SINAMICS DCM: PC Adapter USB A2 Communication and Power Setup

Overview

Establishing a point-to-point engineering link between a SINAMICS DCM DC drive (6RA80 family) and the STARTER commissioning tool through a PC Adapter USB A2 (Siemens order code 6ES7972-0CA00-0XA0) is a routine requirement during commissioning, parameterization, fault diagnosis, and firmware updates on Standard or Advanced Control Units (CUD). The single most frequent fault mode reported by STARTER is the “Communication not established” dialog generated by the Set PG/PC Interface wizard. In the majority of cases the root cause is not a defective cable or a wrong PROFIBUS address but the absence of the external 24 V DC supply that the USB A2 adapter requires at its power terminals in order to feed its RS-485 transceiver.

This reference documents the complete setup: drive-side CUD pinout, adapter-side wiring, the 24 V power input, PROFIBUS termination, the drive parameter that selects the bus protocol, and the STARTER PG/PC interface configuration. It also disambiguates the role of analog reference terminals on the CUD (commonly labeled 31 and 32) so that the 24 V is sourced from the correct power terminals and not mistakenly applied to the ±10 V reference outputs.

SINAMICS DCM Communication Architecture

The SINAMICS DCM is a converter platform designed for the speed and torque control of DC motors in basic and complex applications. Two communications paths are available on each CUD:

For service commissioning with a laptop, the most-used path is the Standard CUD's X9 socket running PROFIBUS. Both the MPI and PROFIBUS protocols share the same physical RS-485 layer on X9, and the active profile is selected by drive parameter p0927 (or, on newer firmware, p2030). The Standard CUD does not include PROFINET; PROFINET requires the Advanced CUD module.

Hardware Required

For a point-to-point link between STARTER and a single drive, a terminator is not strictly required, but it does help signal integrity and avoids false “bus fault” diagnostics in the adapter's LED pattern.

PC Adapter USB A2 Architecture

The PC Adapter USB A2 is a self-contained converter from a USB 2.0 port to either an MPI or PROFIBUS RS-485 segment. It contains an isolated transceiver, a small microcontroller, and an on-board regulator that draws power only from the USB port for the digital side. The RS-485 transceiver on the back of the adapter is fed by an internal DC/DC converter that requires 24 V to be present at the adapter's power terminals for the bus side to operate. Without 24 V, the D-sub 9 socket remains physically present but electrically silent: STARTER sees the bus as missing, and the adapter's PG LED stays on while the PBR LED remains off.

A correct and healthy point-to-point link shows: 24 V on, PG on, PBR flashing (master telegram requests), RCV flashing (slave responses). When 24 V is missing the only LED on is PG; this is the signature fault pattern.

24 V External Power Supply - Critical Requirement

The PC Adapter USB A2 exposes two spring-cage power terminals labeled 24V and 0V on the top face of the housing. These terminals accept 24 V DC at a draw of approximately 100 mA. The adapter ships without any external supply, and Siemens does not supply a dedicated wall-mount PSU with the adapter; the engineer must source one separately. A typical implementation is a Siemens Logo! power supply (6EP1332-1SH43) or any 24 V rail on the cabinet.

Wiring the 24 V supply is straightforward:

1. Route a 24 V DC supply pair (+ and 0V) from the cabinet to the adapter's spring terminals.
2. Observe polarity: reversed polarity is blocked by the adapter's protection diode, but the internal supply will not energize until correct polarity is applied.
3. Verify the 24 V LED on the adapter face before continuing.
4. Connect the USB cable to the PC and wait for the Windows driver to enumerate the adapter (Siemens ships a signed driver in the PC_Adapter_USB_A2 package).

CUD Standard X9 Pinout

The PROFIBUS / MPI port on the Standard CUD is a female D-sub 9 ("socket") on the front of the Control Unit, labeled X9. The pinout follows IEC 61158-2 and is identical to a DP-9-pin PROFIBUS connector.

STARTER PG/PC Interface Setup

Once the adapter is powered, STARTER must be told which physical interface to use. The configuration is held in Windows as the Set PG/PC Interface control panel applet.

1. Open Windows Control Panel → Set PG/PC Interface. (On newer Siemens toolchains the path is Start > Siemens Automation > SIMATIC > Set PG/PC Interface.)
2. In the Access Point of the Application drop-down, select S7ONLINE.
3. In the Interface Parameter Assignment Used list, select PC Adapter (MPI / PROFIBUS) with the proper USB instance (e.g. PC USB A2).
4. Open the Properties dialog. On the PROFIBUS tab set the transmission rate to 1.5 Mbps for a point-to-point link - the simplest configuration. Lower rates (187.5 kbps) are mandatory for MPI.
5. On the PROFIBUS tab, the Node Address of the PG must be unique on the segment. Use 0 for a PG master unless 0 is already used by an automation system on the same physical cable.
6. Confirm and close. STARTER will read this back when it next attempts an online connection.

