x82

   Answer: A. 0x81

   Modbus exception response codes are used to indicate errors in a Modbus communication. The exception response is a 3-byte message that starts with the function code that was requested, followed by the exception code. The exception code 0x01 indicates an illegal function, meaning that the function code received is not recognized or allowed by the slave device. Other exception codes include 0x02 (illegal data address) and 0x03 (illegal data value).

2. In a ladder logic diagram, if a coil is connected in series with two normally open contacts, what is the condition for the coil to activate?

   1. The coil will activate when both contacts are open (i.e., when both inputs are false).
   2. The coil will activate when either contact is closed (i.e., when at least one input is true).
   3. The coil will activate regardless of the state of the contacts.
   4. The coil will activate only when both contacts are closed (i.e., when both inputs are true).

   Answer: D. The coil will activate only when both contacts are closed (i.e., when both inputs are true).

   In ladder logic, a coil is activated when all the contacts in series with it are closed (true). This means that for a coil to activate, all the conditions (contacts) in series with it must be satisfied simultaneously. Other options may describe alternative scenarios, but the correct behavior is that the coil activates when all series contacts are closed.

3. When comparing HMI user permission levels with SCADA systems, which of the following statements is correct?

   1. HMI user permission levels allow operators to interact with the system based on their roles, ensuring that only authorized personnel can make critical changes, while SCADA systems typically manage higher-level processes and data collection, with less focus on user-specific permissions.
   2. HMI user permission levels are used to manage system-wide security, while SCADA systems handle user access at the device level.
   3. HMI user permission levels and SCADA systems are functionally identical in terms of managing user access and permissions.
   4. HMI user permission levels are less secure compared to SCADA systems, which have more advanced security features.

   Answer: A. HMI user permission levels allow operators to interact with the system based on their roles, ensuring that only authorized personnel can make critical changes, while SCADA systems typically manage higher-level processes and data collection, with less focus on user-specific permissions.

   HMI user permission levels are specifically designed to manage user access and interaction at the operator interface level, ensuring that only users with the appropriate roles can perform certain actions. SCADA systems, on the other hand, are typically used for supervisory control and data acquisition at a higher level and do not focus as much on user-specific permissions. This distinction is important for maintaining system security and operational integrity.

4. Which of the following statements is correct regarding Modbus TCP/IP port usage?

   1. Modbus ASCII uses port 4840 for communication.
   2. Modbus RTU uses port 502 for communication.
   3. Modbus TCP/IP uses port 502 by default for communication.
   4. Modbus TCP/IP uses port 4840 by default for communication.

   Answer: C. Modbus TCP/IP uses port 502 by default for communication.

   Modbus TCP/IP operates over Ethernet and uses port 502 by default. This is a standard defined in the Modbus TCP specification. Other options are incorrect because Modbus RTU uses serial communication (not port 502), Modbus ASCII uses the same port as Modbus TCP (502), and Modbus does not use port 4840 (which is used by OPC UA).

5. Which of the following statements about Modbus is correct?

   1. OPC UA uses port 502 by default.
   2. Modbus TCP/IP uses port 502.
   3. Modbus RTU uses port 502.
   4. Modbus ASCII uses port 502.

   Answer: B. Modbus TCP/IP uses port 502.

   Modbus TCP/IP is a variant of Modbus that uses Ethernet as the transport medium and operates on port 502. This is a standard defined in the Modbus specification. The other options are incorrect because Modbus RTU uses serial communication, OPC UA uses port 4840 by default, and Modbus ASCII is a different variant that uses ASCII encoding over serial.