Key takeaways
- Start with the complete I/O module order number and its equipment manual; the module defines the permitted BaseUnit type and terminal assignment.
- A light-colored BaseUnit opens a new potential group, while a dark-colored BaseUnit normally continues the power and AUX buses from the left.
- The short designation describes width, process terminals, AUX access, voltage-bus terminals and whether the unit opens or continues a potential group.
- Temperature-measurement applications may require an A1 /T BaseUnit when internal reference-junction compensation is used; confirm the exact analog module and measuring mode.
- The total current of modules and external loads in a potential group must remain within the limit stated for the selected BaseUnit; common 24 V groups are not unlimited buses.
An ET 200SP BaseUnit is more than a removable terminal block. It defines the process connection, provides or continues internal voltage buses, may expose an AUX bus, and can provide functions such as temperature acquisition for a compatible analog module. Two BaseUnits that look similar can therefore behave differently in the same station.
The correct selection sequence
The safest method is to work from the electronic module outward. Do not begin with a BaseUnit color or a spare already in the cabinet. Siemens identifies the required BU type in the module data, and the process-terminal assignment is determined by the inserted I/O module.
Document the decision in this order
- 01
Record the complete electronic-module order number
Include every suffix. Similar module names can use different terminal assignments, widths, supply arrangements or BaseUnit types.
- 02
Read the required BU type in the module manual
Siemens documentation identifies types such as A0, A1, B1 or U0 for the specific module. Treat that value as a compatibility requirement, not a recommendation inferred from appearance.
- 03
Choose the required connection variant
Confirm BaseUnit width, number of process terminals, AUX terminals and any additional potential terminals. Check the module wiring diagram for the exact channel and sensor or actuator connection.
- 04
Decide whether this position opens or continues a potential group
Select a light-colored D version to open and feed a group, or a compatible dark-colored B version to continue the buses from the left. Apply any type-specific restrictions.
- 05
Calculate the potential-group current
Add the power required by the I/O modules and the external loads supplied through the group. Compare the result with the BaseUnit and bus limits in the current Siemens manuals.
- 06
Verify the assembled station
Check the TIA Portal hardware configuration, physical order numbers, terminal labels, potential-group boundaries and wiring against the released drawings before energization.
How to read a BaseUnit short designation
A short name such as BU15-P16+A10+2D/T is a compact description of physical and electrical features. It is useful for comparing variants after the required BU type is known. It does not, on its own, prove that an electronic module is compatible.
Example: BU15-P16+A10+2D/T
| Element | Meaning | Selection consequence |
|---|---|---|
| BU15 | BaseUnit, 15 mm wide | Must match the module width and station layout |
| P16 | Push-in process connection with 16 terminals assigned through the I/O module | Use the module wiring diagram; terminal numbers do not have a universal signal meaning |
| A10 | Ten terminals connected to the AUX busbar | Useful only when the design needs those additional common or protective-conductor connections |
| 2 | Two terminals provide access to the self-assembling P1/P2 voltage buses | Used to supply or tap the potential group, subject to the documented voltage and current limits |
| D | Light-colored version; opens a new potential group | Buses are interrupted to the left and the new group is supplied here |
| /T | Temperature-acquisition version | Used only where the selected temperature module and compensation method require it |
A corresponding designation ending in B identifies a dark-colored version that continues the power and AUX buses from the adjacent module on the left. Other numeric combinations describe different terminal access—for example, 12 can include two P1/P2 terminals plus additional terminals for another potential. Read each complete short designation rather than translating only its last letter.
Design the potential groups before finalizing the BOM
Potential groups divide a station into electrical supply sections. A typical 24 V DC group begins at a light-colored BaseUnit, which interrupts the buses from the left and accepts the new supply. Compatible dark-colored BaseUnits to the right conduct the internal buses until another light-colored BaseUnit starts the next group.
The first BaseUnit to the right of the CPU or interface module is usually light-colored because it starts the first potential group. Siemens documents exceptions for certain AC I/O and energy-meter arrangements, subject to CPU or IM version requirements. Treat those as documented exceptions, not as a general rule that any dark BaseUnit may be used first.
A potential group needs a current budget
For common BaseUnits that specify a 10 A group limit, the calculation includes both the power requirement of the I/O modules and the external loads connected to the group. The group limit does not mean every process terminal can carry 10 A; per-terminal limits are separate and vary by BaseUnit. Protective devices, conductor sizing, voltage drop and load starting current also remain part of the circuit design.
