S7-224XP to MM420 USS Communication Troubleshooting

Problem Definition

A SIMATIC S7-224XP CPU is configured as USS master for a bus of three MICROMASTER 420 (MM420) drives. The USS_INIT instruction reports a successful execution and the Active output reads 16#0E, but every cycle of the USS_CTRL instruction returns Error = 1. Three MM420 units are visible on the RS485 bus, all of them have been re-parameterised via the AOP with P0700=5, P1000=5, P2010=6, P2011=6, P2012=2, and P2013=3, and the bus is wired through a 6ES7972-0BA51-0XA0 PROFIBUS connector. The drive is never reported as Fault or Inhibit; the master simply cannot exchange a valid response frame with the slave.

The fault is therefore a protocol / parameter mismatch at the USS-PKW and PZD layer, not a hardware or wiring failure. The remainder of this article walks through every parameter, pin, and instruction flag that has to be aligned for the bus to return clean USS_CTRL cycles.

USS Protocol Reference for S7-200 and MM420

USS (Universal Serial Interface protocol) is a single-master multi-slave bus originally defined for Siemens MICROMASTER 4, SINAMICS V20 and COMBIMASTER drives. Each telegram consists of a 14-byte header, an optional PKW area (parameter identifier value, used to read/write parameter numbers) and a PZD area (process data, used to write the control word and speed setpoint and read the status word and actual speed).

For the S7-200 library:

• The master polls a configured set of drives cyclically. The Active word selects which of the 16 possible nodes is enabled (bit 0 = node 1, bit 1 = node 2, and so on).
• The PZD length is fixed at 2 words for both directions, matching the S7-200 USS_CTRL data block (control word + speed setpoint, status word + actual speed).
• The PKW length can be 0 (PKW disabled) or 4 (PKW with one parameter per telegram, 4 words). Any other value forces the drive to wait for a variable PKW length that the S7-200 library never produces.

The drive side mirrors the same fields. The MM420 parameter P2012 sets the PZD length, P2013 sets the PKW length and P2010/P2011 set the baud rate and node address. The MM420 evaluates the same 2-word PZD area by default, which is what the S7-200 library emits.

S7-224XP Hardware and Port 0/1 Specifications

The S7-224XP (order number 6ES7214-2AD23-0XB0 or later) provides two electrically isolated RS485 ports with the following relevant characteristics for USS operation:

For a 3-drive MM420 network running at 9.6 kbit/s, either port is acceptable. With 19.2 kbit/s use Port 1 to leave more time per telegram. The USS_INIT call must be made exactly once on the chosen port, in the first scan, with a one-shot rising-edge trigger to avoid reinitialising the port on every cycle.

PROFIBUS Connector 6ES7972-0BA51-0XA0 Pinout

Order number 6ES7972-0BA51-0XA0 is the SIMATIC DP FastConnect bus connector with PG socket, 90 degree cable outlet, terminating resistor that can be switched in. The D-sub pinout used for USS on the S7-200 is identical to the PROFIBUS standard:

The connector is electrically suitable. The two most common field failures on the 6ES7972-0BA51-0XA0 are (a) the on-board 220 ohm terminator left switched off at the physical end of the bus and (b) the on-board terminator left switched on in the middle of a multi-drop bus. Only the two extreme nodes of the bus must be terminated. The S7-224XP end of the cable is one extreme; the last MM420 is the other.

MM420 Parameter Map for USS Operation

The parameter set published in the original request is close to working but contains two errors: P2011 is set to 6 on all three drives, and P2013 is set to 3 (variable PKW length) which the S7-200 library does not support. The validated parameter map for a 3-drive MM420 network is given below. Each drive must be given a unique address and an identical PZD/PKW configuration.

Notes on the parameter values:

• P2011 must be unique. Three MM420 units all on P2011 = 6 will collide and the S7-200 master will never receive a clean response, so USS_CTRL latches Error = 1.
• P2013 = 0 disables PKW. A value of 3 or 127 tells the drive to expect a variable-length PKW, but the S7-200 library only produces 0 (none) or 4 (4-word) PKW. Variable lengths are not implemented in the S7-200 library and the drive will time out waiting for the remaining PKW words.
• P0700 = 5 and P1000 = 5 must both be set. Either one alone leaves the drive ignoring USS, even if RS485 is wired correctly.
• P0010 = 0. Many parameters cannot be written while P0010 = 1 (quick commissioning) is active. Always leave P0010 = 0 before any USS parameter change.

USS_INIT and USS_CTRL Subroutine Reference

The S7-200 USS library exposes two subroutines that are mandatory on the master side. Their data block layout is given below.

USS_INIT

USS_INIT(MODE, BAUD, ACTIVE, DONE, ERROR, STATUS)

A successful USS_INIT returns DONE = 1, ERROR = 0, STATUS = 0, regardless of whether any drive is connected. Active = 16#0E is valid and tells the master to poll nodes 2, 3 and 4 only. If the user intends to use nodes 1, 2 and 3, Active should be 16#07.

