Siemens CPU 555 Online Access TISOFT, Profibus & S5

Problem Overview

The Siemens CPU 555, originally a Texas Instruments TI555 controller later rebranded and sold under the Siemens SIMATIC TI label, presents a unique connectivity challenge for engineers accustomed to the native S7 platform. Three recurring failure modes appear in field service:

1. PG cannot enumerate Profibus nodes from the CPU 555 master even when the PG address is set to an unused station number.
2. No online connection to the CPU 555 using standard MPI/Profibus cables because the legacy TI serial protocol is required.
3. S5-100U CPU will not go online with a TTY-to-RS232 cable and S5 for Windows, often because of port configuration or null-modem polarity.

This reference consolidates the cable wiring, software versions, addressing rules, and verification procedures needed to bring a TI555-based machine (for example, a 3-layer blown film extruder with ~20 Profibus DP slaves, Simovert and Simoreg drives, and a supervisory S7-400) online for commissioning or service.

Identifying the CPU 555 Platform

Confirm the controller family before selecting any programming path. The Siemens CPU 555 carries a Siemens label but the firmware, instruction set, and serial protocol are inherited from the Texas Instruments TI505 / TI555 series. The Siemens part number printed on the front face typically reads 555-1106 or similar (for example, 555-1105, 555-1107 depending on memory and option mix).

Hardware Requirements: Cables and Interfaces

The CPU 555 communicates through a 25-pin female Sub-D port on the CPU faceplate using the TI NIM (Network Interface Module) serial protocol at 9 600 bps or 19 200 bps. A standard MPI cable (6ES7 901-0BF00-0AA0) or a Profibus DP cable will not establish a programming session.

Required cable

• TISOFT programming cable (null modem): DB-25 male to DB-9 female, crossed TX/RX, crossed RTS/CTS, DCD tied to DTR at the PC end. Commercial equivalents are available from third-party suppliers as "TI505 / TI555 programming cable" or "Siemens 505 to PC serial cable".
• Asynchronous serial port: A true RS-232 port is required. Modern laptops without a DB-9 need a USB-to-RS-232 adapter based on FTDI FT232 or SiLabs CP2102N. Drivers must be configured to expose a standard COM port; software such as S5 for Windows or TISOFT does not enumerate USB-CDC virtual ports reliably on Windows 10/11.

Pinout reference (TISOFT null-modem cable)

For a full schematic and memory configuration details, refer to the SIMATIC TI545/TI555 System Manual (PDF, order code 545-8101-2), section 6 "Starting Up the TI555 System" and section 6.3 "TI555 CPU Memory Configuration".

TISOFT Software Configuration

TISOFT (also shipped as Siemens "SoftShop" in some markets) is the only officially supported programming environment for the TI555. DirectSOFT from AutomationDirect supports the TI505 instruction set up to the TI435; it will not enumerate the TI555 because the latter uses extended memory and enhanced instruction set beyond TI435.

Project setup

1. Launch TISOFT and create a new project. Select PLC type TI555 (not TI545, not TI525).
2. Open PLC > Communications and configure the COM port:
   • Baud rate: 9 600 (default) or 19 200 if the CPU is set for high-speed NIM.
   • Data bits: 8, Parity: None, Stop bits: 1.
   • Port address: COM1, COM2, etc., matching Windows device manager.
3. Set the PLC node address to match the CPU front-panel rotary switches (typically 1).
4. Click Connect. A successful handshake displays the CPU model, firmware revision (for example, "TI555 Rev 5.0"), and current run/stop mode.

Profibus Network Access Considerations

The CPU 555 acts as a Class 1 Profibus DP master. The "0" address printed on the schematic refers to the master's logical address on the Profibus segment, not the PG's target address. When a PG is attached, it must operate as a Class 2 master and therefore cannot read slave I/O unless the active master grants token passing.

Why the PG cannot enumerate slaves

• Token ownership: Only one Class 1 master can hold the token at a time. The CPU 555 holds it continuously during run mode and never yields to a passive PG.
• PG address conflict: Setting the PG to address 0 collides with the master and is rejected by the Profibus stack.
• Diagnostic buffer: Connect via TISOFT first and read PLC > Diagnostic Buffer. A "Master address conflict" entry confirms the address collision.

Supported paths into the Profibus network

1. Through the CPU's serial port with TISOFT — read Profibus diagnostic counters and slave status without disturbing the running master.
2. CP5711 attached as a Class 2 master via a Profibus tap — the CP5711 (Siemens 6GK1571-1AA00) speaks Profibus DP and can co-exist with the active master when correctly configured in STEP 7.
3. IBH Softec NetLink or NetLink PRO with TCP/IP-to-Profibus-DP gateway (IBH Link S7++). The IBH gateway sits on the segment as an additional master and is the typical budget path for service engineers.

