ST20 Compact CPU Data
ST20 is the compact transistor-output alternative to the relay-based SR20. It keeps the same 20-point I/O scale but uses a 24 V DC supply and semiconductor outputs. Three high-speed outputs can support pulse-train tasks, making this CPU a better match for small positioning, indexing and fast-switching applications.
Product Detail & Identification
Fast switching in a 90 mm controller
Transistor outputs avoid mechanical contact wear and respond far faster than relays. They are limited to DC loads and require correct polarity, current protection and suppression. For pulse applications, wiring quality and the drive input specification are just as important as the 100 kHz CPU capability.
ST20 I/O Architecture
Communications & Integration
Ethernet and PROFINET
The isolated RJ45 interface supports 10/100 Mbit/s Ethernet and S7 communication. PROFINET controller or I-Device capabilities depend on firmware; verify roles, update time and device count in the active project.
RS485 serial channel
The integrated RS485 port supports serial integration up to 187.5 kbit/s. Addressing, termination, shield bonding and the selected protocol must match every field device.
ST20 Motion-Ready Checks
Verify the 24 V DC supply capacity and polarity.
Confirm every output load is compatible with transistor switching.
Use shielded wiring for high-speed counter and pulse circuits.
Test homing, travel limits and pulse direction at reduced speed.
ST20 Machine Uses
The I/O and output technology of CPU ST20 make it a practical choice for the following control duties. Final selection must still consider safety architecture, analog requirements, expansion count and environmental limits.
ST20 setup notes for compact pulse applications
ST20 is the smallest transistor-output CPU in this group. Its value comes from combining a 24 V DC power format, twelve inputs, eight solid-state outputs and three 100 kHz pulse channels in a 90 mm-wide controller.
Select ST20 for DC loads and fast edges
Choose ST20 when the actuators and drive inputs are compatible with 24 V DC transistor switching. The outputs do not provide isolated dry contacts like the SR models, so polarity, common reference and load current must be confirmed for every channel. Semiconductor outputs switch quickly and avoid relay contact wear, making them suitable for repetitive commands that would be inefficient for mechanical contacts.
Protect the twenty-four volt supply quality
The CPU operates from 20.4–28.8 V DC. Size the supply for the controller, sensors and any loads that share the distribution while preserving headroom for startup conditions. Separate noisy actuator branches where practical, bond the reference according to the panel design and verify polarity before connection. A stable supply is particularly important when the same controller is producing pulse trains for a positioning device.
Assign the three pulse channels before wiring
ST20 provides three high-speed outputs rated for pulse operation up to 100 kHz. Reserve those terminals in the I/O schedule before ordinary outputs are assigned. Confirm the selected drive accepts the electrical signal and frequency, then document pulse, direction and enable wiring. Do not route high-speed conductors beside contactor or motor cables simply because the distance inside a small enclosure is short.
Commission motion at reduced consequence
Before automatic cycling, verify direction, limit logic, homing sequence and travel scaling with the mechanism unloaded or at reduced speed. Test one axis function at a time and make sure an emergency or software limit stops the command as intended. The controller can generate fast pulses, but safe machine motion still depends on the drive configuration, field devices and mechanical limits.
Use the 20 KB program area efficiently
ST20 is intended for compact machines, so keep sequencing, interlocks and diagnostics organised around the actual mechanism. Store a commented Micro/WIN SMART project, note retentive data and record any pulse instructions and high-speed counter assignments. These details matter when a replacement CPU is fitted because an ordinary I/O checkout will not prove the timing and direction logic used by a feeder or indexer.
Check ST20 identification instead of assuming ST equals ST
Verify 6ES7288-1ST20-0AA1 on the nameplate and order. The ST prefix identifies transistor output technology, while the 20 designation identifies the onboard point scale. Photograph the label and terminals, confirm the DC/DC/DC format and keep the firmware record with the project. This prevents accidental substitution of a relay CPU that cannot reproduce the intended pulse behaviour.
ST20 pulse-control installation record
Confirm that twenty points are enough before panel work begins
List every detector, limit, operator command, enable signal and driven device before choosing the compact CPU. Twelve inputs and eight outputs are a good fit for a focused mechanism, but the design should retain service reserve instead of consuming the final point on day one. Identify any analog measurement or additional communication requirement separately because those functions influence expansion planning even when the basic digital count appears comfortable.
