Controlling 6RA70 Digital I/O via Profibus with S7-414-2DP

This reference explains how to expose and actuate the digital inputs and outputs of a Siemens SIMOREG 6RA70 series DC MASTER drive on a Profibus-DP network controlled by a SIMATIC S7-414-2DP CPU. It covers the connector/binector (K/U/B) routing required in the drive, the PPO/PZD word layout that carries the free digital I/O over the bus, the S7 hardware configuration, the STEP 7 program interface, and a complete commissioning and troubleshooting matrix. The information is written for engineers commissioning rolling-mill lines, stand-alone DC drives, and retrofit applications where the 6RA70 must be controlled bit-for-bit from a Profibus master.

1. System Overview

The 6RA70 is a fully-digital, three-phase (six-pulse or twelve-pulse) DC converter designed for armature and field control of DC motors up to 1200 A and beyond. Communication is handled by one or two Communication Unit boards (CUD1 left slot, CUD2 right slot). Each CUD exposes:

• Profibus-DP slave interface (baud rate 9.6 kbit/s to 12 Mbit/s, automatic detection)
• Up to 6 digital inputs on the CUD terminal strip (X171, X172)
• Up to 4 digital outputs on the CUD terminal strip (X173)
• Additional digital I/O available on the customer terminal strip (X177) and power-interface board
• One serial RS485 port (X200) for commissioning with DriveMonitor / STARTER

The S7-414-2DP is a SIMATIC S7-400 CPU with an integrated Profibus-DP master port (MPI/DP, selectable). It exchanges a parameter-process-data object (PPO) with the drive every Profibus cycle. The 5th PZD word (16 bits) is a "free" 16-bit channel; routing connectors and binectors in the drive is what turns that 16-bit channel into a digital input mirror and a digital output actuator.

2. Prerequisites

2.1 Hardware

• 1 × SIMOREG 6RA70 (any frame size, CUD1 populated, CUD2 optional for redundancy)
• 1 × SIMATIC S7-414-2DP (or any S7-400 with a Profibus-DP master port, e.g. S7-414-3PN/DP via CP443-5)
• Profibus cable (violet, type A, 2-wire, 120 Ω characteristic impedance, terminated on both ends)
• Two Profibus connectors with terminating resistors (last node = ON)
• Shielded I/O wiring 0.5 mm² for the digital signals on the 6RA70 terminals

2.2 Software

• STEP 7 V5.5 + SPx (or TIA Portal V15.1 and higher with HSP for SIMOREG)
• SIMOREG 6RA70 GSD file: SIEM811F.GSD (for firmware < 3.x) or SIEM8117.GSD (for firmware 3.x and newer)
• DriveMonitor or STARTER (Comissioning tool with SIMOREG device package)
• Manual set: SIMOREG DC MASTER 6RA70 Operating Instructions (E31925-K3080-U001 series)

2.3 Required drive firmware parameters

Verify the following parameters are accessible (write protection P053 = 0):

• P051 = 40 (parameter access level — service)
• P052 = 3 (parameter set selection: enable CUD2 parameters when populated)
• P060 = 5 (free PZD selection — see below)
• P918 = <DP node address> (Profibus station address, default 4)

3. Profibus PPO/PZD Layout

SIMOREG 6RA70 supports PPO types 1, 2, 3, 4, and 5. Only PPO type 4 (6 PZD each direction) and PPO type 5 (10 PZD each direction) provide the fifth PZD word that is used as the "free digital channel".

PPO 5 extends the free area with words 7–10. The fifth word is the one most often used because it does not conflict with the standard control word 1 / status word 1 handshake (on/off, fault reset, ramp enable, etc.) and is short enough to read as a single 16-bit pattern.

3.1 PPO type selection in the drive

Set the PPO type in the master HW Config and verify with parameter P060 (PZD configuration) and P927 (parameter write enable). The drive will accept whatever the master sends; mismatches are logged as fault F060.

4. Reading the Digital Inputs (Drive → PLC)

The internal status of the digital inputs is available at the binector K0020. To bring this status onto the bus, route the binector to a connector — the connector name format is U734.nn, where the index nn selects the position of the 16-bit word inside the PZD set. The 6RA70 uses U734 for the free PZD outputs. Each U734.nn corresponds to a 16-bit word:

4.1 K0020 (DI status) — bit map

K0020 is a 16-bit consolidated binector (an aggregation of digital input states). On the standard CUD1 terminal strip the mapping is:

The exact bit-to-terminal mapping is shown on function diagram Z110 in the SIMOREG 6RA70 manual. K0020 is also documented on page 652 of the parameter list.

4.2 Step-by-step: route K0020 to U734.05

1. On the OP1S or DriveMonitor front panel, navigate to P764 (connector destination assignment — index 05).
2. Set P764.05 = K0020.
3. Save with P.
4. Cycle the 6RA70 control board to re-initialise the connector routing, or write P052 = 0 followed by P052 = 3 to force a parameter-set reload.
5. On the S7 side, after the next Profibus cycle, the fifth PZD receive word in the drive's I/O image will contain the live status of the digital inputs.