S5 CPU 100U Connectivity

An S5-100U with no EPROM and no I/O modules will still power up and respond on the serial port as long as the CPU is in RUN or STOP with the battery-backed RAM intact. The presence of EPROM or I/O has no bearing on the ability to go online.

Wiring an S5 TTY cable

The S5-100U uses a 25-pin Sub-D TTY (20 mA current loop) port, not RS-232. The "TTY-to-RS232" cable in widespread use is in fact an active converter (Siemens 6ES5 734-1BD20 or equivalent) that converts the 20 mA loop to RS-232 levels. A passive cable will never work.

Software configuration (IBH S5/S7 for Windows V5.03)

1. Select Online > Set Connection > S5 via AS511.
2. COM port: match the Windows assignment (use COM1–COM4 as above).
3. Baud: 9 600. Handshake: none.
4. Click Connect. The PLC type and memory configuration should populate within 3 seconds.

S7-400 and Drive Communication

On machines built around a TI555 master, the higher-level S7-400 is typically added as a supervisory controller communicating with Simovert (master/slave) and Simoreg (DC drive) units over Profibus DP using PROFIdrive profile. STEP 7 V5.4 SP1 remains the supported engineering tool for this generation.

Going online with the S7-400

1. Connect a PC adapter (USB MPI, 6ES7 972-0CB20-0XA0) to the S7-400 MPI port.
2. In STEP 7, set PG/PC interface to PC Adapter (MPI) with transmission rate 187.5 kbps (default) or 1.5 Mbps if the S7-400 has that capability enabled.
3. Click Accessible Nodes. The S7-400 MPI address and any DP slaves should appear.
4. For Drive ES commissioning of Simovert / Simoreg, install Drive ES V5.x or use DriveMonitor (free download from Siemens support) over the drive's serial X300 port if Profibus is unavailable.

CP5711 alternative

The CP5711 (6GK1571-1AA00) provides Profibus DP and PROFINET access for STEP 7, Drive ES, and STARTER without occupying an MPI slot. It is the recommended modern interface for service laptops running Windows 10/11 where the USB MPI adapter has driver issues.

IBH S5/S7 for Windows Configuration

IBH Softec's "S5/S7 for Windows" (version 5.03 referenced in the source case) acts as a unified front-end for both S5 and S7-300/400 controllers. It can drive the TI555 only through its S7 emulation layer when used with an IBH NetLink gateway — direct serial access to a TI555 is not supported.

For modern laptops without a serial port, refer to the SiePortal article "Go online with S5 program" for USB-to-serial conversion strategies and approved adapter chipsets.

Verification Checklist

1. Confirm TISOFT reports the correct CPU model and firmware revision on Connect.
2. Read the diagnostic buffer — clear pre-existing errors and re-power.
3. Verify all 20 Profibus slave nodes appear in TISOFT Profibus Diagnostics > Live List with green status.
4. From STEP 7 (V5.4 SP1), confirm Accessible Nodes lists the S7-400 MPI address.
5. Open Drive ES or DriveMonitor and read each Simovert / Simoreg parameter set (r001, r002, etc.).
6. Toggle one digital output from each controller and observe the corresponding field device.
7. Backup the TI555 program using TISOFT File > Upload from PLC to a versioned archive directory.

Common Error Scenarios Matrix

Pre-Commissioning Safety and Documentation

Before any online intervention, observe the following field-tested precautions:

• Verify the machine is in a safe state — drives disabled, heaters off, extruder screws isolated.
• Read the machine's E-stop wiring diagram. The TI555 typically maps E-stop to a high-priority discrete input that cannot be bypassed from software.
• Capture the existing program and parameter set before any upload/download.
• Confirm battery health on the TI555; if the lithium battery is below 2.7 V, the RAM-backed program will be lost on power-down.
• For S5-100U, check the buffer battery; a depleted battery may still allow online but prevents program retention.

Spare Parts and Long-Term Support

The TI555 is a legacy product. Spare CPU modules are scarce; consider identifying a TI555-1106 or compatible drop-in replacement before a failure occurs. The SIMATIC TI545/TI555 System Manual remains the definitive reference for memory layout, I/O addressing, and Profibus master configuration.

For an S7-1200 / STEP 7 Basic first-commissioning reference applicable when upgrading adjacent controllers, the SiePortal checklist "S7-1200 / STEP 7 Basic — Check list for first online access" provides a parallel workflow that is useful when an S7-1200 has been retrofitted alongside the legacy TI555.