Siemens ET 200iSP EEXIA I/O Module Selection and Configuration

Selecting intrinsically safe (IS) I/O for Zone 1 / Zone 2 hazardous areas requires module families that carry an Ex i certification, support HART pass-through, deliver channel-level fault diagnostics, and interface cleanly with a Safety Shutdown / Solenoid Operated Valve System (SOVS). Siemens does not offer Ex-rated modules in the SIMATIC S7-300/400/1500 CPU racks. The recommended distributed platform is the SIMATIC ET 200iSP, an intrinsically safe remote I/O station designed for mounting in Zone 1/2 with field connections into Zone 0/1. This reference covers module selection, surge/transient protection, HART integration, fault diagnostics, and SOVS interface wiring for the four module types typically required: 8-channel digital input, 4-channel digital output, 8-channel analog input (4-20 mA) with HART, and 4-channel analog output (4-20 mA) with HART.

1. Hazardous Area I/O Requirements

An EEXIA (Ex i) module must be certified so that the energy delivered to the field loop is limited to levels that cannot ignite a defined gas group. The ET 200iSP family is built around the SIMATIC ET 200iSP Device Manual and the ET 200iSP Fail-Safe Modules Manual. The station mounts directly in Zone 1/2 without a separate enclosure, and its terminals route field wiring to Zone 0 instruments when required.

2. ET 200iSP System Architecture

The ET 200iSP station is a modular, intrinsically safe PROFIBUS DP or PROFINET station. It uses a backplane with Ex e power and Ex i signal slots, terminated by a terminating module. Power comes from a PM 136 power module mounted in slot 0; the IM 152-1 interface module occupies slot 1; I/O modules occupy slots 2 through 15. Up to 32 stations can be operated on a single PROFIBUS DP segment, each station up to 4 m wide.

Cycle times on PROFIBUS DP at 1.5 Mbit/s run 5-10 ms per station. With PROFINET and IM 152-1 PN, update times drop to 1-3 ms per station, suitable for fast SOVS closure. Each I/O module supports hot-swap; the station continues to operate with one slot empty when configured accordingly in STEP 7 / TIA Portal.

2.1 Addressing and Slot Mapping

Slot numbering follows the physical order. For an 8 DI / 4 DO / 4 AI HART / 4 AO HART configuration starting at slot 2, the address map is:

Address gaps depend on the GSD file revision. Always read live addresses from the hardware configuration in TIA Portal / STEP 7 after insertion, never hard-code offset values from printed manuals.

3. SM 131 - 8 DI NAMUR (6ES7131-7RF00-0AB0)

The SM 131 is an 8-channel isolated digital input module that operates in IEC 61131-2 Type 1 or Type 3 characteristic. Each channel can be software-configured for NAMUR sensor inputs, 24 V PNP switches, or dry contacts. The input front end is functionally similar to the Texas Instruments ISO1228 eight-channel isolated 24 V digital input receiver which provides per-channel current limit, deglitching, and wire-break detection. Siemens implements equivalent functions in their ASIC, but the operating principles (threshold detect at 8.0 V / 2.1 mA Type 3, wire-break alarm at < 0.1 mA) are the same.

3.1 Specifications

3.2 Channel-Level Fault Diagnostics

Each of the 8 channels returns a structured diagnostic record. In STEP 7 / TIA Portal the diagnostic interrupts are mapped to OB 82 (diagnostic interrupt) and the channel status can be read via SFC 51 / SFC 59 with the SSL_ID for module status. Key diagnostic bits:

4. SM 132 - 4 DO (6ES7132-7RD20-0AB0)

The SM 132 drives four 24 V DC Ex i loads, typically shutdown solenoid valves. Each channel is current-limited to 45 mA continuous, with short-circuit foldback. Output repetition rate is 100 Hz max for inductive loads; for SOVS valves, configure de-energize-to-trip logic in the application program so the DO goes OFF in a trip event.

4.1 SOVS Interface Wiring

A standard pattern for SOVS trip outputs is:

Each SOV line should be fitted with a transient surge suppressor (varistor or TVS) at the field end to absorb inductive kick. Use Ex i-rated suppressors such as the Phoenix Contact TTC-6-ISD or Dehn BVT-IS-EX. Calculate the suppressor clamping voltage below the SM 132's max off-state voltage of 30 V.

4.2 Specifications

5. SM 134 - 4 AI HART (6ES7134-7TD00-0AB0)

The SM 134 provides four 4-20 mA 2-wire analog inputs with HART pass-through, all on an Ex i front end. Two modules can be installed in adjacent slots and combined logically to give 8 AI HART channels on a single station. The 4-20 mA loop is internally powered by the module from the Ex i bus.

5.1 HART Pass-Through to Controller

HART variables are accessible two ways:

1. PROFIBUS DP / PROFINET cyclic PV: The 4-20 mA current is scaled to 0-27648 and routed as a 16-bit integer in the input image. This is the primary process variable for control.
2. Acyclic record access: HART commands (e.g., 03 = PV, 09 = device variables, 48 = additional device info) are read using SFB 52 / SFB 53 (READ/POLL) in STEP 7, or via the HART master blocks in TIA Portal. Each HART frame is a 30-byte array.

For a 4-20 mA transmitter with HART, the loop load resistor is built into the module (250 Ω, 0.5 W). The maximum Ex i loop voltage is 26 V; the maximum short-circuit current is limited to 93 mA. The analog-to-digital conversion is 16-bit, giving a resolution of 0.4 µA per LSB.