5. Writing the Digital Outputs (PLC → Drive)

The free PZD word coming from the PLC is mapped to binectors B3500–B3515 (one per bit). To make a bit of that word appear on a physical digital output, route the corresponding binector into the parameter that owns that output. The 6RA70 exposes digital outputs through parameters P771–P774:

For each of the four binary outputs, P771–P774 accept a binector source (e.g. B3500, B3501, B3502 …). The default source is the standard status-word bit, so without re-routing the output is meaningless from a free-channel perspective.

5.1 Bit mapping (PLC → drive → output)

5.2 Step-by-step: route B3500–B3503 to P771–P774

1. Navigate to P771; assign P771 = B3500.
2. Navigate to P772; assign P772 = B3501.
3. Navigate to P773; assign P773 = B3502.
4. Navigate to P774; assign P774 = B3503.
5. Save the parameters.
6. Set P052 = 0 then P052 = 3 to commit the routing to non-volatile storage.

After this, setting bit 0 of the fifth PZD send word from the S7-414-2DP will turn on DO0 on the CUD1 terminal strip. Bits 1–3 control DO1–DO3, and bits 4–15 are available for software-side interlocks inside the drive (e.g. wired to additional free binectors for ramp-enable, brake release, fault acknowledge, or warning-lamp logic).

6. S7-414-2DP Hardware Configuration

1. Open SIMATIC Manager and load the station with the S7-414-2DP.
2. In HW Config, double-click the DP master port and open "Properties → Profibus". Set the address (default 2) and baud rate (1.5 Mbit/s is the practical maximum for a stable 6RA70 link, even though 12 Mbit/s is supported).
3. Open the GSD file SIEM811F.GSD (or the equivalent for your firmware) and drag the 6RA70 slave from the catalog into the Profibus subnet.
4. Set the slave Profibus address to match P918 on the drive (default 4).
5. Slot 1 of the slave: select PPO type 4 (or 5) with 6 PZD each direction.
6. Map the I/O addresses. The fifth PZD word will typically land at PEW xxx (input from drive) and PAW xxx (output to drive) within the S7 input/output image of the slave.

6.1 Example address assignment (32-byte aligned)

7. STEP 7 Program — Consistent Read/Write

For reliable, cycle-consistent data exchange with the 6RA70, always use SFC14 DPRD_DAT and SFC15 DPWR_DAT. Direct peripheral access (L PIW 264) returns only process-image data and is not consistent across the Profibus cycle; bit operations inside the S7 CPU can tear a 16-bit value.

7.1 Reading the DI status word (PIW 264)

// Call in OB1, FB or FC
CALL  SFC 14  (
  LADDR    := W#16#0108,            // logical base address = 264 decimal = 0x108
  RET_VAL  := MW 100,               // return code (0 = OK; 80xx/82xx = error)
  RECORD   := P#M 110.0 BYTE 12     // destination: 12 bytes for 6 PZD words
  );

// Bit-level access (mask, do not alias the word)
L     W#16#0001;     // DI0 mask
T     MW 120;        // mask for DI0 in MW 110 low word

L     MW 110;        // 5th PZD word lives in low word of M110..M121
L     MW 120;
UW    ;              // AND
==I   ;              // test
=     M 200.0;       // boolean: DI0 active

7.2 Writing the DO control word (PQW 264)

// Build the 5th PZD control word
L     MW 300;              // free DO control word, set by HMI or logic
T     MW 314;              // copy to PZD5 send position

CALL  SFC 15  (
  LADDR    := W#16#0108,
  RECORD   := P#M 310.0 BYTE 12,   // 6 PZD = 12 bytes
  RET_VAL  := MW 322
  );

The first 12 bytes of M310..M321 (six 16-bit words) hold PZD1..PZD6 in the same order: PQW 256..266. The fifth word is MW 318.

7.3 STL/cyclic style for OB1

// On every OB1 cycle (or in OB35, the cyclic interrupt OB)
      CALL  SFC   14
           LADDR  :=W#16#108         // PIW 264
           RET_VAL:=MW   100
           RECORD :=P#M 110.0 BYTE 12;

      CALL  SFC   15
           LADDR  :=W#16#108         // PQW 264
           RECORD :=P#M 310.0 BYTE 12
           RET_VAL:=MW   322;

8. CUD1 and CUD2 Considerations

The 6RA70 chassis has two CUD slots. Each CUD runs its own parameter set, and the relevant parameters (P764.x, P771–P774, K0020, B3500–B3515) exist separately on each board. If only CUD1 is populated, only the CUD1 parameters apply; the Profibus slave represented on the bus is CUD1. If CUD2 is fitted, both CUD1 and CUD2 must be configured.

8.1 CUD1 vs CUD2 routing

For redundant operation in a rolling-mill line, the S7-414-2DP will see two Profibus slaves (one per CUD) and must hold two parallel routing configurations. The application logic in the PLC should only treat the active CUD as authoritative; the standby CUD can be polled for diagnostics but its outputs are masked.

9. Function Diagram Quick Reference

These four sheets are mandatory reading before configuring anything; the manual numbering varies by edition, but the sheet numbers Z110, G110, G111, G112 are stable across the 6RA70 family.

