Interfacing Siemens 6ES5 921-3WB14 S5-130WB via AS511 TTY Loop

System Overview

The target machine in this scenario is an Index C200 CNC lathe equipped with a SINUMERIK System 3 operator panel and a Siemens SIMATIC S5-130WB programmable logic controller (CPU module 6ES5 921-3WB14). The requirement is to bring the PLC online from a modern laptop running STEP 5 V7.23 + Hotfix 1 for diagnostics, block upload, and block download. The challenge is that the S5-130WB does not expose a V.24 (RS-232C) port at its front connector, and the operator panel PCB 6FX1125-8AA01 (which does carry a DB25 socket marked X111) is part of the SINUMERIK NC/CRT path, not the PLC. Bringing the S5-130WB online therefore requires adding the dedicated AS511 interface module 6ES5511-5AA14, a V.24-to-TTY (20 mA) converter, and the same serial cable used between a PG 7xx or Field-PG and the AS511 port.

The protocol on the wire is AS511 at 9600 bit/s, full-duplex, 8 data bits, no parity, 1 stop bit (8/N/1), carried on a 20 mA current loop (TTY) rather than on V.24 voltage levels. This physical layer is the root cause of most field confusion: a standard DB25 RS-232 cable will not establish the link even though the shells match.

Hardware Identification and Bill of Materials

Every component on the link must be identified by Siemens order number (MLFB) so that cable, converter, and protocol choices are unambiguous. The parts relevant to this interface path are listed below.

Why the 6FX1125-8AA01 PCB Is Not the PLC Port

The 6FX1125-8AA01 is a front-end PCB for the SINUMERIK System 3 CRT/keyboard unit. Its DB25 socket labelled 'X111 / RS232c/TTY' is wired to the operator-panel interface of the NC, not to the S5-130WB backplane. Even where TTY pins are present, the protocol and baud rate on that connector are not AS511. Online access to the PLC must be made on the PLC's own backplane, via the 6ES5511-5AA14 module.

If the 6FX1125-8AA01 connector carries TTY signals only for the SINUMERIK MCP/keyboard, attempting to drive it with the PG can stall the NC keyboard scan or, in older revisions, damage the line drivers. The safe approach is to treat the 6FX1125-8AA01 as out-of-scope for PLC online access and to wire to the AS511 module only.

If the cabinet does not contain a 6ES5511-5AA14, it is a sign that the PLC was never programmed online from outside — common on older Index C200 builds where the program was loaded at the factory and never changed. In that case, fit a 6ES5511-5AA14 in the first free slot of the S5 rack. The card is cold-insert only; always power down the rack before fitting it.

AS511 Interface Module 6ES5511-5AA14

The 6ES5511-5AA14 is the standard Siemens AS511 communications module for the S5-130 and S5-150 families. It occupies a single slot in the S5 rack adjacent to the CPU 6ES5 921-3WB14 and converts the internal SIMATIC S5 backplane bus to an external 20 mA current-loop (TTY) interface. Without this module, there is no way to perform an AS511 online session with STEP 5 from any PG or PC.

Module Layout

• Single Europe-format PCB, occupies one slot in the S5 rack.
• Front-panel DB25 socket for the external TTY cable.
• DIP-switch block for line termination and active/passive current-loop source selection.
• Power is drawn from the S5 backplane (5 V) — no external supply needed for the module itself.
• Front-panel status LEDs (PWR / REC / SND or equivalent — consult the card manual for the exact label set of the -5AA14 revision).

Configuration Switches

The DIP switches on the 6ES5511-5AA14 must be set to match the physical wiring of the current loop. The exact numbering depends on the hardware revision; always consult the printed manual that shipped with the card.

• Switch S1 (line termination): typically ON for the end device, OFF when a device is in the middle of a multi-drop loop. The AS511 link is point-to-point, so S1 is normally ON.
• Switch S2 (active/passive current source): selects whether the PLC side sources the 20 mA (active) or sinks it (passive). With a PG or V.24/TTY converter, the convention is to set the PLC side to passive and the converter side to active.