5.2 Specifications

5.3 Stacking Two SM 134 Modules for 8 AI

To achieve the 8 AI requirement, install two SM 134 modules in adjacent slots. The HART multiplexor treats them as logical channels 0-7. STEP 7 / TIA Portal will assign a contiguous I/O address range. The diagnostic buffer combines both modules' OB 82 events.

6. SM 135 - 4 AO HART (6ES7135-7TD00-0AB0)

The SM 135 closes the SOVS / control valve loop. Four 4-20 mA 2-wire outputs drive Ex i positioners or other actuators. The module provides HART primary variable read-back, allowing the controller to verify that the field device has actually moved to the commanded setpoint.

The Siemens manual entry for the AO HART module is documented at 6ES7135-7TD00-0AB0 product page. This is the module referenced in the field report as a follow-up to the original ET 200iSP recommendation.

6.1 Specifications

7. Surge / Transient Protection for Hazardous Loops

Field wiring in Zone 1 can be exposed to induced transients from VFDs, lightning strike on adjacent structures, and switching of large inductive loads. Each 4-20 mA loop and each digital input/output should pass through a surge protector selected for Ex i service.

Verify each protector carries both an ATEX/IECEx Ex i certificate AND a functional safety certificate (SIL) if the loop is part of a SIF. SOVS trip loops are typically SIL 2 or SIL 3.

8. SOVS Integration Pattern

For a Shutdown / Solenoid Operated Valve System using ET 200iSP, follow this logic pattern in the application program:

8.1 STL Code Snippet (S7-300/400)

// OB 35 - Cyclic Interrupt 100 ms
// SOVS Trip Logic for ET 200iSP SM 132
// Address Q0.0 - Q0.3 = SM 132 in slot 3

      L     "Trip_Any"            // OR of ESD, PSHH, Gas, Manual
      AN    "Override_Coil_Test"  // Permit trip during coil test
      =     L     0.0

      L     "Output_Enable"       // Maintenance / bypass switch
      NOT   ;                      // Off = bypass active
      O     "Heartbeat_OK"        // Heartbeat from SOVS logic
      AN    "Channel_Fault_DI7"   // Health of trip inputs
      =     L     0.1

      U     L     0.0
      U     L     0.1
      =     "SOV_1_Command"       // Internal tag
      =     "SOV_2_Command"
      =     "SOV_3_Command"
      =     "SOV_4_Command"

      // Map to physical outputs (de-energize to trip)
      U     "SOV_1_Command"
      =     Q     0.0              // SM 132 slot 3, channel 0
      U     "SOV_2_Command"
      =     Q     0.1              // Channel 1
      U     "SOV_3_Command"
      =     Q     0.2              // Channel 2
      U     "SOV_4_Command"
      =     Q     0.3              // Channel 3

9. Commissioning Sequence

1. Mount ET 200iSP in its Zone 1/2 enclosure; verify IP54 minimum rating.
2. Insert the terminating module at the end of the backplane; torque terminal screws to 0.6 N·m.
3. Wire the PM 136 power module from a 24 V DC Ex e source. Maximum 5 A from PM 136 to backplane Ex i bus.
4. Insert SM 131, SM 132, SM 134 (×2), SM 135, IM 152-1. Confirm mechanical coding keys match module type.
5. Connect IM 152-1 to PROFIBUS DP or PROFINET head. Set the PROFIBUS address using the rotary switches on the IM 152-1 front.
6. Load the GSD file (Siemens ET 200iSP GSD) in TIA Portal / STEP 7 HW Config. Drag each module into its slot.
7. Configure channel parameters: DI Type 1/3, AI 4-20 mA, AO 4-20 mA, surge detection enable, wire-break alarm enable.
8. For each HART device, run the HART DTM in TIA Portal or PDM to confirm PV/SV/TV read-out. Tag each device with its tag number.
9. Perform a loop check on each SOV: drive Q0.0 ON, verify field current with mA clamp, drive OFF, verify spring return.
10. Force a trip from each input (PSHH test button, ESD pushbutton, gas detector) and confirm SOV closure time < 1 s.
11. Capture the diagnostic buffer (PLC > Module Information > Diagnostic Buffer) and save as commissioning record.

10. Troubleshooting Matrix

11. Verification and Acceptance

After commissioning, perform the following verification checks and archive the results in the SAT (Site Acceptance Test) record:

• For each DI channel: simulate wire break, confirm channel diagnostic bit set within 2 s.
• For each DO channel: measure field current with mA clamp, confirm within 30-45 mA at 24 V.
• For each AI HART: ramp PV in field, confirm controller readback tracks at < ±0.5 % deviation.
• For each AO HART: command 4, 12, 20 mA from controller, confirm field current within ±0.05 mA.
• Run HART DTM scan, confirm all 4 (or 8) devices respond with valid unique ID and PV.
• Inject a forced trip on the SOVS, confirm all 4 SOVs close within 1.0 s, and that the diagnostic buffer records the trip cause.

12. Related Considerations

If the SOVS is part of a SIL 1/2/3 instrumented function, the F-variant ET 200iSP modules (6ES713x-7TF00-0AB0 series) provide PROFIsafe over PROFINET. These are pin-compatible with the standard modules and support SIL 3 with 1001 / 1002 architectures. The same fault-diagnostic model applies; the F-modules additionally use PROFIsafe channel passivation in the F-CPU.

For non-hazardous or Zone 2 panel-mounted installations, alternative 8 AI modules such as the Honeywell PCD3.W200 8-channel 10-bit analog input may be considered, but they are not Ex i rated. The ET 200iSP remains the correct choice when field devices sit in Zone 0/1 and require intrinsically safe wiring.