10. Commissioning Procedure

1. Connect the Profibus cable between the S7-414-2DP and the 6RA70; both ends terminated.
2. Power up the drive, set P051 = 40 and P918 = 4 (or as per your Profibus plan).
3. Configure the PPO type in HW Config; PPO 4 is the recommended default.
4. Bring the CPU up in run. Watch the slave diagnostic LED on the 6RA70: green steady = Profibus up, flashing = data exchange but parameter mismatch, off = no bus.
5. DriveMonitor: open the drive online and confirm that r735.05 (or the read-only back-mirror of U734.05) tracks the 5th PZD receive word from the PLC.
6. Set bit 0 of the S7 fifth PZD word to 1. Verify with a multimeter that DO0 (X173.1/X173.2) closes.
7. Force a single DI high and verify the corresponding bit arrives in the S7 PIW. Toggle every DI to validate the K0020 routing.
8. Toggle every DO bit from the PLC; verify the same bit pattern on the CUD terminal strip.
9. Save parameters to non-volatile (P052 = 0 → 3).
10. Run a full drive enable sequence (STW1 = 047Eh then 047Fh) to confirm standard control-word 1 handshake is not affected by the free PZD work.

11. Verification Checklist

12. Troubleshooting Matrix

13. Standards and Reference Material

• PROFIBUS & PROFINET International (PI) — protocol specification, PPO type definitions, GSD conventions.
• SIMOREG DC MASTER 6RA70 Operating Instructions (Siemens order number 6RX1700-0AD76) — function diagrams Z110, G110, G111, G112 and parameter list including K0020 (page 652).
• SIMOREG 6RA70 List Manual (parameter list) — full descriptions of P060, P764, P771–P774, P918, r723, r735.
• SIMATIC S7-400 CPU 414-2DP Manual — Profibus-DP master configuration, SFC14/SFC15 usage.

14. Edge Cases and Field Notes

14.1 Free PZD vs. control word collision

The Profibus standard for SIMOREG fixes the first two PZD words (STW1/ZSW1 + speed setpoint/actual). Even with PPO 4, the master cannot re-purpose PZD 1 or 2 for free I/O. Do not route K0020 to U734.01 or U734.02 — the master will overwrite it with STW1 every cycle.

14.2 Latency in DI mirroring

Mirror latency is one Profibus cycle plus the S7 OB1/OB35 cycle. At 1.5 Mbit/s and 5 ms S7 cycle, expect ≤ 20 ms total. For high-speed interlocks (e.g. emergency stop over the free PZD), use the dedicated safe inputs on the CUD or wire a hardwired safety chain — do not rely on Profibus round-trip.

14.3 Wire-break on a DO

The CUD outputs are 24 V / 0.5 A sourced from the CUD's internal 24 V. If a wire breaks, the binector still toggles, the parameter still updates, but no current flows. Always confirm with a meter; do not trust the PLC bit alone.

14.4 Adding a redundant Profibus path

Where the 6RA70 is on a Y-link or redundant ring, the 6RA70 Profibus interface does not support Profibus redundancy on its own. Use two CBP2 modules and an external Y-link coupler if full redundancy is required. The free PZD configuration must be repeated on the second CBP2.

14.5 TIA Portal versus STEP 7 V5.5

Configuration is identical at the slave level. In TIA Portal, install the GSD as a generic Profibus device; the slot/PPO mapping dialog is functionally the same. SFC14/SFC15 in TIA Portal is replaced by DPRD_DAT/DPWR_DAT with the same I/O area definition. Bit-level access is identical.

14.6 Brake-control note

For rolling-mill stand drives, DO0/DO1 are typically used for brake release (open/close). Wire the brake contactor through DO0 and use DO1 as a brake-closed feedback or as a secondary release path. Release DO0 only after the drive reports running (ZSW1 bit 2 = 1) and the speed actual exceeds the configured holding speed (default 3 % n_max). Map the enabling condition to bit 4 of the free PZD word (B3504) and then to a free binector, so the application code can interlock DO0 directly in the drive.

15. Roll-up Summary

To control the digital I/O of a 6RA70 from a S7-414-2DP Profibus master:

1. Select PPO type 4 (or 5) in HW Config so that 5+ PZD are exchanged.
2. For inputs, assign P764.05 = K0020 to send DI status to PZD 5 receive.
3. For outputs, assign P771 = B3500, P772 = B3501, P773 = B3502, P774 = B3503 to actuate DO0–DO3 from PZD 5 send.
4. Read with SFC14 and write with SFC15 to ensure consistent PZD exchange.
5. Verify each bit with a meter on the CUD terminal strip before trusting the logic.
6. If CUD2 is also fitted, repeat the entire routing on the CUD2 parameter set; the two CUDs are independent slaves on the bus.

Once the routing is in place and the parameters are saved, the fifth PZD word is a free 16-bit digital channel between the S7-400 and the drive — 8 usable input bits (DIs 0–7, with DI0–DI3 typically reserved for default control functions) and 4 usable output bits (DO0–DO3) in the standard configuration, with the remaining bits available for drive-internal binector logic.