Controlling SINAMICS V20 with LOGO! 8 Analog, Digital, Modbus RTU

Controlling SINAMICS V20 with LOGO! 8 — Analog, Digital, and Modbus RTU Methods

Siemens LOGO! 8 logic modules cannot natively speak the SINAMICS V20's USS or Modbus RTU protocol stack without a dedicated communications interface, but the V20 still drops cleanly into a LOGO! system through three well-trodden paths: a 0-10 V analog link using the LOGO! AM2 AQ expansion, digital hardwiring that exploits the V20's built-in connection macros (Cn002 / Cn003 / Cn005), or a true bus link through the LOGO! CIM Modbus RTU module. Each method trades wiring density, parameter depth, and update latency against cost and program complexity. This reference documents all three, with the exact terminal assignments, parameter sets, register maps, and FBD building blocks needed to ship a working drive control from a single LOGO! 8 base module.

1. System Overview and Communication Options

The SINAMICS V20 is a compact, sensorless vector drive for low-power 1AC/3AC motors (0.12 kW to 30 kW) with two on-board communications paths: RS485 USS and RS485 Modbus RTU, exposed on terminals 14/15 of the control board. It does not carry PROFINET, EtherNet/IP, or PROFIBUS on the base unit. A LOGO! 8 base module (6ED1052-1xx08-0BAx, firmware ≥ V8.0) has on-board RS232 only for the LOGO! TD text display and programming cable; it has no RS485 port. The 0BA7 / 0BA8 generations (sold as "LOGO! 8" and "LOGO! 8.FS4") added Ethernet on certain variants (LOGO! 8.FS4 with 6ED1052-1MD08-0BA0 / 6ED1052-1FB08-0BA0) but still no Modbus RTU master port. The only native Modbus RTU master capability appears in LOGO! 8.FS4 with the LOGO! CIM attachment, an external communications interface module that snaps onto the base module's expansion port and adds a true Modbus RTU master/slave to the LOGO! backplane.

2. Prerequisites and Hardware Selection

Before choosing a topology, confirm the BOM against the actual LOGO! on the bench. A common pitfall is to buy a LOGO! 8 without analog outputs and then discover the AM2 AQ must piggyback on a base module that exposes the right expansion bus pinout. Use the Siemens Product Support portal to validate the order number against the firmware sticker on the side of the module.

• LOGO! 8 base module: 6ED1052-1MD08-0BA0 (12/24 VDC, 8 DI / 4 DO, with display) or 6ED1052-1FB08-0BA0 (230 VAC, 8 DI / 4 DO). Both support the AM2 AQ on the expansion port. A "LOGO! 8.FS4" sticker or firmware ≥ V8.3 is required if you intend to add the LOGO! CIM for Modbus RTU.
• LOGO! AM2 AQ: 6ED1055-1MM00-0BA2 — 2 analog outputs, each 0-10 V or 0/4-20 mA, 12-bit resolution. It is the cheapest way to push a variable speed reference into the V20's AI pin.
• LOGO! CIM: communications interface module for LOGO! 8.FS4 with Modbus RTU master capability. Confirmed for the application case "LOGO! CIM – Modbus RTU communication with SINAMICS V20" (Siemens support entry 109806055).
• SINAMICS V20: 1AC 230 V (6SL3210-5BE21-xUV0) or 3AC 400 V (6SL3210-5BE27-xUV0) family. Any frame size A through D uses the same 14-position control terminal block (1-15). Refer to the SINAMICS V20 Operating Instructions for the exact frame-specific pinout.
• LOGO! Soft Comfort V8.x programming software for the FBD examples below. The free upgrade is available from the Siemens support portal.