The card slots in next to the CPU 6ES5 921-3WB14. On the S5-130WB rack the slot ordering is fixed: power supply, then CPU, then IM/IMs, then I/O. The 6ES5511-5AA14 can sit in any free slot after the CPU. If a free slot is not available, the cabinet must be retrofitted with an additional sub-rack or a bus extension.

Current Loop (TTY) vs V.24 (RS-232)

The AS511 protocol is carried on a 20 mA passive/active current loop, not on V.24. The PC or PG side, however, is almost always V.24 (RS-232C). A converter is therefore required.

Plugging a V.24 cable into a TTY port — or vice versa — does not work, even though the DB25 shell is identical. The voltage levels never reach the optocouplers and the link never establishes.

The optical isolation is also a key EMC benefit. The S5-130WB is mounted in a cabinet that also contains variable-frequency drives, contactors, and a 400 V spindle inverter; the current loop allows the laptop to sit at a different ground potential without dragging the bus into common-mode noise. Ground-potential difference of up to a few hundred volts is tolerated by the optocouplers without damage.

V.24 / TTY Converter Options

Two practical converter choices are common in the field.

1. Siemens 6ES5 734-2... TTY/V.24 adapter

Siemens offered a TTY-to-V.24 converter module under the 6ES5 734-2 family. The -2AF21 variant in particular is widely available on the secondary market and is pin-compatible with the standard PG cable. Power is taken from a 24 V supply or from a plug-in wall PSU. The Siemens-branded unit ships pre-configured for active sourcing on the PG side, which is the correct polarity for an AS511 link to a passive 6ES5511-5AA14.

2. Third-party 20 mA current loop / RS-232 converter

Industrial serial converters with a 20 mA current loop side (e.g. Moxa TCC-80I, Westermo MA-45, Phoenix Contact PSM-TTY/USB) work as well. When using a third-party unit, observe the following:

• Confirm the converter is optically isolated on the TTY side.
• Set the converter to active current source on the PC side, passive on the PLC side (or vice-versa — the polarity must be one active + one passive per pair).
• Use a 4-wire (full-duplex) TTY interface, not a 2-wire (half-duplex) variant.
• Confirm the converter does not require driver software beyond a virtual COM port — many modern units enumerate as a regular COM port on Windows 10/11.

Cable Wiring and Pinout

The pinout for the AS511 cable follows the same assignment as the standard PG 7xx / Field-PG TTY programming cable. The official pin mapping is documented in Siemens Knowledge Base article 217845 — TTY interface pin assignment (PG/PC ↔ SIMATIC S5 AS511).

A '1-to-1' DB25 male-to-male cable with only the four signal pairs (4/5 and 9/10) plus GND (7) and shield (1) wired is sufficient. The remaining pins are not used by the AS511 protocol. If the converter has a DB9 output, use a DB9-to-DB25 adapter wired to the standard TIA-574 assignment on the DB9 (pin 2 = TX, pin 3 = RX, pin 5 = GND) and let the converter's DB25 handle the current loop.

Figure 1 — Physical topology from laptop (USB/serial) through a V.24/TTY converter to the 6ES5511-5AA14 module and the S5-130WB CPU 6ES5 921-3WB14. The 6FX1125-8AA01 board on the SINUMERIK System 3 is intentionally not part of this path.

STEP 5 V7.23 + Hotfix 1 Software Setup

STEP 5 V7.23 + Hotfix 1 supports the AS511 protocol natively. The relevant configuration steps are listed below.

1. Install STEP 5 V7.23, then apply Hotfix 1 (consult the V7.23 release notes on the install media for the corrections it provides; refer to the README for the affected AS511 timing and long-pathname support).
2. Launch STEP 5. Open or create a project that matches the S5-130WB (CPU 921S target).
3. From the menu, choose File → Set PG → Interface…
4. Select AS511 as the interface type.
5. Set the COM port (e.g. COM1 or the COM number assigned to the USB-to-RS-232 adapter).
6. Set the baud rate to 9600 bit/s, 8 data bits, no parity, 1 stop bit (8/N/1).
7. Confirm by clicking Test; the test should report 'AS511: OK' once the loop is closed.