USS_CTRL

USS_CTRL(RUN, OFF2, OFF3, F_ACK, DIR, DRIVE, SPEED_SP, RESP_R, DONE, ERROR, STATUS, SPEED, RUN_EN, D_DIR, INHIBIT, FAULT)

The original configuration calls USS_CTRL with DRIVE = 6. Combined with the P2011 = 6 set on every drive, this means the master is trying to talk to a node that does not exist (the master never enabled drive 6 in Active = 16#0E). The library returns Error = 1 because the request is queued but no response frame is received within the telegram off time.

USS Error Code Reference Table

The original report (USS_CTRL Error = 1, no STATUS value exposed in the snippet) is the most common USS fault and is almost always caused by either an address mismatch or a missing drive on the bus.

Cable Wiring to MM420 Terminal 14 / 15

The wiring example published in the Siemens S7-1200 USS example (and which is also valid for the S7-200, since both CPUs use the same RS485 electrical standard) is reproduced below:

Reference: SIMATIC S7-1200 Manual Collection - Example: USS general drive connection and setup

Use shielded, twisted pair PROFIBUS cable (Siemens 6XV1830-0EH10 or equivalent, 150 ohm characteristic impedance). Connect the shield to earth at the cabinet entry gland on the S7-200 side; on the drive side the shield is normally connected to the drive PE terminal through the gland plate.

Bus Termination and Shielding

RS485 requires termination at both ends of the bus, typically 120 to 220 ohms between the + and - lines in series with a 1 nF capacitor to 0 V (the PROFIBUS standard 220 ohm / 1 nF). The 6ES7972-0BA51-0XA0 has a switchable 220 ohm termination; switch it on at the S7-224XP connector and at the connector wired to the last MM420. Switch it off on every intermediate node.

The MM420 has no internal RS485 terminator. There is no DIP switch on the MM420 for bus termination. External termination is mandatory.

Step-by-Step Reconfiguration Procedure

1. Power off the S7-224XP and the three MM420 units.
2. Connect the bus: S7-224XP Port 0 (or Port 1) → 6ES7972-0BA51-0XA0 → PROFIBUS cable → MM420 #1 terminal block (14, 15, 13) → MM420 #2 → MM420 #3. Switch the terminator on at the S7-224XP connector and at MM420 #3 only.
3. Power on the MM420 units first. On each AOP, run quick commissioning (P0010 = 1, set motor data, P3900 = 1 to calculate). Exit commissioning with P0010 = 0.
4. On each drive, set:
     - P0003 = 3 (expert access)
     - P0700 = 5
     - P1000 = 5
     - P2010 = 6 (9 600 bit/s)
     - P2011 = 1 on MM420 #1, 2 on MM420 #2, 3 on MM420 #3
     - P2012 = 2
     - P2013 = 0
     - P2014 = 2000
     - P2024 = 52100 (control word + NSET)
     - P2025 = 53200 (status word + NACT)
     - P2027 = 0
5. Press the green OK button to save. P0002 must show the value just written. Repeat for every drive.
6. In STEP 7 Micro/WIN, open Library → USS Protocol. Place USS_INIT and USS_CTRL in the main program OB1.
7. Build a one-shot rising-edge of the first scan bit (SM0.1) to trigger USS_INIT exactly once. Set MODE = 1, BAUD = 3 (9 600 bit/s), ACTIVE = 16#07 (nodes 1, 2, 3).
8. Call USS_CTRL three times, one per drive, with DRIVE = 1, 2, 3 respectively. Drive 1 controls MM420 #1, and so on.
9. Download the project to the CPU. Switch the CPU to RUN.
10. Toggle RUN = 1 on the USS_CTRL for drive 1. The MM420 #1 output frequency should ramp to SPEED_SP. STATUS should read 0, FAULT = 0, INHIBIT = 0.

Common Configuration Mistakes

Verification and Diagnostics

After the parameter change and a CPU power cycle, the following signals confirm a healthy bus:

• USS_INIT: DONE = 1, ERROR = 0, STATUS = 0, ACTIVE = 0x07 (or the bit pattern matching the enabled nodes).
• USS_CTRL for each drive: ERROR = 0, STATUS = 0, RUN_EN follows RUN, INHIBIT = 0, FAULT = 0, SPEED tracks SPEED_SP within one telegram cycle.
• On the MM420 AOP, r0000 displays the drive state (e.g. 7 = run), r0021 (output frequency) tracks the setpoint and r0052 (status word) bit 2 (operation enabled) is set.

If USS_CTRL still returns Error = 1, isolate the bus to a single drive and a single address. With only MM420 #1 connected and P2011 = 1, ACTIVE = 16#0001, DRIVE = 1, a clean response is expected within one cycle. If that succeeds, re-add MM420 #2 and #3 one at a time. The first drive that reintroduces Error = 1 is the offending node, typically because of a duplicate P2011 or a missing terminator on the segment between the two drives.

Frequently Asked Questions