Build the DC supply plan around motion stability
ST20 accepts 20.4–28.8 V DC. Calculate the supply allowance for the controller and all connected sensor branches, then consider what happens when valves, brakes or drive enables operate. Keep noisy loads on appropriately protected branches and maintain the intended reference between controller and drive electronics. Record the measured voltage at the CPU during a representative operating cycle rather than relying only on the power-supply nameplate.
Resolve sourcing and common conventions on the drawing
Transistor systems depend on correct polarity and common reference. Mark the field-device convention, terminal common and expected active-state voltage for each channel. A wiring diagram that simply labels a point as output does not show enough information for a future replacement. During checkout, measure the off and on states at the terminal and confirm that the connected interface interprets them correctly before allowing repeated machine movement.
Reserve pulse terminals before ordinary outputs are assigned
Three high-speed output channels can produce pulse trains up to 100 kHz. Place those functions at the beginning of the allocation exercise and document pulse, direction and enable relationships for each drive. Only after the motion resources are fixed should the remaining outputs be assigned to routine DC commands. This order avoids a late-stage change in which a lamp or valve occupies the terminal needed for a positioning function.
Route timing-sensitive conductors deliberately
Keep pulse and high-speed counter wiring away from contactor coils, motor conductors and unsuppressed switching. Follow the drive and sensor requirements for shielding, reference bonding and cable type. In a small enclosure it is tempting to take the shortest route, but a slightly longer controlled path can provide more reliable edge detection. Photograph the finished routing so that later cabinet work does not unknowingly move the signal beside a noise source.
Match the drive interface, not just the frequency figure
A 100 kHz CPU capability is useful only when the receiving drive accepts the voltage levels, pulse format and maximum rate selected in the program. Compare the drive input specification with the controller wiring and configure acceleration, deceleration and pulse scaling before the first powered test. The PLC command frequency, mechanical travel and drive parameter set must describe the same physical movement.
Commission one direction and one limit at a time
Begin at reduced speed with the mechanism clear. Prove enable, direction and a small commanded movement, then verify home and travel limits individually. Test that a removed permissive or active limit stops further command in the expected direction. Once those responses are reliable, run the complete index or feed sequence and monitor online values. This layered process keeps a sign or scaling error from becoming a full-speed mechanical problem.
Archive evidence for the next ST20 service visit
Store the 6ES7288-1ST20-0AA1 label image, firmware note, final Micro/WIN SMART project, pulse assignment table and measured supply record together. Include the drive parameters that determine pulse scaling and direction. A future technician should be able to identify the exact DC/DC/DC hardware and reproduce the motion setup without reverse-engineering the original commissioning decisions.
ST20 Pulse-System FAQ · 6 Checks
Why choose ST20 over an SR20 relay CPU?
ST20 uses 24 V DC transistor outputs and supports three pulse channels up to 100 kHz, which better suits fast-switching or compact positioning tasks.
Can ST20 outputs switch AC loads directly?
No. The semiconductor outputs are for compatible DC loads. Use an appropriate interface when a field load requires isolation or another voltage.
What supply range is required?
The controller operates from 20.4–28.8 V DC; polarity, available current and noise separation should be checked before connection.
How should pulse wiring be routed?
Reserve the pulse terminals, use wiring compatible with the drive input and keep timing-sensitive conductors away from contactor and motor cables.
What should be tested before automatic motion?
Confirm direction, homing, limits, scaling and stop response at reduced speed before combining the functions into the full cycle.
Which complete part number identifies ST20?
Use 6ES7288-1ST20-0AA1 and verify its DC/DC/DC format on the nameplate rather than relying on the short ST20 designation.
ST20 Reference Catalogue
SIEMENS · Published Models
Others · Selected Models
PLC EXPORT ST20 Assistance
For ST20 projects, PLC EXPORT reviews the complete transistor-CPU reference, photographed markings and requested delivery details before quotation. Our team checks complete article numbers, photographed labels, condition and requested quantities before quotation and shipment coordination.
ST20 identity review · Pulse-capability check · Quotation response
Email CristinaWhatsApp ↗ASK ABOUT ST20
Quote
Hot Tags: siemens 6es7288-1st20-0aa1 simatic s7-200 smart cpu st20, China siemens 6es7288-1st20-0aa1 simatic s7-200 smart cpu st20 manufacturers, suppliers, 6ES7 315 2AH14 0AB0, 6ES7 315 2AH14 0AB0 CPU 315 2 DP, 6ES7 315 2AH14 0AB0 Siemens, 6ES7317 2EK14 0AB0 Central processing unit, 6ES7317 2EK14 0AB0 Siemens, siemens
