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EENG421 Ch. 3
Introduction to PLC Programming
| Question | Answer |
|---|---|
| What number systems are used in PLC automation? | decimal, binary, octal, binary coded decimal, hexidecimal |
| What is the relationship between bits, bytes, and words? | A single binary digit is a bit, eight bits are a byte, two bytes (16 bits) are a word, two words (32 bits) are a double word |
| How is PLC memory organized? | bytes, single words, or double words |
| What are the three binary interfaces of PLC memory? | data bus, control bus, address bus |
| What does the data bus do? | Carries parameter values and PLC instructions |
| What does the control bus do? | Provides the logical control for movement of instruction and data |
| What does the address bus do? | Carries the binary address number for all the binary values stored in memory |
| What are the Allen-bradely processor modules? | PLC5, SLC500, Logix |
| What are the two different memory structures? | Rack-based, tag-based |
| Which processor modules use rack-based memory? | PLC5 and SLC500 |
| What is the memory structure for rack-based memory? | 2-drawer filing cabinet with one drawer for program files and one for data files. The program and data file groups are subdivided into instruction-specific files and file types |
| How many designated file types and user defined file types are there for data files in a rack based system? | 9 (0-8) designated 247 (9-255) user-defined |
| What do the data files contain in a rack based system? | Folders with data bits of memory in rows that are 8, 16, or 32 bits wide |
| What are stored in the program files of a rack-based system? | Ladder logic programs. The size is set by PLC memory size |
| What are the two program file types of a rack-based system? | system and program. They are subdivided into system functiosn (0 and 1), main program (2) and subroutine programs (4-255) |
| What processor module(s) use tag-based memory? | Logix |
| How does tag-based memory work? | All data are assigned a variable name called a tag, so knowing the location memory is not necessary. Programs are developed using only tag names |
| What is I/O addressing? | I/O addressing connects the physical location of a field device at a terminal on an I/O module to a bit location in the processor memory |
| What type of I/O addressing does PLC 5 use? | Rack-group |
| How is the system configured in a PLC5 with rack-group based memory? | - Select input and output modules - Determine field device terminations to the modules - Identify the memory address of the field device and data |
| What is a terminal? | I/O points that attach field devices to PLC modules and to bits in processor memory |
| What is a group? | 16 input terminals (one 16 bit input word) and 16 output terminals (one 16 bit output word) |
| What is a logical group? | 8 groups (0-7) |
| How does half-slot addressing work? | the PLC has two groups to a slot (two input words and two output words) |
| How does single-slot addressing work? | One group to a slot (one input word and one output word) |
| How does two slot addressing work? | One group distributed over two slots |
| How does rack-group I/O addressing work? | I/O:xy/z I/O: input/output x: rack # y: group # z: terminal # |
| Which PLC processor uses rack-slot based addressing? | SLC500 |
| How does rack-slot I/O addressing work? | I/O:x.y/z I/O: input/output x: I/O slot number (0-30) y: word number (0-255) z: bit number (0-15) |
| How does tag-based memory work? | Tag-based memory assigns variable names or tags to field device inputs and outputs, internal relays, and data values |
| What is a tag? | A text-based name for a byte (16 bit) area of the controller's memory where data is stored |
| How many tasks does Logix permit? What are their functions? | 32 - Holds the information necessary to schedule the program's execution - Sets the execution priority for one or more programs |
| What types of tasks are there for Logix pricessor module? | - Continuous - Periodic |
| What does a continuous task do? | Executes non-stop |
| What does a periodic task do? | Interrupt the continuous task and execute for a fixed length of time at given intervals. |
| How many programs does each task require? How many are they allowed to have? Can they execute at the same time? | Requires one Can have 32 executing one at a time |
| What is a routine? | Provides the program code for the project, using a programming language such as ladder logic |
| What is the difference between program and controller tags? | Program tags are local, controller tags are global |
| What is the location for the phyiscal address for a tag-based system? | Location:Slot:Type.Member.Submember.Bit |
| What is the difference between retentive and non-retentive memory? | Retentive memory bit permits when the rung is true, remains set when the rung is false, or when the PLC power is removed. Non-retentive memory bit becomes true when the rung is true and returns to false when the rung is false or the PLC is turned off. |
| What is an application for retentive memory? | Warm restarts, control of process machinery components (conveyors, pneumatic and hydraulic actuators, motors) that may present safety hazards |
| What determines the scan time? | The number of rungs in the program, or the amount of memory used to store the program |
| What is a rule of thumb for scan time? | 1 ms per 1000 bytes |
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