S7-200 to ABB ACS800 Modbus RTU Communication Setup

Overview

The Siemens SIMATIC S7-200 (CPU 222, 224, 224 XP, 226) and ABB ACS800 industrial drive can be integrated over a serial fieldbus without requiring a dedicated Siemens PROFIBUS/USS stack when the application is point-to-point PLC-to-drive. The two most field-proven paths for an S7-200 talking to a third-party drive such as the ACS800 are:

1. Modbus RTU on RS485 using the ABB RMBA-01 Modbus RTU Adapter and the Siemens Modbus Master library (MBUS_CTRL / MBUS_MSG subroutines) loaded into the S7-200 with STEP 7 Micro/WIN.
2. PROFIBUS DP using the ABB RPBA-01 PROFIBUS DP Adapter on the drive and the EM277 PROFIBUS DP slave module on the S7-200 (a configuration that effectively turns the S7-200 into a PROFIBUS slave and pairs it with a PROFIBUS master, which is rarely the right choice for a single PLC-to-drive link).

For a one-PLC-to-one-drive link, Modbus RTU is almost always selected because it requires no extra hardware on the S7-200 beyond the on-board Port 0 (or Port 1 on CPU 224 XP/226), and the ACS800 only needs the RMBA-01 plugged into Slot 1 of the drive's DDCS chain. The same hardware topology also works for an S7-200 driving an Allen-Bradley PowerFlex, Schneider Altivar, Yaskawa A1000, or Danfoss VLT 5000/6000 — only the register map changes.

Communication Architecture Options

The USS protocol is Siemens-proprietary and is NOT available on the ACS800; this is the single most common mistake when migrating from a MicroMaster-based line to an ACS800-based line. Do not load the Siemens USS_INIT / USS_CTRL libraries and try to address an ACS800 — the drive will never respond.

S7-200 Communication Port and Protocol Capabilities

The S7-200 family ships with one or two RS485 ports that can each be configured in STEP 7 Micro/WIN (V4.0 SP9 or later recommended) for one of four modes:

• PPI (Siemens point-to-point, default)
• MPI (multi-point, Siemens master/slave)
• Freeport (user-defined protocol, ASCII or RTU)
• Modbus RTU master/slave (via library instructions)

For Modbus RTU master, the S7-200 uses two subroutine calls from the Modbus Master library:

Per the SIMATIC S7-200 Programmable Controller System Manual (6ES7298-8FA30-8BH0), the on-board RS485 ports support baud rates 1.2k, 2.4k, 4.8k, 9.6k, 19.2k, 38.4k, 57.6k, and 115.2 kbps with half-duplex 2-wire connection. Modbus RTU master mode is supported on Port 0 and Port 1.

ABB ACS800 Fieldbus Adapter Options

The ACS800 was supplied from 1995 to roughly 2014 and uses the DDCS (Distributed Drives Communication System) optical/serial backplane. Each ACS800 can accept one of the following optional adapter cards in Slot 1 (and an additional one in Slot 2 in some firmware versions):

Fieldbus option modules are documented in the ABB Drives Library under the ACS800 product family. The relevant handbooks are linked from the ABB ACS800 product page and the RPBA-01 PROFIBUS DP Adapter User's Manual (3AFE64504215). For Modbus RTU, the RMBA-01 manual is 3AFE64498833.

Hardware Wiring — Modbus RTU Path (RMBA-01)

Both the S7-200 Port 0 (or Port 1) and the RMBA-01 use a 9-pin D-sub female with the same RS485 2-wire pinout (A = D−, B = D+ on the Siemens side; the equivalent on the RMBA-01 is also marked A/B, with shield on Pin 1 / housing). Cable recommendations follow the Modbus over serial line specification Modbus_over_serial_line_V1_02.pdf:

• Shielded twisted pair, characteristic impedance ≈120 Ω, capacitance < 100 pF/m, e.g. Belden 3106A or Lapp ETHERLINE 6XS.
• Maximum length without repeater: 1000 m at ≤ 19.2 kbps, 600 m at 38.4 kbps, 200 m at 115.2 kbps.
• 120 Ω termination resistor at BOTH physical ends of the trunk; ABB typically ships a termination jumper on the RMBA-01 that is enabled for the end-of-line drive.
• Shield grounded at one end only (drive end) to avoid ground loops.

ACS800 Communication Parameter Configuration

With the RMBA-01 fitted and the drive powered, navigate to the control panel (ACS-CP-A or similar CDP / DCS panel) and set the following parameters. Refer to the ACS800 Firmware Manual (3AFE64671194) for canonical definitions and full parameter list.

