Resolving Profibus 'Wait Prm' Fault on Ramsey Micro-Tech

Overview

The Ramsey Micro-Tech 3101 and 3105 belt-scale integrators are Profibus-DP slaves used to accumulate flow rate, total weight, belt speed, and load from weigh frames in continuous-bulk-material applications. When paired with a Siemens SIMATIC S7-300 CPU 319-3 PN/DP over a Profibus-DP segment, a recurring commissioning fault surfaces as the integrator's diagnostic line Test Scroll 9 -> Test -> Profibus reading "1.5 Mb Wait Prm". The CPU simultaneously reports a DP slave fault on the configured node.

The root cause is almost always a mismatch between the GSD file installed in the STEP 7 / TIA Portal hardware catalog and the actual Ident Number (Ind_Num) inside the GSD, or a wrong slot/I/O configuration. This article documents the complete diagnostic path, the parameter-assignment handshake, GSD repair, and hardware configuration for the Micro-Tech 3101/3105 family. It also covers the parallel cases for the legacy 2101/2105 devices and the modern Thermo Scientific 3101/3105 successors because the GSD file numbering convention is shared across generations.

Problem Description

Observed Symptoms

• Siemens S7-300 CPU 319-3 PN/DP raises a Profibus-DP fault for slave node 7. SF / BF LEDs on the CPU or IM 153/IM 360 interface blink or remain steady, depending on firmware revision.
• The Micro-Tech 3101 diagnostic page Test Scroll 9 -> Test -> Profibus reports "1.5 Mb Wait Prm". The baud-rate indication confirms physical layer activity but the slave is in the Wait_Prm state of the Profibus state machine.
• No cyclic I/O exchange occurs. Input process image (PIW) and output process image (PQW) for the slave stay at zero or initial values.
• The PLC project was configured for 48 input words / 48 output words; the integrator was set to the same.

Why the Baud Rate Is Reported Correctly

The baud-rate indication inside the Profibus test page is decoded from frames the CPU transmitted during the baud-rate search phase. Detection of the segment's actual 1.5 Mbit/s speed proves the RS-485 physical layer is healthy, termination is correct on both ends, and the slave's Profibus ASIC is receiving and decoding telegrams. The fault is therefore strictly in the data-link layer (Layer 2) parameter assignment / configuration exchange, not in the physical layer.

Profibus-DP State Machine and the Wait_Prm State

Every Profibus-DP slave implements the state machine defined in IEC 61158-6 / IEC 61784. The relevant states are:

A slave that remains in Wait_Prm is waiting for a Set_Parameter telegram that contains its own Ident Number. The Ident Number is the manufacturer-assigned 16-bit identifier compiled into the GSD file. The Siemens master only sends a Set_Prm for a slave whose Ident Number matches the entry installed in the STEP 7 / TIA Portal hardware catalog. If the GSD supplied by the integrator vendor uses an Ind_Num for the wrong model, the master transmits Set_Prm for a different device, and the physical 3101/3105 firmware ignores it and remains in Wait_Prm indefinitely.

Root Cause

The field incident in this article was traced to a GSD file mismatch:

1. Vendor supplied a GSD for the Micro-Tech 2101/3101 family of integrators.
2. The site actually has a Micro-Tech 3105 integrator.
3. The GSD's Ident_Number (= Ind_Num inside the file) carried the 2101/3101 ident value, e.g. 0x0E15.
4. The 3105 firmware expected ident value 0x0E18 (or the equivalent per the 3105 GSD release notes).
5. The CPU's Set_Prm telegram contained the wrong ident, so the 3105 rejected it and stayed in Wait_Prm.

Editing the GSD file's Ident_Number = 0x0E15 line to Ident_Number = 0x0E18 (or replacing the GSD with the correct one from the manufacturer) resolved the fault and the slave transitioned to Data_Exch on the next master poll.

Solution: Step-by-Step Procedure

The following procedure assumes STEP 7 V5.5 with the S7-300 / S7-400 optional package installed. Equivalent TIA Portal V15..V18 steps are noted in italics.

Step 1 - Verify Physical Layer

1. Confirm Profibus cable type: Profibus-DP cable, 150 Ω nominal impedance (e.g. Siemens 6XV1830-0EH10, Belden 3079A).
2. Verify termination: ON at both physical ends of the segment only. The Micro-Tech 3101/3105 ships with a built-in terminating resistor enabled by an internal switch; disable termination on every other node.
3. Check shield bonding: cable shield clamped to grounding bar at cabinet entry, low-impedance connection to panel ground.
4. Measure A-B line voltage with slave powered: differential ≥ 1 V under idle, ≥ 200 mV during polling. Both lines should swing around 0 V; do not see DC offset.

Step 2 - Verify Baud Rate and Node

1. On the Micro-Tech, navigate Scroll 9 -> Test -> Profibus and confirm reported baud rate matches the configured 1.5 Mbit/s.
2. Set or verify slave node address on the integrator (typically in Scroll 1 or Scroll 9). The address must match the STEP 7 hardware configuration and be unique on the segment. Valid Profibus addresses: 0..125 (0 and 126 are reserved for class-2 master and service use).

Step 3 - Install the Correct GSD File

1. Open the GSD text file in a plain-text editor (Notepad++, not Word).
2. Locate the line: Ident_Number = 0x????
3. Compare against the table below for the actual installed unit. If the Ident_Number does not match, obtain the correct GSD from the vendor.

Step 4 - Install the GSD in STEP 7 / TIA Portal

1. STEP 7 V5.5: HW Config -> Options -> Install GSD File... -> select *.GSD and any bitmap (*.BMP) -> OK. The slave appears in the catalog under PROFIBUS-DP -> Additional Field Devices -> Ramsey.
2. TIA Portal: Options -> Manage general station description files (GSD) -> browse to *.GSD -> install. The slave appears in the device catalog under Other field devices -> Profibus DP -> Ramsey.