3. Method 1 — Analog 0-10 V Speed Control with AM2 AQ

The V20 exposes a 0-10 V / 4-20 mA analog input on terminals 7 (AI+) and 8 (AI-, signal ground) with an internal 10 V potentiometer supply on terminal 6. Driving that AI with the LOGO! AM2 AQ's 0-10 V output gives a clean proportional speed reference that tracks P2000 (the normalization reference, default 50 Hz) and uses the V20's internal ramp generator (P1120 / P1121) for the start/stop ramp. Run/Stop and direction are handled with three ordinary digital outputs from the LOGO! 8 base module wired to V20 digital inputs.

3.1 Wiring — LOGO! AM2 AQ → V20 AI

Digital wiring for run/stop and direction is overlaid on the V20's 24 V sourcing inputs:

The 24 V jumper from V20 terminal 12 to terminals 1, 2, 3 commons is set on the control board and is required because V20's digital inputs are 24 V sinking/source depending on the jumper position. With the jumper in "Source" mode (the default for EU variants), an open switch is 0 V (false) and a closed switch to +24 V is true; this matches the LOGO! 8's 24 V sourcing outputs.

3.2 V20 parameter set for analog control (Cn002 / Cn005 hybrid)

Apply connection macro Cn002 for a pure analog reference or Cn005 for analog with two fixed frequencies as a backup. The exact parameters applied by each macro are listed in the V20 Operating Instructions; the key values for the LOGO! analog path are:

0 V at the LOGO! AM2 AQ M1 terminal and 10 V at U1+ corresponds to a setpoint of P2000 (default 50 Hz). Scaling is linear. To slow the maximum speed, lower P2000 (reference). The drive ramps internally on P1120 / P1121 whenever the setpoint changes by more than 1 % or the run input toggles.

3.3 LOGO! Soft Comfort program for the analog path

The minimum useful FBD in LOGO! Soft Comfort V8.x looks like this:

1. Block B001 — Analog Input off an internal potentiometer (I7 on the base module, or a remote potentiometer on AI1 of the AM2 AQ if used in reverse). Range 0-1000 represents 0-10 V on the AQ.
2. Block B002 — Analog Amplifier (Gain = 1.0, Offset = 0) to pass the setpoint through.
3. Block B003 — PI Controller (optional). Set SP = the analog input, PV = a 4-20 mA feedback signal from a pressure sensor, Output → AQ1. The V20's internal ramp then governs the actual motor acceleration.
4. Block B004 — AND gating the run command with the safety interlock. Input I1 = start, I2 = stop (NC), I3 = safety door. Output → Q1 (drive ON/OFF1).
5. Block B005 — RS Flip-Flop latching the run state. Set input from the AND of I1, reset from the OR of I2 + drive fault (read via I4 = V20 relay output).
6. Block B006 — Off Delay of 2 s on the fault input to debounce the V20 fault relay.

The output wiring is then straightforward: Q1 → V20 DI0 (run), Q2 → V20 DI1 (reverse), Q3 → V20 DI2 (fault acknowledge pulse generated by a 0.5 s pulse relay on Q3), AQ1 → V20 AI+ on terminal 7.

4. Method 2 — Digital I/O with Connection Macros

If the application only needs three or four discrete speeds plus run/direction, the AM2 AQ module can be dropped entirely. The V20 has a connection macro called Cn003 (three fixed frequencies) and Cn004 (four fixed frequencies) that wires DI0-DI2 to run, direction, and fixed-speed selection, with the setpoint selected by a binary code on the remaining DIs. The factory-default fixed speeds are P1001 = 5 Hz, P1002 = 10 Hz, P1003 = 15 Hz, P1004 = 20 Hz; these are freely re-programmable.

4.1 Connection macro Cn003 — three fixed frequencies

Cn003 configures the drive as follows:

Applying Cn003 sets P0700 = 2, P0701 = 1, P0702 = 15 (fixed setpoint bit 0), P0703 = 16 (fixed setpoint bit 1), P0704 = 12 (reverse), and writes the default fixed setpoints P1001, P1002, P1003. The exact mapping is documented in chapter 6.1 of the V20 Operating Instructions.