Modbus Register Map for ABB ACS800

With the profile ABB DRIVES (the default and most widely deployed profile) selected, the ACS800 exposes a standard 16-word process data area plus parameter access on Modbus holding registers. The convention follows Modicon-compatible 4xxxx addressing; PLCs typically send the address as 4xxxx-40001 = zero-based in the MBUS_MSG block.

The Control Word is the master→drive command set. Bits per the ABB Drives profile:

The Status Word returned by the drive (reg 40005):

S7-200 Modbus Master Programming

The Modbus Master library is shipped with STEP 7 Micro/WIN as the example project "Modbus Master". It consists of two instruction blocks to be called from a Main OB1 cycle. The library pre-allocates 1 KB of V-memory for the stack (default VW0…VW1023); relocate it to a free range with V-memory Overview.

Initialization (Network 1, first-scan only):

NETWORK 1 // Initialize Modbus Master on Port 0
SM0.1  MBUS_CTRL
        Mode: 1 // 0 = PPI, 1 = Modbus RTU master
        Baud: 9600 // 9.6 kbps
        Parity: 2 // 0=none, 1=odd, 2=even, 3=no parity (2 stop)
        Timeout: 1000 // ms response timeout
        Done: VB100.0
        Error: VB100.1

Cyclic command to write Control Word + Reference 1 (Network 2, called every 50 ms via a timer):

NETWORK 2 // Start command + reference to drive at address 1
T37 (50 ms pulse)  MBUS_MSG
        First: 1 // first call, 0 for subsequent
        Slave: 1 // Modbus address of the ACS800
        RW: 1 // 1 = write multiple (FC16), 0 = read (FC03), 2 = write single (FC06), 3 = read input (FC04)
        Addr: 0 // 4xxxx register 40001 minus 40001 (zero-based)
        Count: 2 // write 2 words: Control Word + Reference 1
        DataPtr: &VW200 // pointer to holding buffer in V memory
        Done: VB101.0
        Error: VB101.1

Build the two-word payload in V memory BEFORE calling MBUS_MSG. In a separate network, move the control-word pattern and the scaled reference into VW200 and VW202:

NETWORK 3 // Build control word 0x047F (Run enable, no off2/off3, ramp2, no jog, dir = fwd)
LDB= V10.0, 1 // V10.0 = "RunRequest" from HMI/operator
        MOVW 16#047F, VW200
        // Hex 047F = bits: ON=1, OFF2=1, OFF3=1, /RESET=0, HOLD=0, RAMP2=1, REF2=0, JOG=0, DIR=0
        // Bit 5 (RAMP2) = 1 selects accel pair 2; bit 6 (REF2) = 0 uses REF1
        // Bit 0 (ON) = 1 starts the drive after a 0→1 transition with no fault active

NETWORK 4 // Build reference: 1500 rpm * 20000 / 11.05 default 1500 = 20000 dec
MOVW +20000, VW202 // 20000 ≡ 100 % of 11.05 REF1 MAX (default 1500 rpm)

Read-back of Status Word + Actual 1 (Network 5, called once every 100 ms):

NETWORK 5 // Read Status Word and Actual 1 from slave 1
T38 (100 ms pulse)  MBUS_MSG
        First: 1
        Slave: 1
        RW: 0 // FC03 read holding registers
        Addr: 4 // 4xxxx reg 40005 minus 40001 = 4
        Count: 2 // read SW + ACT1
        DataPtr: &VW210
        Done: VB102.0
        Error: VB102.1

After Done, the S7-200 has 16-bit Status Word in VW210 and 16-bit Actual 1 in VW212. Actual 1 is interpreted per parameter 4.01 SELECT ACTUAL 1 in the drive (default 0 = SPEED, scaled 1 = 1 rpm; default 4.02 = MOTOR SPEED FILTERED).

Fault reset is performed by toggling bit 3 of the control word. The typical sequence in S7-200 ladder:

NETWORK 6 // Edge-trigger fault reset when operator pushes reset
EU  V10.1 // "ResetRequest" from HMI
        // XOR the current control word with 0x0008 to invert bit 3 for one cycle
        MOVW VW200, VW204
        XORW 16#0008, VW204
        MOVW VW204, VW200 // overwrite for next write transaction

Alternative PROFIBUS Path (RPBA-01 + EM277)

If PROFIBUS is mandatory because the rest of the machine already runs DP, the ACS800 can be equipped with an RPBA-01 PROFIBUS DP Adapter. The S7-200 does not natively act as a PROFIBUS master — only the S7-300/400/1200/1500 do. To still drive an RPBA-01 from an S7-200, the typical workarounds are:

1. Add an EM277 PROFIBUS DP slave module to the S7-200 and pair it with an S7-300/400 or a third-party PROFIBUS master such as a Siemens S7-1200 CM 1243-5 master; the S7-200 then becomes a DP slave and exchanges V-memory with the master, while the master is the one talking RPBA-01.
2. Add a third-party PROFIBUS master card to a PC, which defeats the purpose of having a S7-200.

