Selecting a Siemens SIMATIC CPU for Parker SSD 635 Profibus

Problem Context and Drive Identification

Parker SSD 635 series servo drives (model code 635-K-DER-03) are brushless servo amplifiers designed for point-to-point and continuous-path positioning of three-phase servomotors. The unit communicates with an external controller through a command interface that, in the base product, is hardwired analog reference plus encoder feedback; for networked installations, Parker offers an optional Profibus-DP communications board that replaces the front-panel command path with cyclic process data. Whether a given 635 on a machine floor actually supports Profibus is therefore conditional: the option card must be fitted, and the drive firmware must be at a revision that recognises the option. Always read the option-slot label inside the drive lid and cross-check the firmware version printed on the keypad display against the Profibus manual before commissioning.

The question on the engineering bench is which Siemens SIMATIC CPU should be specified as the Profibus-DP master for a 635-based axis. Parker's official application literature explicitly supports the Siemens S7-300 family as the master side: any S7-300 CPU with an integrated Profibus-DP master port (CPU 31x-2DP), or a CPU paired with a CP 342-5 external DP master, will exchange data with the 635's option board once the Parker-supplied GSD file is installed in STEP 7. Parker's Technical Application Notes for Communications (PDF) walk through a real configuration between an S7-315 DP master and an SSD Profibus communication module (L5353 family) using SIMATIC Manager STEP 7 as the engineering tool.

The 635 servo drive belongs to the wider SSD/Parker legacy product line that includes the AC650, AC650V, and 690+ drives. The Profibus manual for the AC650V (HA469761) describes the device-description (GSD) registration under "PROFIBUS DP > Additional Field Devices" in the SIMATIC hardware catalog and the parameter-channel layout used on every SSD Profibus slave. The 690+ service guide (HA465492) confirms that the same SSD Profibus conventions apply across the family, so a CPU selection that works for an AC650V master will integrate a 635 servo with the same parameter mapping.

Communication Protocol Selection

Profibus-DP (Decentralised Periphery, IEC 61158 / EN 50170) is the only industrial fieldbus the 635 option card supports. Profinet (the Ethernet-based successor) is not available on this hardware, and a Profibus-to-Profinet gateway is not an engineered path for a closed-loop servo axis because the gateway's update latency is incompatible with the cycle times the drive expects for position control. Treat the 635 option as a Profibus-DP slave with a fixed cyclic data length and commit the rest of the network to Profibus-DP on RS-485.

The protocol operates on a polled master-slave model. The master (the SIMATIC CPU) holds the token, polls each slave in turn, and reads/writes the configured Process Data Words (PZD). Up to 244 bytes of input and 244 bytes of output are exchanged per slave per cycle; the 635 servo uses a small subset (typically 4 to 8 PZD words in each direction) plus an 8-word Parameter Channel (PKW) for non-cyclic parameter read/write. The master can also broadcast a global-control telegram to freeze or clear outputs on all slaves simultaneously, which is useful for grouped E-stop behaviour.

Baud rates from 9.6 kbit/s up to 12 Mbit/s are supported on Profibus-DP. The SSD drives default to auto-baud detection, so the master can be set to any standard rate up to 12 Mbit/s. Field practice is to choose 1.5 Mbit/s for short panel wiring (up to 200 m) and 187.5 kbit/s or 93.75 kbit/s for cabinet-to-cabinet runs that exceed 500 m; the chosen rate must be the same on every node. Cable length limits per Profibus specification at 12 Mbit/s are 100 m segment, at 1.5 Mbit/s are 200 m, and at 187.5 kbit/s are 600 m, with repeaters used to extend beyond a single segment.

Siemens SIMATIC CPU Selection Criteria

The selection decision is constrained by two factors: the CPU must physically own a Profibus-DP master port, and the CPU's program execution budget must keep the DP cycle shorter than the servo drive's position-loop update time. Parker's recommended answer for the 635 option is the S7-300 family. S7-1200 and S7-1500 are not in scope because the 635 Profibus GSD is registered as a DP-V0 slave and the user base for this drive family is overwhelmingly STEP 7 / S7-300 / S7-400.

