"trap" of Siemens S7-300 PLC process image - Database & Sql Blog Articles

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It is well known that in the HW Config window of the SIMATIC Manager, you can configure the currently connected hardware devices, including hardware modules, communication connections, and so on. If the configuration is incorrect, “Save and Compile” will not pass. But in today's knowledge explosion, if you think that the process image area of ​​the S7-300 series plc is fixed, then you may be "OUT". The following story may be beneficial to engineers.
A customer uses Siemens S7-300 series controller CPU313C-2DP (6ES7 313-6BG04-0AB0) to achieve its control requirements. Since multiple valves are added on site, a new 32 points need to be added after the original controller. DO digital output module. When the DO module is added to the rack, in order to keep the address of the digital output continuous, the address of the DO module needs to be set. The CPU313C-2DP body integrated DO address range is 124-125, so set the DO module address "Start" to 126, as shown below:

When you click "OK", the warning shown below will pop up, indicating that the set address is invalid and automatically assigning a new address starting from 128.

What is the cause of the above phenomenon? Open the properties window of CPU313C, select the tab "Clock/Cycle Memory", you can see that the current CPU input / output IO image area range is 128 bytes by default, as shown below:

The 32-point DO module needs to occupy 4 bytes, and the DO output address of the ontology is 124-125. If the address is kept consecutive, the address of the DO module should be 126-129, and the default process image area 128 is exceeded. For a digital module, its output address cannot "cross" the process image, ie a part of the address is in the process image area and a part of the address is outside the process image area. If you want to keep the digital output address continuous, you can change the IO image area of ​​Figure 3 to 256, as shown in the figure below. If there is no requirement for the same address, the recommended starting address 128 can be selected, so that the address of the DO module is 128-131.

Not all PLCs support the modification of the process image area, which can be confirmed in two ways: by checking the technical specifications of the PLC to confirm whether the process image area extension is supported, or by viewing the relevant PLC technology via the Siemens China Product Support window. data. As the saying goes, "Daily is not afraid of thousands of miles", timely understanding and mastering the new features of PLC is not a daily routine, as long as you insist on moving forward every day, how can you be afraid of thousands of miles.