4.2 Fixed-speed selection truth table (Cn003)

4.3 Connection macro Cn005 — analog with two fixed frequencies

If the application wants the AM2 AQ's smooth setpoint but the digital I/O simplicity, Cn005 configures the V20 for analog main setpoint with two fixed frequencies as backup, and keeps DI0 = ON/OFF1, DI1 = fixed-setpoint select 0, DI2 = fault ack. The analog input drives the motor whenever DI1 is 0. This is the recommended hybrid for the "gradual ramp with discrete speed floor" requirement.

4.4 LOGO! program for the digital-only path

Use a binary decoder to translate the discrete push-buttons (low speed, mid speed, high speed) into the bit pattern on Q2 / Q3, and a latching run circuit on Q1. A minimal program:

1. Block B001 — RS Flip-Flop for run latch (I1 = start, I2 = stop).
2. Block B002 — Counter Up/Down (0-3) incrementing on I3 = speed-up, decrementing on I4 = speed-down.
3. Block B003 — Decoder 2 → 4 (or 3 → 8) producing the bit pattern on Q2 / Q3 from the counter value.
4. Block B004 — Off Delay of 0.5 s on Q1 to ensure the run command is the last to drop on stop.
5. Block B005 — AND of Q1 (run) with the decoded speed bit, then OR with Q2 / Q3 outputs to drive DI1 / DI2.

5. Method 3 — LOGO! CIM Modbus RTU (LOGO! 8.FS4)

The Siemens application example "LOGO! CIM – Modbus RTU communication with SINAMICS V20" (Siemens support entry ID 109806055) provides a tested program, parameter list, and wiring diagram for direct Modbus RTU control of the V20 from a LOGO! 8.FS4 with the LOGO! CIM module. This is the only path that exposes the V20's full control word (STW) and status word (ZSW) and lets the LOGO! read back motor current, DC bus voltage, fault codes, and actual frequency.

5.1 Hardware

• LOGO! 8.FS4 base module, firmware ≥ V8.3 (displayed as "LOGO! 8" with the FS4 sticker).
• LOGO! CIM (Communications Interface Module) — adds a 4-pin RS485 terminal block to the LOGO! backplane.
• SINAMICS V20, firmware ≥ V1.21 (the bus firmware revision is visible on P0003 / r0018 in the drive BOP).

5.2 RS485 wiring (LOGO! CIM ↔ V20)

Termination is enabled on the LOGO! CIM DIP switch for the last (or only) node, and disabled on intermediate nodes. The V20 has no on-board RS485 termination resistor; if the V20 is the end of the segment, fit a 120 Ω resistor across terminals 14/15.

5.3 V20 Modbus RTU parameters (Cn011 or manual)

Apply the connection macro Cn011 (MODBUS RTU control) or set the parameters manually. The full set is:

The default P2010 = 6 is 9600 bit/s. For higher update rates bump P2010 to 11 (115200 bit/s) provided the LOGO! CIM is set to the same baud rate in the LOGO! program. Always match baud rate, parity, and stop bits exactly on both sides; mismatched parity is the most common cause of "drive does not respond" symptoms on a fresh install.

5.4 Modbus register map (SINAMICS V20)

5.5 Control word bit map (40100)

5.6 LOGO! Soft Comfort program outline

The LOGO! CIM is configured in the LOGO! Soft Comfort "Network" project as a Modbus RTU master on a serial port. Inside the FBD, two function blocks handle the bus traffic:

1. Block M001 — Modbus Master reading register 40110 (ZSW), 40111 (HIW), 40112 (current). Function code 03, slave address = 1, scan every 100 ms.
2. Block M002 — Modbus Master writing register 40100 (STW) and 40101 (HSW). Function code 06, slave address = 1, updated every 100 ms.
3. Block B001 — Build the STW: 0x047E base + bit 0 (run) + bit 7 (direction) + bit 11 (reverse enable). Output → M002 STW value.
4. Block B002 — Build the HSW: scale the 0-1000 setpoint to 0x0000-0x4000. Output → M002 HSW value.
5. Block B003 — Decode ZSW bit 2 (running), bit 3 (fault), bit 7 (alarm). Output → Q1 (ready), Q2 (run lamp), Q3 (fault lamp).