Save the project to a path shorter than 64 characters. STEP 5 V7.23 has known issues with Windows long-path support; Hotfix 1 partially mitigates them, but block names longer than 8 characters (e.g. FB200) can still be problematic on FAT-formatted archives.

If the test fails with 'Interface not responding', check that the converter is powered (its power LED is lit), that the TTY cable is plugged into the AS511 card and not into X111 on the 6FX1125-8AA01, and that the laptop's COM port is not already open in another program (e.g. HyperTerminal, PuTTY).

Going Online with the PLC

With the cable and STEP 5 configured, the online session is straightforward.

1. Power up the Index C200 in NC-STOP (the NC must be in stop, the PLC in RUN or STOP per the application).
2. Confirm the 6ES5511-5AA14 status LEDs: power lit, REC flickering when there is link activity, SND flickering when the PLC is responding.
3. In STEP 5, select PLC → Online… or press the toolbar 'Online' button.
4. Choose Reset, Status, Force or Upload to PG as required.
5. To download a new program, choose Block → Transfer → PC → PLC and select the FB/PB/DB/OB blocks to send.

During the first session always perform a block upload to PG and back up the AG (PLC) program. The S5-130WB retains its program on power-down in battery-backed RAM (or, in later builds, on Flash). A failed battery or a corrupted Flash will erase the program; an upload is the only way to recover.

The upload procedure is:

1. PLC → Upload to PG
2. Select the block types to upload (OB, PB, FB, DB, SB, FX).
3. Save the upload as a separate project — never overwrite the original.

For a download, always first perform an Overall Reset on the PLC (PLC → Overall Reset) before sending the new blocks. The S5-130WB will not overwrite blocks that are currently in RUN state.

Verification, Diagnostics and Troubleshooting Matrix

If the link does not establish, work through the matrix below. The rows are ordered from 'most likely' to 'least likely' based on typical field experience on Index C200 retrofits.

Loop-Current Measurement Procedure

If the link fails to start, measure the 20 mA loop directly.

1. Disconnect the cable at the AS511 end.
2. Place a multimeter in mA mode in series with the + and − of one pair (e.g. pin 4 and pin 5).
3. With the converter powered and the loop closed, the meter should read approximately 20 mA.
4. Repeat for the second pair (pins 9 and 10).

If both pairs read 0 mA, the active side is not sourcing. If one pair reads 0 mA and the other reads 20 mA, the active/passive polarity is reversed on that pair. If both pairs read 20 mA but the link still fails, the issue is in the protocol layer (baud rate, wrong interface type in STEP 5, or wrong COM port).

Optical Isolation Test

To verify that the converter and the AS511 card are isolated from each other, measure the resistance between the converter's TTY shield (pin 1) and the PLC's backplane ground. A reading above 1 MΩ confirms proper isolation; a reading below 100 Ω indicates a shorted opto-coupler, in which case the converter or the card must be replaced.

Field Commissioning Checklist and Safety

Use this checklist when commissioning the link on a customer machine.

• Lockout/tagout the cabinet before opening it. The S5-130WB is on 24 V and 120/230 V; both are present on the backplane connector.
• Power down the CNC before inserting or removing the 6ES5511-5AA14 module. Hot-plugging a SIMATIC S5 module can blow the backplane fuse.
• ESD strap when handling the AS511 card. CMOS components on the card are sensitive.
• Verify the 24 V supply feeding the converter is regulated and isolated.
• Label the AS511 cable at both ends with 'PLC ONLINE — DO NOT UNPLUG UNDER POWER' once installed.
• Record the DIP switch positions of the 6ES5511-5AA14 in the machine logbook.
• After going online the first time, back up the PLC program to disk and to a printed listing.
• Verify the Index C200's PLC mode selector returns to RUN after the session, otherwise the NC will halt on the next cycle.

Frequently Asked Questions