CPU 315T-2DP I/O Addressing: Troubleshooting SM323 Output Wiring

Problem Overview

The CPU 315T-2DP is a Siemens Technology-CPU from the SIMATIC S7-300 family that combines standard PLC functionality with integrated motion-control technology objects. A common field failure involves an engineer attempting to drive a digital output on a separately-racked SM 323 digital input/output module while observing that Q0.0 remains at 0 V despite the logic program setting the bit to 1. The same symptom appears when the on-board 4DI/8DO of the Technology-CPU is addressed as a standard I/O point (for example QB66) because that address space is reserved for the technology firmware and is invisible to the standard user program.

This reference covers both failure modes:

1. SM 323 outputs are logically true but physically 0 V — root cause is the missing 24 VDC / ground reference at the front connector of the digital module.
2. On-board I/O of the CPU 315T-2DP cannot be set or read from OB1 — root cause is that the I/O area starting at byte 66 is owned by the technology objects and is not part of the standard process image.

Both problems are resolved by correct wiring and correct interpretation of the STEP 7 hardware configuration. A working configuration and a non-working configuration differ by as little as two jumpers on the front connector of the SM 323.

Hardware Identification

Before any address decision is made, confirm the exact MLFB (Siemens order number) of every device. The 6ES7 315-2xxx and 6ES7 315-6xxx families are not interchangeable.

Verify the on-board I/O count in HW Config: open the CPU 315T-2 DP station, double-click slot 2, and inspect the dialog "Properties — CPU 315T-2 DP". If addresses are listed starting at byte 66, the I/O is owned by the technology firmware. For a standard CPU 315-2 DP the same slot will show "no integrated I/O".

SM 323 Digital Module — Pin Map and Power Requirements

The SM 323 is a 16-channel digital input / 16-channel digital output module that occupies one slot in the S7-300 rack. It is delivered in several variants; the most common for the 16DI/16DO configuration is 6ES7 323-1BL00-0AA0. The front connector is a 40-pin screw or spring-type connector. The two halves of the module — inputs and outputs — each require their own 24 VDC supply.

The two supplies (pins 21/30 and 31/40) are galvanically isolated on the module and can be fed from separate 24 VDC circuits. Many cabinet builders place a single 24 VDC terminal and bridge 21–31 and 30–40 with a short jumper — this is acceptable as long as the wire gauge supports the cumulative load (0.5 A nominal per group, 1.0 A peak).

Front Connector Wiring Diagram

Root Cause Analysis

Two distinct root causes produce the symptom "logic says 1, terminal says 0 V":

The reported case on the field — a CPU 315T-2DP with an SM 323 and outputs that would not drive despite S Q 0.0 in OB1 — was caused by the missing ground reference on the front connector of the SM 323. The 24 VDC load supply must have a 0 V return path; without it the output transistors cannot complete a circuit and the channel reads 0 V even though the logic state is true.

Step-by-Step Resolution

Step 1 — Inspect Hardware Configuration in STEP 7

1. Open SIMATIC Manager and load the offline / online project for the station.
2. Open HW Config and double-click the SM 323 in slot 4 (or wherever it is racked).
3. Confirm the starting address matches the address you are using in the program. The default is 0 for both input and output bytes, giving IW0 / QW0 and bit access I0.0 / Q0.0.
4. Open the Addresses tab. Note the entry under Process image: "OB1 PI" means the module is part of the OB1 process image update (automatic I/O refresh each cycle). "----" means the module must be read/written using SFCs (SFC 14/15 for consistent access, or direct load/transfer for non-consistent access).
5. Repeat for the CPU slot to identify the on-board I/O starting address of the CPU 315T-2 DP (typically IB 66 / QB 66).

Step 2 — Wire the SM 323 Front Connector

1. Power down the rack (PS 307 24 VDC) before touching the front connector.
2. Open the front cover and remove the connector from the SM 323.
3. Strip and crimp the conductors for the following pins:
     • Pin 20 — load ground return (0 V, output side)
     • Pin 21 — +24 VDC supply group 1
     • Pin 30 — M (0 V) of supply group 1
     • Pin 31 — +24 VDC supply group 2
     • Pin 40 — M (0 V) of supply group 2
4. Re-seat the connector and torque the mounting screw.
5. Restore 24 VDC to the PS 307 and verify with a multimeter: 24 VDC must be present between pins 21 and 30, and between pins 31 and 40.
6. Check the SF LED on the SM 323. It must be off.