If the S7-200 is the only controller on the machine, PROFIBUS is the wrong choice — switch to Modbus RTU instead. The cost of the EM277 plus a DP master easily exceeds the cost of an RMBA-01 plus a Modbus scanner for any S7-300/1200 replacement that may come later.

Common Fault Codes and Diagnostics

The ACS800 reports faults on its CDP panel and in status word bit 3. The most common faults encountered during S7-200 commissioning:

The Modbus side error codes returned by MBUS_MSG Error word (V-memory per library documentation):

Commissioning Verification Procedure

1. Disconnect the motor from the drive output terminals (U2, V2, W2) or lock the mechanical load out. Move parameter 21.03 to STOP=COAST if you want a safe coast on Stop.
2. With the drive in local panel control (LOC mode on CDP), confirm the drive starts and runs the motor locally. This isolates motor + drive wiring from fieldbus issues.
3. Set 10.01 = COMM.CW and 10.03 = COMM.REF. Save with 16.07 = 1, then re-power the drive to activate the fieldbus profile (98.02 must be FIELDBUS).
4. Connect the S7-200 Port 0 to the RMBA-01 X2. Pull the connector and re-seat it to verify a good contact.
5. From the S7-200 program, force MBUS_CTRL Mode=1, Baud=9600, Parity=2, Timeout=1000, and monitor VB100.1 Error. If Error is 0, the link is alive.
6. Run a single MBUS_MSG to read register 4 (Status Word) with Count=1. If Status Word is non-zero, the drive is responding. A common starting value is 0x0467 (Ready_On + Ready_Run + Ready_Ref + Remote).
7. Run a single MBUS_MSG to write register 0 (Control Word) with Count=1, Data = 0x047F (or 0x047E for coast; 0x047E means ON=0, OFF2=1, OFF3=1). Monitor Actual 1 to confirm the drive is at zero speed and the status word transitions.
8. Write 0x0000 to the Control Word, then 0x047F to command a start. Actual speed should ramp from 0 to Reference 1 with accel per 22.02 (default 10 s for ACS800 standard macros).
9. Write 0x0000 to the Control Word to ramp stop, or 0x047E to coast. Watch the Status Word bit 0 and bit 12 transitions on the HMI.
10. Re-attach the motor and run a no-load spin check before coupling the load.
11. Run a full ID run (parameter 99.10 = ID MAGN or ID RUN per the motor nameplate) so the drive learns the motor model. Without it, torque and current loops are not optimised and nuisance over-current faults are common on first production runs.
12. Document the working program and lock the parameters with 16.08 PARAM LOCK = 1 if required by the end user.

Notes and Best Practices

• For multi-drive applications (one S7-200 controlling two or more ACS800 units on the same RS485 trunk), set 51.01 MODBUS NODE to a unique value per drive (1, 2, 3 …) and use the S7-200 ladder to multiplex the Slave parameter of MBUS_MSG through a routine. Total cycle time is roughly (N × 50 ms) for a typical 4-word read/write, so for 4 drives keep the poll rate above 200 ms per drive to avoid stack overflow in the S7-200.
• If the application is high-speed (e.g. winder dancer / unwinder tension control with closed-loop PID), the 50–200 ms latency of Modbus RTU is too slow. Migrate the controller to an S7-1200/1500 with PROFINET and a Yaskawa GA500 / Sinamics S120 / ABB ACS880 with PROFINET adapter (RPNT-01 / FENA-11).
• Use a USB-to-RS485 isolator such as the FTDI USB-RS485-WE for PC-side commissioning, with ABB's DriveWindow Light or Drive Composer entry tool. Never connect the drive's RS485 directly to a non-isolated PC port during commissioning — the ground potential difference will damage the RMBA-01.
• If the S7-200 is a CPU 224 XP or 226 with Port 1 free, use Port 1 for Modbus and Port 0 for programming/HMI. This decouples the fieldbus traffic from the Micro/WIN upload.
• For S7-200 replacement: the modern Siemens replacement is the S7-1200 (CPU 1214C / 1215C) with the Modbus_Comm_Load and Modbus_Master instruction blocks, or the S7-1500 with Modbus_Master in the user library. The ACS800 register map remains identical.
• The S7-200 is at end-of-life as of 2017, and Siemens officially stopped support on 1 Oct 2023. For new installations consider the S7-1200/S7-1500 + ACS880 platform.

Frequently Asked Questions