The full FBD with analog conversion tables and scaling is provided in the Siemens application example PDF. Reuse that program as-is if your motor and rated current match the V20 defaults; otherwise, scale P2000 (reference) and P0305 (motor rated current) to match your hardware.

6. SINAMICS V20 Parameter Quick Reference

The full V20 parameter list runs to over 600 parameters; the subset that touches a LOGO! application is in Table 11.

7. Topology and Wiring Diagrams (SVG)

Two schematics encode the wiring. Render them in any modern browser or paste into a vector editor.

8. Commissioning, Verification, and Test Procedure

Run the following checklist before energizing the motor. Each step is a pass/fail gate; do not proceed if a step fails.

1. Power off. Verify all wiring with a meter. Confirm 24 V common is bonded at one point only (typically at the LOGO! 8 power supply).
2. Power on the LOGO! 8 only. The base should reach the RUN indicator (green). Download the program from LOGO! Soft Comfort; confirm the LOGO! Soft Comfort online view shows the expected I/Q/AQ state on a manual input toggle.
3. Power on the V20. The drive should display "ready" within 3 s. If the drive shows F0001 (overcurrent on power-up) immediately, check for a wiring short on the power stage, not the control board.
4. For analog control: with the LOGO! in STOP, command AQ1 = 0 %. Measure V20 terminal 7 to terminal 8 with a meter. Reading should be < 50 mV. Command AQ1 = 100 %; reading should be 9.95-10.05 V. If the voltage is reversed, swap U1+ and M1 on the AM2 AQ.
5. For digital control: with the LOGO! in RUN and Q1 asserted, measure V20 terminal 1 to terminal 13. Reading should be 22-24 V. If reading is 0 V, the 24 V jumper is missing between terminals 12 and 1.
6. Issue a small setpoint (5 Hz) and command run. Verify on the BOP (V20 basic operator panel) that r0024 (output frequency) ramps up at the P1120 rate. Reverse command should toggle r0026 (output frequency × sign) negative.
7. For Modbus RTU control: from the LOGO! Soft Comfort online view, force STW = 0x047E (no run). Read back ZSW. Bit 0 (ready to switch on) should be 1. Force STW = 0x047F (run command). Read back ZSW. Bit 2 (operation enabled) should transition 0 → 1 within 1 s. If ZSW remains 0x0000, the bus is not communicating — check termination, address, parity, and baud.
8. Fault test: command a fault condition (e.g., open the enable interlock while running). The V20 should display F0001 or the appropriate fault code. Q3 (fault output) on the LOGO! should latch. Reset with a 0.5 s pulse on Q3 (digital) or set STW bit 15 (Modbus).

9. Performance and Update-Rate Considerations

The three methods have very different update latencies between an input change on the LOGO! and a corresponding change on the motor. The analog path is the slowest in absolute terms because the V20's internal ramp (P1120/P1121) dominates, but it produces the smoothest mechanical response. The digital macro path is the fastest at the digital I/O layer (one LOGO! scan, ~10 ms) but produces stepped setpoint changes that the drive must ramp. The Modbus RTU path is configurable, with the LOGO! program and the bus baud rate both setting the cycle time. At 9600 bit/s and a 100 ms scan, expect 4-5 telegram roundtrips per second on a single slave; at 115200 bit/s that rises to ~40. For closed-loop pressure or flow control, push P2010 to 11 (115200) and set the LOGO! Modbus Master block to scan every 20 ms.

10. Troubleshooting Matrix

11. Frequently Asked Questions