Drive-Side Protocol Configuration

For a brand-new drive that ships with a default profile, the Standard CUD's X9 port is preconfigured for PROFIBUS DP-V1 as a slave. The following parameters should be verified before going online.

If the drive was previously in an MPI picontext and STARTER keeps reporting an MPI mismatch, change p2030 to 1 (PROFIBUS) and cycle the drive electronics power so the new protocol takes effect. This is the second-most-common cause of communication not established reports after the 24 V supply issue.

Connection Procedure

1. Power the SINAMICS DCM and let the CUD complete its boot-up. The RDY LED stays green when the CUD is healthy.
2. Apply 24 V DC to the PC Adapter USB A2 power terminals. Confirm the 24 V LED.
3. Plug the USB cable into the laptop. Wait for Windows to enumerate the Siemens PC Adapter USB A2 COM/USB device.
4. Plug the PROFIBUS cable into the CUD's X9 socket and into the PC Adapter's D-sub 9 socket. Screw the connector thumbscrews on both ends.
5. Open STARTER, open the project containing the SINAMICS DCM target, right-click the drive node, and select Target system > Go online. The first attempt may take up to 30 seconds while the baud rate is auto-detected.
6. If STARTER reports Online connection established, navigate to the Diagnostics > Buffer to read the latest drive-side history.

PROFIBUS Termination Considerations

For a permanent point-to-point link, termination is not strictly required, but reflection margins improve with a closed segment. The PC Adapter USB A2 carries an internal, DIP-switch-1-selectable bus termination resistor. When the adapter is the only device at one end of the cable, and the CUD is the only device at the other end, enable the adapter's internal termination. The Standard CUD does not include built-in termination resistors; a 6GK1500-0DA00-style D-sub 9 PROFIBUS terminator must be placed on the CUD end of the cable.

Maximum bus length versus baud rate for typical installations:

If the laptop is more than a few meters from the cabinet, drop to 187.5 kbps and use a Siemens-approved PROFIBUS cable.

Troubleshooting Matrix

Verification Checklist

1. 24 V LED on the PC Adapter USB A2 is on.
2. USB enumeration succeeded in Windows Device Manager (the adapter appears as a USB device, not as an Unknown device).
3. PROFIBUS continuity confirmed by multimeter between pins 3 and 8.
4. CUD RDY LED is green, no red or yellow fault code on the CUD seven-segment display.
5. STARTER access level is set to “Expert” or higher for full parameter visibility.
6. STARTER reports Online connection to … established and the Buffer tab lists drive-side faults with timestamps.

Spare Cable & Conversion Notes

Engineers sometimes substitute an MPI/PROFIBUS cable with an RJ45 patch lead or a generic D-sub 9 cable. This fails because: an RJ45 cable does not map to the PROFIBUS pinout; a generic serial RS-232 cable does not provide the RS-485 differential pairs required by PROFIBUS. Always use a cable type A or type B PROFIBUS lead for production connections, even for short service links. The cable is typically Siemens 6XV1830-0AH10 (violet sheath, fast-connect) for permanent wiring and 6XV1830-3DH10 for highly flexible trailing applications.

The PC Adapter USB A2 is RoHS-compliant and operates over 0 °C to +60 °C. It is firmware-upgradable through Siemens support for newer protocols, but for SINAMICS DCM service the firmware version that ships from the factory is sufficient.

Related Functions and Migration

Older PC Adapter (RS232) (6ES7972-0CA00-0XA0 predecessor), and the current PC Adapter USB A2 are the most-used service adapters for SINAMICS DCM. The newer TS Adapter family provides remote access over Ethernet and is rarely needed for an SINAMICS DCM service laptop link. For production networks, the recommended path is to interface through PROFINET on the Advanced CUD at ≥ 100 Mbit.

Standards and Reference Documents

Refer to the SINAMICS DCM Operating Instructions (12/2010 and later editions) and the PROFIBUS technology pages for full connector specifications and commissioning parameters. The PROFIBUS product finder categorizes SINAMICS DCM under the drives technology ring, confirming PROFIBUS and PROFINET certification.

Useful engineering links:

• SINAMICS DCM on PROFIBUS Product Finder
• Siemens Application & Support portal (manuals for 6RA80 and STARTER commissioning tool)

FAQ