SIWAREX U 7MH4950 Status Bits: Reading Module Health in STEP 7

Overview

The SIWAREX U weighing module, order number 7MH4950-1AA01, is a single-channel electronic weighing system designed for the SIMATIC ET 200S distributed I/O platform. It replaces the legacy 7MH4601-1*A01 module without changes to the wiring or footprint, providing direct integration into the SIMATIC S7-300 / S7-400 / ET 200S I/O bus. Once configured, the module occupies 16 bytes of process input image (here IB 304 … IB 319) and supplies both measurement data and a structured status / diagnostic word map.

This reference documents how to read the module health bit from input address I304.0, how the surrounding byte carries fault and operating-state flags, and how to surface those flags into a WinCC runtime screen. It also covers the SIWAREX diagnostic buffer entries generated when the firmware detects a checksum, parameter, or weighing-sensor fault.

Prerequisites

• SIMATIC Manager (STEP 7 V5.5 / V5.6) or TIA Portal V15.1 or higher with the SIWAREX U HSP / GSD file installed.
• ET 200S head module (IM 151-1 or IM 151-3 PN) configured with a SIWAREX U slot occupied.
• The SIWAREX U device manual and the "Ready_for_Use" example project from Siemens Online Support.
• An FB / FC for periodic evaluation of the SIWAREX status word (typically FC "SIWA_Status" inside the Ready_for_Use example).
• WinCC flexible / WinCC Comfort / WinCC Professional with a configured connection to the S7 CPU.

Input Address Layout (16 Bytes)

When the SIWAREX U module is inserted and parameterised in HW Config, it consumes 16 bytes of input process image. In the example installation referenced in the source, those bytes occupy IB 304 through IB 319. The first two bytes are reserved for the SIWAREX status and error word; the remaining 14 bytes carry scaled weight, gross/net, tare, and raw ADC data as 32-bit floating point values.

IB 304 / IB 305 Status and Error Bit Assignment

The status word is bit-coded. Each bit reflects a hardware, firmware, or weighing-process condition. Polling this word is the recommended method for surfacing SIWAREX health into STEP 7 logic and the HMI.

Reading the Module Health Bit (I 304.0)

For a binary "module healthy / not healthy" status used as an LED on a WinCC screen or as a permissive in an FC, evaluate I 304.0 directly. The Ready_for_Use sample FB exposes this as the contact SIWA_OK inside the cyclic portion of OB 1.

// LADDER - cyclic evaluation in OB 1
NETWORK 1   Module health
   A    I 304.0          // Module_OK
   =    "SIWA_OK"        // BOOL tag, wincc-visible
   JCN  M001             // jump on FALSE

NETWORK 2   Group error (SF)
   A    I 304.7          // Group_Error (mirror of SF LED)
   S    "SIWA_SF"        // latched fault flag

M001: NOP 0

If you need the fault state to be latching (so a transient SF event is not missed between the OB 1 cycle and the WinCC update rate), set the bit with S on the falling edge of I 304.0 or on the rising edge of I 304.7. Reset the latched fault after the operator acknowledges it on the HMI.

ST equivalent

IF "SIWA_DB".Module_OK THEN
   "SIWA_OK" := TRUE;
   "SIWA_SF" := FALSE;
ELSE
   "SIWA_OK" := FALSE;
   "SIWA_SF" := TRUE;       // latched group error
END_IF;

Mapping the Status into WinCC

Create an internal WinCC tag of type BOOL for each bit you want to expose, then link the PLC tag to it via the WinCC tag management. The minimum set is:

1. Open the WinCC project and select the connection to the S7 CPU.
2. Add external tags: SIWA_OK, SIWA_SF, SIWA_SCALE_ERR, SIWA_PARAM_ERR, SIWA_STANDSTILL, SIWA_OVERLOAD.
3. On the screen, drop a "Status display" or LED object and bind the "On / Off" appearance to each BOOL tag.
4. For the operator-visible fault light, wire it to the negated value of SIWA_OK or the direct value of SIWA_SF (latched in the PLC).
5. For limit / overload warning, bind I 305.4 (Above_Max) to a red bar or "Overload" lamp.

To save HMI licensing and reduce tag traffic, evaluate the eight bits as one BYTE tag IB 304 in WinCC and break them out in the screen's "Bit selection" property — WinCC can drive up to eight lamps from one byte tag.

