ET200SP Base Unit Selection Guide (2025 Edition) – Complete, Practical & Engineer-Ready

Selecting the correct base unit for Siemens ET200SP modules is one of the most frequently misunderstood aspects of system design. With dozens of module variations, multiple electrical behaviors, and different potential groups, choosing the wrong base unit can lead to:

modules failing to start,
incorrect sensor/actuator wiring,
overloading a potential group,
inaccurate analog or thermocouple measurements,
or unnecessary project costs.

This guide distills the complete ET200SP base-unit logic into a single, engineer-ready reference.
It is enhanced with practical insights, wiring considerations, and current-load analysis to help you design safer, more reliable systems.

1. What Makes ET200SP Base Units Unique?

Each ET200SP module consists of two parts:

Electronic Module (DI, DQ, AI, AQ, FM, Safety, etc.)
Base Unit (BU) — determines wiring, AUX terminals, and power routing

This separation allows flexibility—but also causes design confusion.

Below is a complete explanation of every base-unit type and how to select the correct one.

2. A0 & A1 Base Units

A0 is the most commonly used base unit for standard digital and analog modules.

Overview of Siemens ET200SP base unit types, including A0, A1, B0, B1, C0, C1, and U0 base units.

A0 Base Unit Variants

A0 units are divided by two characteristics:

1) Power color (light vs dark)

Light A0 — starts a new potential group (supplies 24V)
Dark A0 — extends an existing group (no internal power)

2) With or without AUX terminals

Long type = with AUX
Short type = without AUX

Why it matters

Light A0 → next modules share a new 24V/0V group
Dark A0 → continues the same group
Light A0 can supply 10 A through the backplane to dark A0 units

This affects how you group sensors, actuators, and high-load modules.

3. A1 Base Units (Special Case for Thermocouples)

A1 looks identical to A0.
You can ONLY distinguish them by the ordering number.

A1 is required only when:

Using thermocouple modules
Using internal cold-junction compensation (CJC)

For all normal I/O and analog modules, A0 is preferred to avoid installation mix-ups.

4. B0 & B1 Base Units

Used for relay modules, AC input modules, high-current loads, and modules requiring their own supply.

B0 vs B1 Summary

Feature	B0	B1
Own power terminals	❌ No	✔ Yes
Can be placed immediately after IM?	❌ No	✔ Yes
Passes supply to the right	✔ Yes	❌ No
Typical usage	Relay / AC DI	High-current, isolated loads

Electrical wiring diagram comparing Siemens ET200SP B0 and B1 base units and their power isolation behavior.

Because B1 does not pass 24V to the next base, it behaves differently inside potential groups. Understanding this is essential for power-group design.

5. C0 / C1 Base Units (AS-i & Safety)

Type	Color	Usage
C0	Light	AS-i, AS-i Safety, F-PM-E
C1	Dark	AS-i Safety only

Like A0, C0 starts a potential group; C1 must be powered from the left.

6. U0 Base Units (High-Voltage & New Generation)

The newest, increasingly common base type.

Used with:

UC modules
Relay outputs
Weighing modules
DALI
F-TM servo / stepper modules
Many future ET200SP expansions

U0 supports DC120V or AC240V, depending on module.
Light U0 can supply dark U0 units (10 A).

This family is gradually becoming the standard for advanced modules.

7. Module–Base Compatibility Table

This is one of the most valuable references for engineering design.

Below is the fully reconstructed, copy-friendly version:

Module → Base Unit Compatibility

✔ = Supported

Module	B0	B1	C0	C1	D0	F0	U0
DI 4×120..230VAC ST		√
DI 8×24VDC/48VUC BA		√
RQ 3×120VDC–230VAC/5A CO ST	√	√
RQ 3×120VDC–230VAC/5A CO n.i. ST	√	√
DQ 4×24..230VAC/2A HF	√	√
DQ 4×24..230VAC/2A ST	√	√
RQ 4×120VDC–230VAC/5A NO MA ST	√	√
RQ 4×120VDC–230VAC/5A NO ST	√	√
AI Energy Meter 400VAC ST			√		√	√
AI Energy Meter 480VAC ST			√		√	√
AI Energy Meter 480VAC/CT HF			√		√	√
AI Energy Meter 480VAC/RC HF			√		√	√
AI Energy Meter CT HF					√	√
AI Energy Meter RC HF					√	√
AI Energy Meter CT ST					√	√
AI Energy Meter RC ST					√	√
TM Pulse 2×24V	√	√
TM SIWAREX WP341 HF	√	√
TM SIWAREX WP351 HF	√	√
CM 1×DALI		√
CM AS-i Master ST		√	√	√
F-RQ 1×24VDC/24..230VAC/5A		√		√
F-CM AS-i Safety ST		√	√	√
F-PM-E 24VDC/8A PPM ST		√	√	√
F-TM ServoDrive ST 1×24..48V 5A		√					√
F-TM ServoDrive HF 1×24..48V 5A		√					√
F-TM StepDrive ST 1×24..48V 5A		√					√
SITRANS TM FCT070		√				√
SITRANS TM FST070		√				√
TM ECC PL ST		√					√
TM ECC 2×PWM ST		√					√

8. Electrical Behavior & Potential Groups

Electrical wiring diagram of Siemens ET200SP A0 base unit showing P1, P2, AUX bus behavior and light versus dark base-unit functions.

Typical Siemens ET200SP station configuration showing potential group segmentation and backplane power distribution.

A potential group is created whenever a light-colored base appears.

Inside a potential group:

P1/P2 are powered from the leftmost light base
All dark bases share the same 24V/0V bus
Maximum load capacity = 10 A
AUX terminals (long versions) share a unified AUX potential

Which modules consume the most current?

24 V sensor supply (up to 700 mA)
Analog measurement modules (up to 0.7 A)
Digital outputs with actuators (up to 4 A total)
External devices powered through dark-base L+/M terminals

You must calculate all of these to avoid overload—not simply count modules.

9. Power-Related Tables

9.1 Power Loss

Parameter	Value
Power loss (typical)	1 W

9.2 24 V Sensor Power Supply

Parameter	Value
Voltage	24 V
Short-circuit protection	Yes
Max per channel	700 mA
Max per module	700 mA

9.3 Encoder Power Supply

Parameter	Value
24 V encoder supply	✔
Short-circuit protection	✔
Maximum total current	0.7 A

9.4 Analog Module Current Consumption

Parameter	Value
Max input current	150 mA

9.5 Digital Output Module Current Consumption

Parameter	Value
Consumption (unloaded)	35 mA

10. Practical Engineering Recommendations

Use A0 whenever possible (except for TC modules using internal CJC).
Use B1 for isolated high-current or AC applications.
U0 is ideal for new designs (servo, weighing, DALI, UC).
Always compute current per potential group—never use fixed “5 modules per group” rules.
Separate high-noise modules (energy meters, AC modules) into independent potential groups for EMC stability.