PLC Programming Example: Automatic
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PLC Programming Example, simple manual and automatic control.
This PLC programming example below was written to answer a specific request by a learner, so we thought we would also share it with our website visitors. This PLC programming example is for manual control and automatic control. The manual rung jogs motor only when a momentary jog push button is pressed. The automatic control rung is a simple seal-in circuit. We use even this most simple PLC programming example to pass along and/or re-enforce some of the PLC programming best practices we teach. If you would like to have your employees learn more best practice PLC programming, please request a quote for our on-site PLC training.
Best practice PLC programming would have all real-world input and output addresses (like I:1/0 [I0.0] or O:3/6 [Q0.0]) in their own Cross-Reference subroutine (XREF Sub). But for our classroom purpose, in this programming example, we used real-world I/O in a single subroutine so all logic can be seen on one screen.
Rung 0 comment: Boolean Logic for this rung below is ... If JOG_PB or JOG_HMI then JOG_MOTOR_1
Instruction List - { BST XIC I:1.0/0 NXB XIC B3:0/10 BND OTE B3:0/9 }
Best practice PLC programming requires the seal-in element (instruction/contact) to be the last element in the branch of the seal-in circuit. We used seal-in for this logic because best practice states NEVER use a latching instruction (or set/reset or negate) unless needed. Always defaulting to a seal-in type circuit/logic which helps ensure a safe start-up condition on machine power-up.
Rung 1 comment: Boolean Logic for this rung below is ... If AUTO_PB or AUTO_HMI or RUN_MODE and not STOP_PB, then RUN_MODE
Instruction List - { BST XIC I:1.0/1 NXB XIC B3:0/12 NXB XIC O:3.0/6 BND XIO I:1.0/2 OTE O:3.0/6 }
We will end with a couple more best practice PLC programming tips that will save end-users (company using the finished equipment and PLC program for the next 10 years) millions of dollars in reduced downtime…
#1 Best Practice PLC programming rule: Keep your program easy for the layman to understand.
#2 Best Practice PLC programming rule: Document, document, document!
Every rung should have a detailed description, every element/instruction in rung should have a detailed descriptor too.
#2.a Best Practice PLC programming rule: Do NOT use acronyms in descriptions, spell them out.
Rung 2 comment: Boolean Logic for this rung below is ... If JOG_MOTOR_1 or RUN_MODE and not ESTOP_1, then MOTOR_1_RUN
Instruction List - { BST XIC B3:0/9 NXB XIC O:3.0/6 BND XIO B3:0/8 OTE O:3.0/7 }
Legend: BST=Branch Start, NXB=Next Branch, BND=Branch End
The video below will show you this PLC programming example running in a real PLC for you to better understand the ladder logic processing.
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