SF LED, Diagnostic Buffer, and Operating Errors

The SF (red) LED on the front of the SIWAREX U module is hardware-driven by the same condition that sets I 304.7 (Group_Error). When the LED is on, a diagnostic interrupt is queued and visible in the CPU diagnostic buffer. Typical entries seen in the buffer are:

SIWATOOL U and Scale DB Diagnostics

The "Ready_for_Use" project ships with a scale DB (DB_SIWAX) that contains a structured set of UDTs. Among them are SIWA_STATE (mirroring IB 304/305) and SIWA_DIAG which holds the last diagnostic code pushed by the module. Polling DB_SIWAX.SIWA_DIAG.Code in addition to the bit-level health flag gives you both a single-bit "is something wrong" and a numeric code that WinCC can render on a fault screen.

// SCL - read diagnostic code from scale DB
"DIAG_CODE" := "DB_SIWAX".SIWA_DIAG.Code;
"DIAG_TEXT" := "DB_SIWAX".SIWA_DIAG.Text[1..32];
IF "DB_SIWAX".SIWA_DIAG.New THEN
   "DIAG_NEW" := TRUE;          // HMI acknowledges
END_IF;

Troubleshooting Matrix

Field Commissioning Procedure

1. Insert the SIWAREX U module in the ET 200S slot designated in the project. Power the station.
2. Confirm the SF LED is OFF and the DC 24 V supply LED is ON.
3. Open HW Config / device configuration and confirm the module order number is 7MH4950-1AA01. Save and download.
4. Open SIWATOOL U, connect via RS-232 to the front service port of the module.
5. Upload the current DR 3 / DR 4 and verify zero, span, unit, filter, and standstill window match the design spec.
6. Drive the CPU to RUN. In VAT or a watch table, observe IB 304 and confirm I 304.0 = TRUE.
7. Apply a known test weight and verify IB 306 … IB 309 reflects the expected net value.
8. Force a wire break at the load cell terminals — confirm I 304.1 and I 304.7 set, SF LED on, and the diagnostic buffer records the expected entry.
9. Restore the wire, clear the diagnostic, and re-zero the scale.
10. In WinCC Runtime, verify that the operator screen shows the green "Module OK" lamp on the first poll.

Cross-Platform Notes

The same 7MH4950-1AA01 module can be deployed under STEP 7 V5.x and under TIA Portal V15.1+ with the SIWAREX HSP. In TIA Portal, the status bits become available as "SIWAREX_U".StatusWord in the IO tag table, with the individual bits exposed as StatusWord.%X0 through StatusWord.%X15. The mapping is identical to I 304.0 … I 305.7 described above.

If you migrate to an ET 200SP TM SIWAREX WP351 HF, the diagnostic-interrupt messaging model is updated and the SF bit is exposed via ChannelError rather than the legacy group-error bit. The TIA Portal help documents the new diagnostic-message text and hex codes; the diagnostic-message text "Checksum of electronic weighing system FW no longer matches — Remedy: load factory settings" applies directly to WP351 HF as well.

Safety and Wiring Constraints

• Use only shielded, twisted-pair cable from the load cell junction box to the SIWAREX terminals. Ground the shield at the cabinet entry only.
• Do not route load cell cables in the same conduit as VFD output cables. Maintain ≥ 200 mm separation, or use a dedicated steel conduit.
• Verify the cell excitation voltage (typically 10 V DC at the SIWAREX U terminals) with no load connected — if it is below 9 V, the sense lead may be open.
• The SIWAREX U module does NOT participate in PROFIsafe. If the application requires a SIL-rated weighing function (e.g. legally-for-trade overfill protection), use SIWAREX WP251 / WP351 with an F-CPU.
• Replace the lithium backup battery only with the specified type (e.g. Renata 2450N or equivalent). Reverse polarity will destroy the NV-RAM.

Verification Checklist

• I 304.0 = TRUE in VAT while the scale is idle and healthy.
• SF LED off; the CPU diagnostic buffer contains no SIWAREX-related entries.
• All eight status bits in IB 304 are FALSE except during a deliberate fault injection.
• WinCC operator screen shows green "Module OK" lamp within one refresh cycle of the connection.
• A wire-break simulation triggers both the SF LED and the Scale_Error tag on the screen.
• Removing the wire-break condition and clearing the diagnostic buffer returns I 304.0 to TRUE without a module power-cycle.