Migrating Siemens Masterdrives 6SE7021-8TB20 to 6SE7021-8TB61

Migrating Siemens Masterdrives 6SE7021-8TB20 to 6SE7021-8TB61 (CB1/CU2 → CBP2/CUVC)

This field-procedure document covers a controlled drive swap on a running Siemens SIMOVERT MASTERDRIVES Vector Control (VC) line. The legacy unit is a 6SE7021-8TB20 with a CB1 Profibus communication card and a CU2 control board; the replacement is a 6SE7021-8TB61 with a CBP2 Profibus card and a CUVC (Vector Control Unified) control board. The PLC is a Siemens S5 acting as a Profibus DP master, and the configuration tool is COM PROFIBUS. The objective is a drop-in replacement with minimal S5 program change while keeping the same PPO type, Profibus address, and drive behavior.

1. Hardware Identification and Compatibility

Both 6SE7021-8TB20 and 6SE7021-8TB61 are members of the SIMOVERT MASTERDRIVES VC compact family. The "8TB" prefix indicates the compact form factor with integrated line supply; the trailing suffix (20 vs 61) defines the control/communications option installed at the factory.

The power stage (IGBT inverter, rectifier, gate drivers, and current sensing) is electrically and mechanically compatible between the two MLFBs. The functional change is concentrated in the control board and the communication option.

2. Three Valid Migration Paths

Before performing a CU2→CUVC parameter conversion, evaluate the following three options. Each option changes the scope of work.

Option A — Hardware migration only (zero S5 program change)

If the legacy CB1 card and the legacy CU2 board are still functional, they can be physically transplanted into the new drive chassis. The new drive acts as a like-for-like replacement of the power section only. No S5 program change, no COM PROFIBUS change, no parameter conversion. This is the lowest-risk path but it sacrifices the benefit of the newer CUVC firmware.

Option B — Mixed migration (keep CB1 in a CUVC drive)

The CB1 card can be carried over to a CUVC drive by replacing the LBA backplane with the variant that accepts CB1 on a CUVC. In this case the S5 program remains unchanged but the parameter set must be re-entered through the CUVC menu because the CUVC firmware will not read a CU2 parameter backup.

Option C — Full migration (CBP2 + CUVC)

The new CUVC is used with the new CBP2 Profibus card. This is the path the rest of this document covers. It requires: (1) updating the GSD file in COM PROFIBUS, (2) parameter conversion CU2 → CUVC, (3) PPO type confirmation, and (4) optionally, a small adjustment in the S5 program if any control word / status word bit assignments changed.

3. Prerequisites

Before starting the swap, gather the following items and information.

1. Replacement drive 6SE7021-8TB61 with CBP2 installed and the matching LBA backplane.
2. Current parameter backup of the existing 6SE7021-8TB20. Use DriveMonitor to read all CU2 parameters (P000 to P999) and save the project to disk. Do not rely on the PMU panel only; the panel backup is incomplete.
3. The GSD file for the CBP2 board (SIEM8181.GSD or SI018180.GSD depending on firmware version). The correct GSD identifier must be selected in COM PROFIBUS — this is the most common failure point in the swap.
4. COM PROFIBUS V5.x or higher with the latest GSD library installed. Restart the tool after copying the GSD into its library directory.
5. The S5 Profibus master configuration and a printed copy of the S5 program that handles the drive's PKW and PZD areas. The S5 typically uses IM308C or CP5431 as the Profibus master; both handle parameter (PKW) and process (PZD) data as separate DBs or as flagged words.
6. Motor nameplate data, encoder specification (if fitted), and the original CU2 parameter printout for cross-reference.
7. The Siemens Masterdrives Compendium, specifically chapter 8.2 (CUVC + CBP2 + Step7 Profibus). Reference: SIMOVERT MASTERDRIVES Compendium.
8. Sample DP-communication project for CUVC + CBP2: SIMOVERT VC DP communication with CBP2.

