S7-1200 Guided Learning Q5: – How many IO points are available

Automation and Control Industrial Automation and Control
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This article is part of a guided learning series on building our understanding of the Siemens S7-1200 PLC.

:pushpin: Canonical Article: Navigating Siemens S7-1200 PLC model numbers
:blue_book: Learning Companion (Q&A): Learning Companion Navigating Siemens S7-1200 PLC model numbers

You are reading: Question 5

How many IO points are available?

Clarification

The term “point” is defined as the total number (summation) of PLC’s inputs and outputs.

  • The term is generally limited to the built-in I/O.
  • The term typically refers to digital I/O
  • The term is a proxy for the capability in a PLC family such as the S7-1200. We recognize that the larger PLCs with more points have increased computation and memory resources.

Tech Tip: Personally, I like to use the word “screw terminal” as there is a physical wire associated with each point.

Answer

There are currently five core members in the S7-1200 family. The number of points range from:

  • The smallest CPU 1211 has 8 inputs and 6 outputs making it a 14-point PLC
  • The largest CPU 1217 has 14 inputs and 10 outputs making it a 24-point PLC.

By comparison to other PLC families, this is a small range. However, the S7-1200 was designed with expansion in mind. For example, consider the TIA Selection tool configuration shown in this picture. With the expansion modules the system now has 46 inputs and 58 outputs. If we stretch out definition we could say that this system has 104 points.

Limitation of expansion

We could expand the system using additional modules and even networked remote I/O. However, we must be careful as it takes time to service the additional I/O. This includes:

  • computational time for the user program to monitor and control the large system

  • network time to send and retrieve data to the I/O

As complexity increases, we run up against real-time constraints where the system is no longer responsive enough to handle the machine or process. At this point, we can shift to a larger PLC such as the S7-1500 or divide the system with multiple responsive PLCs coordinating the tasks.

Image of a S7-1200 PLC with multiple expansion modules
Image of a S7-1200 PLC with multiple expansion modules1290×378 45.8 KB

:writing_hand: Article by Aaron Dahlen, LCDR USCG (Ret.), Application Engineer at DigiKey