LOGO! 0BA7 Input Numbering: Mapping AI/DI with DM16 and AM2

Overview

The Siemens LOGO! 0BA7 (LOGO! 8 generation) handles digital and analog input numbering using a fixed, left-to-right slot-based scheme. With a LOGO! 12/24 RCE base module, a DM16 24/R digital expansion, and an AM2 analog expansion mounted in that order, the LOGO! Soft Comfort (LSC) V8 program maps the physical terminals to logical I1..I24 (digital) and AI1..AI4 (analog) in a predictable sequence. Misreading this scheme leads to two common field errors: (1) attempting to read a 4-20 mA signal on a base-module terminal that is hard-limited to 0-10 V, and (2) addressing the AM2's first analog input as AI1 when the base module has already reserved AI1 and AI2 for I7 and I8.

This reference documents the exact I/O map produced by the configuration LOGO! 12/24 RCE > DM16 24/R > AM2 with the AI count flag set to 2 AI (the default), shows how to confirm the mapping live in LSC's I/O Status tool, and clarifies the rule that controls how many analog input slots the base module reserves even when I7/I8 are wired as digital inputs in the circuit program.

Base Module (12/24 RCE) I/O Map

The 12/24 RCE provides 8 digital inputs (I1..I8) and up to 4 analog inputs that are physically shared with four of the digital inputs. The dual-use terminals and their analog ranges are:

With the AI count flag set to 2 AI (the default, configurable under File > Properties > AI Count or the on-device menu), only I7 and I8 are exposed as AI. In this configuration the program-level mapping becomes:

• AI1 = base I7 (0-10 V)
• AI2 = base I8 (0-10 V)
• I1, I2 remain digital-only (no AI shadow, no AI3/AI4 on the base)

The "2 AI" configuration is the most common choice when the application uses 0-10 V transducers directly on the base and offloads any 4-20 mA sensor to an AM2 module.

Digital Expansion (DM16 24/R) I/O Map

The DM16 24/R provides 16 digital inputs at 24 V DC, organized as two 8-bit groups. Because it contains no analog channels, the entire block of 16 terminals is mapped to consecutive digital addresses immediately following the base module's I8.

The DM16 24/R has no influence on AI numbering. AI slots are reserved exclusively by base-module dual-use inputs (I7, I8 in the 2 AI configuration) and by the analog expansion modules themselves.

Analog Expansion (AM2) I/O Map

The AM2 module adds two analog inputs, each independently configurable for 0-10 V or 0/4-20 mA via the LSC parameter dialog (per-channel Type field). The two AM2 inputs occupy the next free AI addresses after the base reservation.

For the configuration 12/24 RCE > DM16 24/R > AM2 with the AI count flag left at the default 2 AI, the canonical mapping is:

• AM2 I1 = AI3
• AM2 I2 = AI4

If the application requires all four base analog inputs (AI1 = I1, AI2 = I2, AI3 = I7, AI4 = I8), change the AI count flag to 4 AI; in that case the AM2 inputs shift to AI5 and AI6 respectively.

The AI Reservation Rule

A recurring source of confusion is whether using I7 or I8 as a digital input in the circuit program (e.g. by placing a make-contact --| |-- block instead of an analog-threshold block) frees up AI1 and AI2 for the AM2. It does not.

The LOGO! 0BA7 firmware reserves the AI slots assigned to the base module at circuit-program compile time, based solely on the AI count flag. The reservation is independent of how the program subsequently uses those terminals. A digital coil or contact on I7 still consumes the AI1 slot; AI1 is not re-assigned to the AM2.

Consequence for the user's case: with the AI count flag at 2 AI, even if I7 and I8 are wired only to mechanical contacts and read exclusively as I7/I8 in the program, the AM2's first physical input is still AI3 in the LSC program. Treating it as AI1 will return 0 forever and the threshold block will never fire.

How LSC Numbers Inputs and Analogs

LOGO! Soft Comfort assigns input addresses using the following deterministic rules:

1. Start with the base module. I1..I8 are assigned to the base's digital terminals in physical order. AI slots are reserved for the base's dual-use terminals according to the AI count flag.
2. Walk the expansion bus left to right. Each digital expansion contributes its DI count to the next free DI addresses. Each analog expansion contributes its AI count to the next free AI addresses.
3. Slot count is fixed by module type, not by what the circuit program actually uses. A digital expansion always reserves DI addresses; an analog expansion always reserves AI addresses.
4. Address gaps never occur. The DM16 24/R always takes I9..I24; the AM2 always takes AI3..AI4 (with 2 AI flag) or AI5..AI6 (with 4 AI flag).

Determining the Correct AIx with I/O Status

The fastest way to confirm the analog address of an AM2 input is to use the LSC online I/O Status tool. The procedure is identical for all 0BA7 generations and does not require a running circuit program.

1. In LOGO! Soft Comfort, connect to the LOGO! 0BA7 base via Ethernet or USB.
2. Open the menu Tools > Transfer > I/O Status (German LSC: Extras > Übertragen > I/O-Status).
3. In the I/O Status dialog, enable the Analog Inputs column via the dropdown or the column-picker.
4. Apply a known signal to the AM2 I1 terminal (e.g. 12 mA from a calibrator or 5 V from a potentiometer).
5. Watch the AI1, AI2, AI3, AI4 columns. The column whose value tracks the applied signal is the correct address. For the configuration described above, the AM2 I1 will show movement on AI3.
6. Repeat for AM2 I2 to confirm AI4.