Two CPU configurations are valid:

1. CPU with integrated DP master port (CPU 31x-2DP): the DP port is on the CPU itself. Examples include 6ES7313-2BE01-0AB0 (CPU 313C-2 DP), 6ES7314-2AG14-0AB0 (CPU 314C-2 DP), 6ES7315-2AH14-0AB0 (CPU 315-2 DP), and 6ES7317-2AK14-0AB0 (CPU 317-2 DP). The "-2DP" suffix in the Siemens catalog always denotes the integrated DP master interface.
2. CPU without DP port plus CP 342-5 external master: any standard S7-300 CPU (CPU 312, 314, 315, 316, 318) plus a CP 342-5 (6GK7342-5DA02-0XE0) or CP 342-5 FO (6GK7342-5DF00-0XE0) on the backplane. The CP handles the DP master protocol independently of the CPU's main scan.

For a single 635 servo axis, a CPU 313C-2 DP or CPU 314C-2 DP is the cost-effective choice. For 2-4 axes, the CPU 315-2 DP is the most common selection because its bit-instruction time (~0.1 µs) and work-memory budget (256 KB) cover multi-axis interpolation comfortably. For systems with 8 or more 635/650/690 drives or substantial motion logic on top of the I/O scan, the CPU 317-2 DP (work memory 1 MB, bit-instruction 0.05 µs) is the appropriate headroom.

Hardware Prerequisites

Profibus-DP runs on RS-485 with a shielded twisted-pair cable. Use Profibus cable type A (two-core, 24 AWG, characteristic impedance 150 Ω, shield coverage > 60 %) for all panel wiring. Belden 3079A, Lapp EBM-003, and Siemens 6XV1830-0EH10 are field-standard references. Type B cable (lower impedance) is permitted for short links but should not be mixed with type A in a single segment.

Every Profibus node uses a 9-pin D-sub connector with integrated bus terminator that can be switched in for the two end nodes of a segment. Termination is a 220 Ω resistor between pins 3 and 8 plus a 390 Ω pull-up on pin 6 and pull-down on pin 5; this is built into the connector and only enabled on the physical segment ends. Failure to terminate exactly two nodes per segment is the single most common field fault and produces reflections that drop slave comms intermittently.

Maximum segment length is determined by the chosen baud rate:

Shield bonding follows the EMC practice: bond the cable shield at both ends to the cabinet ground bar using 360° low-impedance clamps. Parker recommends a clamp directly at the drive gland plate; do not pigtail the shield to a terminal block because the high-frequency impedance of the pigtail defeats the bonding. Avoid running Profibus in the same trough as VFD output cables; if a crossover is unavoidable, cross at 90° and maintain at least 200 mm of parallel separation.

Software Prerequisites

Engineering of the Profibus network is done in STEP 7 (SIMATIC Manager), not TIA Portal. The 635 / AC650V / 690+ Profibus GSDs were published before the TIA Portal hardware catalog supported them and the historical path remains STEP 7 Classic. Specifically:

• STEP 7 V5.5 + SP2 or later (e.g. V5.5 + SP4 with HF1) for stable operation on Windows 7 / Windows 10.
• SIMATIC S7 Technology option package V4.2 or later for any motion blocks beyond the standard SFC14/SFC15 cyclic I/O.
• The Parker-supplied GSD file for the drive family. The AC650V GSD references the directory path "PROFIBUS DP > Additional Field Devices > Drives > Parker SSD Drives 650V" after installation, and the 635/690+ family shares the same installation branch.
• Optional: Parker DSD-PC commissioning software (or its successor SSD Drives Manager) for drive-side parameter setup. The Profibus slave address and the PZD mapping must be set in the drive keypad before the PLC can see the slave on the network.

Obtain the GSD directly from Parker's product page for the specific drive, not from third-party repositories, because the GSD includes vendor-specific PZD definitions that differ between firmware revisions. Parker's Technical Application Notes for Communications (PDF) lists the exact filename pattern (PPR_XXXX.GSD) and the STEP 7 menu path "Options > Install GSD File" used to load it.