A0 and A1: the common source of confusion
A0 is widely used with 15 mm digital, fail-safe, communication, technology and analog modules that do not require the temperature-measurement BaseUnit function. Current Siemens documentation lists both light and dark A0 variants, with and without AUX terminals. The module manual still determines whether a particular variant supports the required connection.
A1 is associated with compatible analog modules that measure temperature. A /T BaseUnit provides temperature acquisition used for reference-junction compensation where the module and selected thermocouple mode call for it. Do not assume every thermocouple installation automatically uses the same compensation method: check whether the design uses the BaseUnit's internal reference junction, an external reference junction or another method supported by the exact module.
A0 versus A1 decision
| Question | A0 path | A1 /T path |
|---|---|---|
| What does the electronic-module manual specify? | BU type A0 or A0/A1 without the temperature function | BU type A1 for the documented temperature-measurement connection |
| Is BaseUnit temperature acquisition required? | No | Yes, when required for the selected compensation method |
| What remains to choose? | Light/dark group behavior and AUX-terminal variant | Light/dark group behavior, AUX-terminal variant and /T version |
What about B0, B1, C0, C1, D0, F0 and U0?
The current Siemens module overview lists several specialized 20 mm BaseUnit types in addition to A0 and A1. Their short names expose different process-terminal counts and bus access, but their application is module-specific. For example, Siemens documents B1 BaseUnits for defined AC I/O modules, while U0 variants are used only with modules whose documentation calls for U0. Do not turn the type list into a universal substitution table.
Current BaseUnit type families in Siemens product information
| BU type | Representative current short name | How to use this information |
|---|---|---|
| A0 | BU15-P16+A0+2D or +2B | Common 15 mm family; verify the exact I/O module and desired group/AUX variant |
| A1 | BU15-P16+A0+2D/T or +2B/T | Temperature-acquisition family; verify the module and compensation method |
| B0 / B1 | BU20-P12+A4+0B / BU20-P12+A0+4B | Special 20 mm arrangements; use only for a module that specifies the type |
| C0 / C1 | BU20-P6+A2+4D / +4B | Special six-process-terminal arrangement; follow the module manual |
| D0 / F0 | BU20-P12+A0+0B / BU20-P8+A4+0B | No generic interchangeability; confirm type, terminals and module assignment |
| U0 | BU20-P16+A0+2D or +2B | Dedicated U0 family with type-specific voltage and mixing rules |
A practical engineering and procurement audit
Before releasing the BaseUnit BOM
- 01
Identity
Electronic module and BaseUnit order numbers are complete, current and tied to the same released design revision.
- 02
Compatibility
The electronic-module equipment manual explicitly permits the selected BU type; no choice is based only on width, color or a legacy cabinet example.
- 03
Wiring
Every field conductor is mapped to the module-specific terminal diagram, including shields, sensor supply, compensation conductors and AUX usage.
- 04
Potential groups
Each group boundary, supply source, protective device and calculated continuous or starting load is shown on the schematic.
- 05
Configuration
The released TIA Portal hardware configuration agrees with the physical sequence and required CPU or interface-module version.
- 06
Spares
A proposed spare matches the full function and article number or has documented engineering approval as an alternative—not merely the same BU type.
Common selection mistakes
Choosing by color first. Light and dark indicate potential-group behavior, but they do not establish module compatibility. Determine the BU type from the electronic module before choosing D or B behavior.
Treating AUX terminals as extra module channels. AUX terminals are a separate bus connection; their intended use and limits must be designed explicitly.
Copying a 10 A figure to every terminal. The potential-group or AUX bus limit and the process-terminal current rating are different specifications.
Assuming A1 is a universal thermocouple BaseUnit. The exact analog module and compensation method determine whether BaseUnit temperature acquisition is required.
Using a historical compatibility screenshot as the final BOM authority. Product information and module revisions change. Keep the official document number and revision with the design record.
Bottom line
The BaseUnit should be the result of the I/O selection, wiring design and potential-group calculation—not a part chosen independently. If the electronic-module manual, BaseUnit designation, potential-group behavior, terminal assignment and current budget all agree, the choice is reviewable. If one of those checks is missing, the apparent fit of the part is not enough.