The same I/O Status approach also validates the digital map: pressing a switch on DM16 terminal I1 should toggle LSC bit I9, terminal I2 toggles I10, and so on through I24.

Module Ordering Recommendation

Siemens explicitly recommends installing digital expansion modules before analog expansion modules on the bus. For the configuration in this reference, the correct order is:

1. LOGO! 12/24 RCE (base, slot 1)
2. DM16 24/R (digital expansion, slot 2)
3. AM2 (analog expansion, slot 3)

Mixing analog modules ahead of digital modules does not break the addressing rules (numbering is still left to right) but can increase the bus scan time of the digital modules because the analog conversion results are interleaved with the digital scan. With only one analog module the effect is small, but in cabinets with several AM2 / AM2 RTD / AM2 AQ modules the recommendation becomes a real performance constraint.

Maximum supported expansion count for the LOGO! 0BA7 base is 8 standard modules plus up to 4 analog modules, with a hard cap of 24 digital inputs, 8 digital outputs, 8 analog inputs, and 4 analog outputs on the base plus expansions combined. Always verify that the planned configuration does not exceed these limits.

AI Signal-Type Constraint (0-10 V vs 4-20 mA)

The 12/24 RCE base analog inputs are voltage-only (0-10 V). A 4-20 mA loop cannot be connected directly to I1, I2, I7, or I8. The AM2 is the lowest-cost 0BA7 module that provides a true current input. Each AM2 channel can be configured independently:

Configuration is performed in the LSC block parameters for the analog threshold or analog comparator: set the input type to "0-20 mA", "4-20 mA", or "0-10 V" as required. The AM2 has no internal shunt resistor to enable; the channel reads current when the input type is set to current mode and the wiring provides a current source into the terminal.

Specifications and Module Parameters

For firmware baseline, LOGO! 0BA7 devices are produced as hardware revision 0BA7 (= LOGO! 8 / FS:04). The companion LOGO! 0BA8 (= LOGO! 8.1 / 8.2) extends analog to 0-10 V only on the 12/24 RCE without changing the addressing rules described above. Always record the device's printed MLFB plus the FS (firmware state) when troubleshooting analog behavior; the FS is the two-digit suffix after the "-" in the order number (e.g. 6ED1052-1MD08-0BA0 is FS:01 production).

Common Pitfalls and Field-Proven Tips

1. Assuming AI1 is free when I7/I8 are used as DI. The base reservation is compile-time. Even an unused AI slot is still numbered. Always use I/O Status to confirm.
2. Wiring a 4-20 mA loop to the base. The base inputs are 0-10 V only. A 4-20 mA transmitter must terminate on the AM2 (or AM2 RTD).
3. Setting AI count to 4 AI when only two sensors exist. This consumes two extra AI slots (AI3, AI4) on the base that point to I7 and I8. The AM2 then shifts to AI5 and AI6, which breaks any pre-written LSC program that references AI3 / AI4. Decide the flag before drafting the circuit program.
4. Forgetting to re-transfer after changing AI count. The AI count is part of the program; an online edit alone is not enough. Use Tools > Transfer > PC → LOGO! after the change.
5. Reversing polarity on the AM2 current input. A reversed 4-20 mA loop reads as 0 mA equivalent (4 mA minimum) and triggers a wire-break if the underrange detection is enabled in the LSC block parameters.
6. Expecting the AM2 AQ variant to provide outputs. The AM2 (6ED1055-1MA00-0BA2) is input-only. The AM2 AQ (6ED1055-1MA00-0BA3) is input+output. The two are not interchangeable on the bus address plan, though the input addressing rules are the same.
7. Adding a TDE (text display) between the base and the DM16. The TDE occupies a bus slot and shifts all downstream DI/AI addresses by the TDE's count. With a TDE inserted, the DM16 24/R becomes I25..I40 and the AM2 shifts to AI5/AI6. The TDE has no DI/AI of its own, but it consumes one of the eight expansion slots.

Verification Procedure

Run this checklist after the wiring is complete and the program has been transferred to the LOGO! 0BA7 base:

1. Power-cycle the base with all expansions connected. The base performs a bus discovery; the front-panel menu Card > Options > Modules should list three modules: base, DM16 24/R, AM2.
2. Connect LSC V8 to the base and open Tools > Transfer > I/O Status.
3. Toggle each DM16 input with a wire jumper or test switch and verify the matching LSC bit changes (I9..I24 in order).
4. Apply a known analog signal to AM2 I1 and confirm movement on AI3 in I/O Status.
5. Apply a known analog signal to AM2 I2 and confirm movement on AI4 in I/O Status.
6. From the LOGO! onboard menu, navigate to AI > AI3 and AI > AI4; both should display the engineering value configured for the channel (0-1000 for 0-10 V, 0-1000 for 0-20 mA, 0-1000 for 4-20 mA scaled). The on-device display mirrors the LSC address plan, so a passing display is strong evidence the addressing is correct.
7. Run the circuit program in test mode for at least 10 minutes to confirm stable analog readings across the AM2 update interval (typically 50 ms per analog bus cycle).

If any step fails, return to the File > Properties > AI Count dialog and confirm the flag matches the intended configuration. A wrong flag is the single most common cause of "AM2 reads zero" symptoms in the field.