Step-by-Step: STEP 7 Project Configuration

1. Open SIMATIC Manager and create a new project. Insert a SIMATIC 300 Station under the project node.
2. Open HW Config. Insert a Rack-300, then a PS-300 power supply sized for the CPU and any I/O (e.g. 6ES7307-1EA01-0AA0, 5 A).
3. Insert the selected CPU in slot 2. Match the order number exactly (e.g. 6ES7315-2AH14-0AB0 for CPU 315-2 DP). STEP 7 will auto-create the integrated DP master interface (IF1) as a Profibus-DP master port.
4. Right-click the DP master port on the CPU and select "Properties". Set the Profibus address (typically 2, since 1 is reserved for the PG and 0 is for diagnostic repeaters). Set the baud rate to 1.5 Mbit/s unless long cable runs demand a lower rate. Enable "DP master" and assign the bus profile "DP".
5. Save and compile the hardware configuration. Close HW Config.
6. Open NetPro. The CPU appears as a Profibus master node on the configured subnet.

GSD File Installation and Slave Integration

Close all running STEP 7 components. Copy the Parker GSD file (and its accompanying bitmap files) into a working folder. In HW Config, choose Options > Install GSD File..., browse to the folder, and confirm. STEP 7 catalogues the device under "PROFIBUS DP > Additional Field Devices > Drives > Parker SSD Drives".

Open the catalog, drag the drive slave symbol onto the Profibus subnet. STEP 7 prompts for a slave Profibus address (1-125; 126 is the broadcast-only "auto baud" address). Use 3 for the first 635 axis, increment by 1 for each subsequent axis. The drive's physical address must match; set it on the drive keypad under "Profibus address" or via DSD-PC.

Double-click the slave to open its properties. In the "Configuration" tab the GSD exposes the available slot configurations. For the 635 servo axis use:

• Slot 1 (PKW): Parameter channel, fixed length 8 words (PKW in/out, type "PKW 8 words").
• Slot 2 (PZD): Process data, 4 words in / 4 words out (or 6/6, depending on the drive's application data set).

The slot order is fixed by the GSD; changing it produces "module configuration error" diagnostics on the slave. Save the configuration and compile HW Config again so the PII/PIQ addresses for the drive are written to the CPU's process image.

PKW Parameter Channel (Non-Cyclic)

The PKW (Parameter-Kennung-Wert) channel is the non-cyclic read/write path for drive parameters. It occupies 8 words (16 bytes) per direction and is structured as follows:

Function codes follow the Profidrive profile: 1 = read parameter value, 2 = write parameter value (word), 6 = read array element. Response codes are 1 = transfer OK, 7 = cannot execute (parameter busy), 8 = invalid parameter number, 17 = invalid value. A request-response handshake takes one or two DP cycles; the drive-side response is synchronous with the next master poll.

Common 635/SSD servo parameters accessed via PKW include:

• P0.001: Setpoint source (0 = analog, 2 = fieldbus)
• P0.002: Control source (0 = terminal, 2 = fieldbus)
• P0.010: Profibus node address (1-126)
• P3.001: Motor code
• P4.001: Position controller Kp
• P4.002: Velocity controller Kp
• P4.003: Velocity controller Ti

PZD Process Data Channel (Cyclic)

PZD words are exchanged every Profibus cycle and carry the real-time command and feedback for the axis. For the 635 servo the typical mapping is:

Control word 1 follows the Profidrive AC drive profile: bit 0 = ON/OFF1, bit 1 = OFF2 (coast stop), bit 2 = OFF3 (quick stop), bit 3 = enable operation, bit 4 = enable ramp generator, bit 5 = enable ramp freeze, bit 6 = enable setpoint, bit 7 = fault acknowledge (rising edge), bit 10 = control by PLC. Status word 1 mirrors the drive state machine: bit 0 = ready to switch on, bit 1 = ready to operate, bit 2 = operation enabled, bit 3 = fault present, bit 6 = switch-on inhibited, bit 10 = speed within setpoint window.

The drive only accepts commands when bit 10 (control by PLC) is set, which prevents back-driving from the analog input while Profibus is the active source. Configure P0.001 = 2 and P0.002 = 2 in the drive before commissioning so the fieldbus path is selected. Speed setpoint is signed 16-bit (N2 format) with a parameter-defined normalisation; the engineering units per bit are typically 0.1 rpm or 0.01 % of rated speed depending on P0.020.