Step 5 - Configure the DP Slave

1. Drag the Micro-Tech 3105 from the catalog onto the DP master system of the CPU 319-3 PN/DP.
2. Set the slave address to 7 (or whatever the integrator was set to).
3. Open the slot configuration. The GSD typically pre-defines slot 0 = a global control/status word and slots 1..N = process data modules. Configure to 48 input words / 48 output words total, distributed across the available slots as the GSD defines. Common slot allocation:

4. Compile and download the hardware configuration to the CPU.

Step 6 - Watch the State Machine Transition

1. On the Micro-Tech, refresh Test Scroll 9 -> Test -> Profibus. The text should now read "1.5 Mb Data Exch" (or equivalent Data_Exch indicator).
2. In STEP 7, PLC -> Monitor/Modify the input words. Rate, total, and status values should appear as live numbers, not zero.

Verification

Technical Details: GSD Anatomy for the Micro-Tech Family

A Profibus GSD is a structured ASCII file. The lines relevant to this fault are:

; Profibus Device Description of the Micro-Tech 3105
; File: RAM3105.GSD

#Profibus_DP

GSD_Revision          = 5
Vendor_Name           = "Thermo Scientific / Ramsey"
Model_Name            = "Micro-Tech 3105"
Ident_Number          = 0x0E18          ; <-- THIS LINE must match the unit
Protocol_Ident        = 0               ; 0 = Profibus-DP
Station_Type          = 0               ; 0 = DP-Slave
FMS_supp              = 0
Hardware_Release      = "Rev. 2.0"
Software_Release      = "V3.12"

; Supported baud rates
9.6_supp              = 1
19.2_supp             = 1
45.45_supp            = 1
93.75_supp            = 1
187.5_supp            = 1
500_supp              = 1
1.5M_supp             = 1
3M_supp               = 1
6M_supp               = 1
12M_supp              = 1

MaxTsdr_9.6           = 60
...

; User parameters (byte-by-byte in Set_Prm)
User_Prm_Data         = 0x00,0x00,0x00,0x00
Ext_User_Prm_Data_Const(0) = 0x00,0x00,0x00,0x00

; Module definitions
Module               = "Status/Control  " 0xC0,0xC0
Module               = "Rate (REAL32)   " 0x44,0x44
Module               = "Total (REAL32)  " 0x44,0x44
...

The key fields for the Wait_Prm fault:

• Ident_Number - Unique per device model. Master cross-checks against its own hardware catalog. If master has different ident, no Set_Prm is sent -> slave stays in Wait_Prm.
• MaxTsdr_* - Maximum response time the master should wait at each baud rate. Out-of-range values can also cause the master to declare the slave failed.
• User_Prm_Data - Application parameters (e.g. units, scaling). If user parameters are required and the master does not send them, the slave may stay in Wait_Prm as well.

Sample Ladder Logic: Reading Rate and Total

Once the slave is in Data_Exch, the configured input words appear in the process input image at offsets set by HW Config. With the slot mapping above (slot 0 = 2 input words, slot 2 = 4 input words for Rate), and assuming the slave starts at IB 256 (configurable), the Rate is at ID 260 and Total at ID 264.

// STL excerpt - S7-300 OB1
// Ladder equivalent: NETWORK 1 - read scale status word
// Assume start address = 256 (configurable in HW Config)
NETWORK 1 // Read status word into MW200
      L     IW 256          // Status word from slot 0
      T     MW 200

NETWORK 2 // Rate (t/h) as REAL32
      L     ID 260          // 4 bytes of REAL32 rate
      T     MD 204          // Store in MD204 for HMI

NETWORK 3 // Total (t) as REAL32
      L     ID 264
      T     MD 208

NETWORK 4 // Conditional reset of master total
      A     M 100.0         // Operator reset pulse
      JCN   END1
      L     W#16#0001       // Set bit 0 in control word (reset total)
      T     QW 256          // Output to slot 0
END1: NOP 0

For SCL / Structured Text on S7-300 with firmware >= V3.0:

// SCL FB example - read the scale data structure
FUNCTION_BLOCK FB_ScaleRead
VAR
  Status  : WORD;        // Slot 0 input
  Rate    : REAL;        // Slot 2 input (t/h)
  Total   : REAL;        // Slot 3 input (t)
  BeltSpd : REAL;        // Slot 4 input (m/s)
END_VAR
BEGIN
  Status  := WORD_TO_INT("iSlave".Status_W);
  Rate    := "iSlave".Rate_R;
  Total   := "iSlave".Total_R;
  BeltSpd := "iSlave".BeltSpeed_R;
END_FUNCTION_BLOCK

Diagnostic Decode Reference

When the Micro-Tech Profibus test page cannot move beyond Wait Prm, use this decode matrix to localize the fault. The Micro-Tech firmware displays one of the following state strings:

Troubleshooting Matrix

Adjacent Equipment: GSD Cross-Reference

When commissioning adjacent Thermo Scientific / Ramsey DP devices on the same PLC, keep this quick reference:

Standards and Reference Documents

• IEC 61158-6 / IEC 61784 - Profibus DP protocol - state machine, Set_Prm, Chk_Cfg definitions.
• GSD Specification (PI Group) - structure of the GSD file, Ident_Number field semantics.
• Siemens S7-300 CPU 319-3 PN/DP manual (6ES7318-3EL01-0AB0) - DP master interface specifications.
• Thermo Scientific Micro-Tech 3101 / 3105 Profibus interface manual - GSD, slot definitions, and user parameter bytes.

FAQ