Step 3 — Download and Test the Logic

1. In OB1, add the segment:
   SET     // sets RLO = 1
   = Q 0.0     // assign to first SM 323 output
2. Download the program to the CPU (PLC — Download).
3. Switch the CPU to RUN. The status LED for channel 0 of the SM 323 must illuminate and the terminal voltage must rise to 24 VDC (less 0.5 V drop on the output transistor).
4. Open a Variable Table (VAT) and monitor Q0.0 in online view. The value must toggle between TRUE and FALSE as the program executes.

Step 4 — Address the On-Board I/O of the CPU 315T-2 DP (if required)

If the application needs to read the technology inputs or override a technology output from the user program, do not attempt a direct bit access. Use the technology function blocks and SFCs provided by the "S7-Technology" option package.

Process Image OB1 PI — What It Means

The "process image" (German: Prozessabbild) is a memory area in the CPU that holds a copy of all input and output signals at the start of every OB1 cycle. The benefit is that the user program reads a consistent snapshot of all inputs at the same instant, eliminating race conditions that would occur if inputs were read directly from the bus multiple times within one cycle.

In HW Config, each digital module shows one of two settings for the process image:

• OB1 PI — the module is automatically refreshed at the beginning (inputs) and end (outputs) of every OB1 cycle. Access with A I 0.0 or = Q 0.0 from the user program. This is the default and the recommended setting for modules with a small number of bytes.
• None (----) — the module is not part of the OB1 process image. Access is only possible with SFC 14 (DPRD_DAT) for consistent reading and SFC 15 (DPWR_DAT) for consistent writing. This is mandatory for modules that exceed the maximum process-image size of the CPU and is also required for some distributed I/O where consistent data is needed (e.g. analogue modules or complex slaves).

For a 16DI/16DO SM 323 the setting should always be OB1 PI. If STEP 7 has marked the module as "----", the user program cannot set the output with = Q 0.0; the SFC 15 call must be inserted in OB1 with the correct record length and the start address of the output area on the slave.

Verification Procedure

Run the following checklist after any wiring or configuration change:

1. CPU in RUN — the green RUN LED is steady.
2. SM 323 SF LED off — no group fault.
3. SM 323 24 V LED on — the supply is present (where fitted on the module variant).
4. Channel status LED on when the bit is set in the program.
5. Terminal voltage between the output terminal and M measures 24 VDC ± 3 V under load.
6. Diagnostic buffer clean — STEP 7 — PLC — Module Information — Diagnostic Buffer — shows no entries.
7. Force test — temporarily force Q0.0 in VAT, observe the channel LED, then remove the force and confirm the program can still toggle it.

Troubleshooting Matrix

Field Commissioning Notes

• Wire the front connector of every digital output module before applying 24 VDC. A floating ground is the single most common cause of "logic true / physical 0 V" on a Siemens S7-300 digital output.
• Use a common 24 VDC busbar (or a fused distribution terminal) so that the cabinet has one defined 0 V reference. Multiple independent 24 VDC supplies without a shared 0 V return can produce mystery faults that change when cabinet doors are opened or closed.
• For the CPU 315T-2DP, never place general-purpose I/O on the front connector of the CPU. Wire all standard digital I/O to an SM 321 / SM 322 / SM 323 in the rack. The on-board I/O is reserved for the technology package.
• If the technology functions are not used, the on-board I/O area is still mapped in HW Config but cannot be read or written by the user program. To save debugging time, place a comment in OB1 indicating that addresses IB66 / QB66 are owned by the technology firmware.
• When extending a station, remember that the maximum process image of the CPU 315-2 DP is 128 bytes for inputs and 128 bytes for outputs. Anything beyond that must be read with SFCs (SFC 14 / SFC 15) and is not part of the OB1 PI.

Related Configuration Checks

Three additional checks are worth performing in HW Config before declaring a station healthy:

1. Slot rule: the SM 323 can occupy slots 4 to 11 in the S7-300 rack. Slot 3 is reserved for interface modules (IM 360/361) when used in a multi-rack configuration.
2. Diagnostic interrupt: leave the default "Diagnostic interrupt" enabled. A blown fuse or missing supply generates a diagnostic entry that points directly to the channel.
3. Substitution value: in the CPU properties — "Behaviour when CPU starts up" — define a substitution value of 0 for the output bytes so that the SM 323 outputs are forced to a defined state on STOP-to-RUN transitions.

FAQ