4. Control Board Parameter Mapping: CU2 → CUVC

The CU2 and CUVC boards both control the same power stage, but the parameter numbering and the default value of certain parameters changed when the CUVC consolidated the VC and MC (Motion Control) firmware lines. The migration is therefore not a copy operation — it is a re-entry with cross-reference.

4.1 Parameter groups that map directly

4.2 Parameters that require remapping or re-entry

4.3 Quick parameter re-entry procedure

1. Open DriveMonitor and connect to the new CUVC via RS232 (PMU) or Profibus.
2. Download a factory reset (P060 = 2 for VC with encoder, P060 = 1 for V/f open-loop), then re-enter motor nameplate into P100–P107.
3. Transfer ramps (P462, P464), limits (P420, P421, P642, P643), and speed controller values (P235, P236) from the CU2 backup.
4. Re-wire the control word source using BICO: P554.1 = 3100 (control word from CBP2 word 1), P555.1 = 3101 (bit 1 of control word mapped to OFF2), etc. The CUVC defaults differ from the CU2 defaults and must be set explicitly.
5. Verify P918 (PPO type), P711 (bus address), and P797.01 (fieldbus timeout).
6. Save to non-volatile memory (P060 = 0) and disconnect DriveMonitor.

5. Communication Board Differences: CB1 → CBP2

The CB1 was a Profibus DP slave for the older CU1/CU2 control boards. The CBP2 is a Profibus DP slave for the CUVC control board. The protocol on the wire is the same (Profibus DP-V0 with PPO telegrams), but the slave identifier and the GSD file differ.

If the same PPO type and the same Profibus slave address are used on the CBP2 as were used on the CB1, the S5 program does not need to be modified. The PKW/PZD structure is byte-compatible.

6. GSD File Selection in COM PROFIBUS

The single most common cause of a "drive does not come up on Profibus" after a CB1 → CBP2 swap is an incorrect or outdated GSD file in COM PROFIBUS. Follow this exact sequence.

1. Locate the CBP2 GSD file on the installation medium (CD shipped with the new drive) or download from Siemens Product Support matching the CBP2 firmware version printed on the card label (e.g. V2.x).
2. Copy the GSD into the COM PROFIBUS GSD library directory (default: C:\SIEMENS\COMPB5\GSD). Do not place it in a subfolder; the tool scans only the top level.
3. Start COM PROFIBUS. From the menu Options > Update GSD Library. The tool re-scans the directory and adds the new GSD to the device catalog.
4. Open the project, delete the old 6SE7021-8TB20 (CB1) slave, and insert a new slave. The catalog entry is now under Drives > Siemens AG > SIMOVERT MASTERDRIVES VC > CBP2.
5. Set the slave address to the same value the CB1 used (e.g. 4, 5, 6 — whatever the S5 master expects).
6. Select the same PPO type as before. If the S5 program reads 6 PZD words, choose PPO2 or PPO4. If it reads 2 PZD words, choose PPO1 or PPO3.
7. Compile and download the new master configuration to the S5 Profibus master (IM308C or CP5431).

7. PPO Type Selection and Telegram Structure

The PPO (Parameter Process Object) defines the cyclic telegram size. The S5 reads/writes fixed-length data blocks, so the PPO type must be identical on the master configuration and on the drive (P918).

The PKW area carries the parameter ID (PNU), the sub-index, and the read/write value. The PZD area carries the control word (STW1) and setpoint (HSW) from master to drive, and the status word (ZSW1) and actual value (HIW) from drive to master.

8. S5 Program Considerations

The S5 Profibus master (IM308C or CP5431) reads/writes the drive's I/O area as a contiguous block of words in a data block (DB). With CB1 and PPO2, for example, the S5 would have 10 words — 4 PKW + 6 PZD. With CBP2 and the same PPO2, the layout is byte-for-byte identical.