Sample STEP 7 Code: SFC14 / SFC15 for Consistent Data

The PZD and PKW data for a Profibus-DP slave must be read and written consistently because the slave updates the entire image atomically. Use SFC14 ("DPRD_DAT") for read and SFC15 ("DPWR_DAT") for write; direct I/O access (L PIW / T PQW) does not guarantee consistency if the data length exceeds one word.

// FB100 - Parker SSD 635 Profibus Cyclic Read
// Called in OB1 every scan, writes PZD/PKW to instance DB

CALL  SFC   14                       // Read consistent data
       LADDR  := W#16#100            // Logical address of slave PZD start
       RET_VAL:= MW100               // Return code (0 = OK)
       RECORD := P#DB100.DBX0.0 BYTE 32  // 8 PKW + 8 PZD = 32 bytes

// Evaluate response code
      L     MW100
      L     0
      <>I
      S     M 101.0                  // Slave diagnostic flag

// Extract control-relevant words from DB100
      L     DB100.DBW 16             // PZD word 1 = STW1 (status from drive)
      T     MW 110
      L     DB100.DBW 18             // PZD word 2 = NIST actual speed
      T     MW 112

// FB101 - Parker SSD 635 Profibus Cyclic Write
// Compose control word and speed, write consistently

      L     1                        // Bit 0 ON, bit 10 PLC control
      T     DB101.DBW 16             // STW1 to drive (PZD out word 1)

      L     1500                     // 1500 rpm setpoint (scaled per P0.020)
      T     DB101.DBW 18             // NSOLL to drive (PZD out word 2)

      CALL  SFC   15
       LADDR  := W#16#100            // Same logical base as read
       RECORD := P#DB101.DBX0.0 BYTE 32
       RET_VAL:= MW102

Wire SFC14 and SFC15 to the same LADDR so the read and write data buffers are tied to the same slave slot configuration. RET_VAL should be checked every cycle; a non-zero value indicates a slave-side diagnostic event (configuration mismatch, watchdog timeout, or group fault).

Verification and Commissioning

1. Power up the drive. The keypad should show no fault (no "F" code) before joining the bus.
2. Bring the PLC to RUN. From PG, open the slave's "Module Information" dialog (right-click slave in NetPro or HW Config > Module Information).
3. Confirm the slot configuration matches what was downloaded. A "Slot 1 module incorrect" diagnostic indicates a GSD-version mismatch between the project and the drive firmware.
4. Check the slave's DP state. The diagnostic byte "DP State" must read "0" (data exchange). "1" = master not in token ring, "2" = master-slave handshake failed, "3" = data exchange active but with error.
5. Trigger a PKW read of P0.010 from the PG (Monitor/Modify). The response code should be 1 and the returned value should match the drive's set node address.
6. Write STW1 = 0x047F (control word ON, enable ramps, enable setpoint, PLC control). ZSW1 should return 0x0231 (operation enabled, no fault). If ZSW1 returns 0x0230, the drive has accepted the word but is waiting for bit 4 (enable ramp generator).
7. Write NSOLL = 1500. NIST should ramp to within ±2 % of the setpoint within the configured ramp time (P0.040).

Diagnostic Indicators and Troubleshooting Matrix

Field Commissioning Notes

Wire the cabinet ground bar first, then the Profibus shield clamps, then the connectors. Parker's Communications Application Note emphasises that the 650V/635/690+ Profibus option card shares a common 0 V with the drive's control electronics, so a missing chassis bond can inject noise into the bus that intermittently trips slave diagnostics. The application note also recommends keeping the bus cable clear of any VFD motor conductors by at least 200 mm and crossing them only at 90°.

Back up the finished HW Config and the GSD-equipped project to a project archive on the engineering PG. Future axis additions reuse the same catalog entries; only the slave Profibus address and the I/O base address change. Parker's HA469761 manual provides the device-description path used to verify the installed GSD, and the 690+ service guide (HA465492) details the line-choke / DC-link hardware options that are often mistaken for Profibus issues during commissioning.

For the 635 servo, plan the network with the drive's position-loop update in mind. The drive expects a Profibus cycle at or below 4 ms for tight position control; at 1.5 Mbit/s with one or two PZD words per slave this is comfortable. If the segment includes additional high-traffic slaves (remote I/O stations with many bytes), measure the actual cycle time with the PG's "Profibus Diagnostics" view and verify it stays inside the budget before enabling the axis.