What is the difference between SR20 and ST20 PLC from Siemens ?

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Accepted Answer

The difference between Siemens S7-200 SMART SR20 and ST20: which one to choose as a reserve?

Target group: Maintenance managers, process technologists and suppliers in pharmaceutical and food industry who operate packaging machines with SIMATIC S7-200 SMART controllers (often installed by Chinese manufacturers) and want a reliable reserve that the line does not stand.

Psychological Pain Points and How to Save them

Fear of line failure: wrong choice of reserve means that the program or binding does not work the same way and the startup is delayed. The right choice of SR20 or ST20 reduces switching time and start.
Uncertainty in order from China: The models are very similar by name, but functionally different. Clear distinction eliminates doubts and concerns in purchasing.
Risk in motion control: if the machine uses PTO/PWM for tap or servo, the wrong type of output causes twitching, uneven dosing or engine alarms. Correct choice eliminates poor quality and complaints.

SR20 vs ST20 – concise comparison

Property	SR20	ST20
Power CPU	AC power supply	DC power supply
Output type	Relay Do	Transitory DO
Suitable load types	Valves, contactors, lamps, AC/DC loads	Fast signals, pulses, driver control
Switch speed	Slower switch, no stable rapid pulses	Fast switch, stable PTO/PWM
Application PTO/PWM	Not recommended on relay exits	Recommended — destined for PTO/PWM
Output lifetime	Mechanical relays – wear of contacts on frequent switching	Semiconductor output – high cyclic endurance
Typical installation	Use without advanced positioning	Machinery with dosing snails, transporters, displacements, robots

When to select SR20 and when ST20

Select SR20, if the exits control mainly valves, contactors, lights and machine no he needs rapid pulses. Relejski DO are robust for various loads, binding is simple and tolerates electrical disturbances.
Select ST20if the programme applies PTO/PWM ed step or servo drives or if you need fast, repeatable commutation. Transitory DOs are necessary for smooth movement and accurate dosing.

Examples from practice (packaging in pharmacy and nutrition)

Dosing snail: if the snail drives the step engine through the driver and CPU generates pulses, ST20 it's the right backup. SR20 will cause irregular appetite or will not work properly at all.
Fan batteries and contactors: if control is predominantly on/off without positioning, SR20 is a rational and robust choice.
Hybrid: if pulses are generated by an external jamming controller and CPU only controls on/off, it can be sufficient SR20If the pulses are generated by CPU, take ST20.

Quick guide for proper reserve order

Check the type designation on the existing controller (e.g. 6ES7288-1)SR20- 0AA0 or 6ES7288-1ST20- 0AA0).
Check the application for use PTO/PWM exits. If YES — ST20.
Focus on the charges: valves/contactors — SR20, step/servo - ST20.
Check power supply: AC power is favored by SR20, DC power is favored by ST20.
Check extensions: if you have EM modules or board signals, stay with standard versions and maintain the same type of output.

Why Difference Matters

SR20 and ST20 are not interchangeable. SR20 is mechanically robust for universal loads, but not for quick positioning. ST20 is electronically fast and accurate for drives, but requires proper power and compatible drivers. The right choice means faster exchange, fewer risks, fewer scrap products and less stress on the night shift.