Cases where the S5 program must be changed:

1. Byte/word order of PKW — see the byte-order note above.
2. Different PPO type chosen — if the new master configuration uses a different PPO type, the S5 DB lengths and word offsets must be updated.
3. Different control word source — if the CUVC BICO defaults are not corrected, bits like "OFF2", "OFF3", "enable", "ramp enable" may not be wired to the control word from CBP2, and the drive will not start. The S5 program is not changed, but the drive's response is wrong until P554 ff. is corrected.
4. Fault acknowledgement path — CUVC can acknowledge faults via a specific bit in the control word (bit 7 of STW1). CU2 used a different bit. Verify the S5 logic sets the correct bit.

9. Step-by-Step Replacement Procedure

1. Document the running drive. Print or save the active fault buffer, the current parameter set, and the Profibus slave address. Note the PPO type currently used in the S5 master configuration.
2. Lock out and tag out (LOTO) the line feeding the drive. Verify zero voltage on the DC bus (P700 in DriveMonitor shows the DC link voltage; wait until it drops below 50 V).
3. De-energize the S5 Profibus master if the master configuration is being changed, or leave it online if only the drive is being swapped (Option A).
4. Remove the legacy 6SE7021-8TB20. If the CU2 and CB1 boards are kept as spares (Option A), set them aside in ESD-safe packaging.
5. Install the new 6SE7021-8TB61. Mount the LBA backplane appropriate for CUVC + CBP2. Insert the CUVC into the right slot and the CBP2 into the option slot. Torque all power and control terminals to the spec in the drive installation manual.
6. Power up the new drive without connecting the S5 Profibus cable. Use DriveMonitor via the RS232 PMU port to perform a first sanity check: read the drive order number (P070), the firmware version of CUVC, and the firmware version of CBP2.
7. Run the parameter re-entry following section 4.3 above. Set the Profibus slave address in P918 to match the old address. Set the PPO type in P918 to match the master configuration. Set the fieldbus watchdog P797.01 to a value greater than the master's poll cycle (typical: 200 ms).
8. Save parameters with P060 = 0. Cycle the 24 V control power to confirm the parameters are non-volatile.
9. Update COM PROFIBUS per section 6. Recompile and download the new master configuration to the S5 Profibus master.
10. Reconnect the Profibus cable to the CBP2. Watch the CBP2 LEDs: BF (bus fault) should be off, SF (slave fault) should be off after the master polls. ON and RDY should be solid green.
11. Cycle the line and test from the S5 HMI: a small jog at low speed, then a full ramp test. Verify the control word is reaching the drive (read P554 in DriveMonitor) and the status word is reaching the S5 (read the S5 status word DB).

10. Verification Checklist

11. Troubleshooting Matrix

12. Field-Proven Caveats

Default differences. The CUVC defaults to V/f open-loop control (P290 = 0). Most Masterdrives VC applications are configured for vector control with encoder (P290 = 1) or without encoder (P290 = 2). If the application was running in vector mode on the CU2, set P290 to the same value or the motor will not develop rated torque.

Encoder supply voltage. CB1 and CBP2 supply the encoder from the drive's 24 V rail. Some 5 V TTL encoders require the 5 V tap on the encoder interface card. Verify the encoder interface card installed in the CUVC matches the encoder (e.g. SBP for HTL, SBR for resolver, SBM for sin/cos).

Watchdog behavior. The CB1 had a fixed hardware watchdog; the CBP2 watchdog is configurable in P797.01. If P797.01 is left at the default of 0 ms, the drive will trip immediately on the first comms loss. Set it to 100–500 ms as a baseline.

Save before power cycle. The CUVC has a buffered parameter save (P060 = 0). The CB1 had an automatic save on power-down. If the parameter save is not executed explicitly, the new values are lost on the next power cycle and the drive reverts to defaults, which is one of the most common "it worked yesterday, it doesn't work today" field complaints.

S5 master compatibility. The IM308C and CP5431 S5 Profibus masters support both CB1 and CBP2 without hardware or firmware change, provided the GSD file on the master is updated. If the S5 master is older firmware (pre-V3), update the master firmware or use a CP5431 with the latest firmware.

FAQ