STEP 5 Program Transfer: Downloading and Uploading S5 PLC Programs

This reference documents the complete engineering workflow for transferring automation programs between a programmer/PC and a Siemens SIMATIC S5 PLC using STEP 5 software (V7.x family). It covers the AS511 point-to-point protocol, the TTY current-loop interface, PG cable part numbers, project structure, online/offline operations, and the verification steps required to confirm a successful download or upload. The guide is engineered to support commissioning engineers maintaining legacy S5 systems, with explicit attention to S5-90U, S5-95U, S5-100U, S5-115U, S5-135U, and S5-155U CPU variants.

1. Overview of the STEP 5 Transfer Workflow

The SIMATIC S5 family uses a single-master, point-to-point serial link for program transfer. The PC or programming device (PG) acts as master and initiates all block read/write operations. Two flow directions exist:

• Download (PC → PLC): Transfers a project archive, individual blocks (OB, PB, SB, FB, FX, DB, DX), or a complete program image from the STEP 5 file system into PLC user memory.
• Upload (PLC → PC): Reads the live program image from the PLC back to the PG for backup, comparison, or offline editing.

Both operations run over the AS511 protocol layered on a 20 mA current-loop physical interface (TTY). The PG cable includes an integrated TTY-to-RS232 converter for connection to a standard PC serial port, or a USB-to-TTY adapter for modern hardware without a native COM port.

2. Prerequisites

2.1 Hardware Requirements

2.2 Software Requirements

• STEP 5 base package (STL, CSF, LAD editors)
• COM port driver (for native RS-232: Windows COM1–COM4; for USB: vendor driver plus virtual COM mapping)
• Sufficient disk space for project archive (typical S5-115U project: 200 KB to 4 MB)
• PLC password (if active block protection is enabled)

2.3 Linkage Topology

The reference linkage is:

PG (or PC + USB-TTY) --- [6ES5 734-1xxx] --- [S5 PLC front AS511 port]
   RS-232 (or USB)         TTY 20 mA loop         15-pin D-sub AG S5

3. AS511 Interface Architecture

3.1 Physical Layer (TTY 20 mA Current Loop)

AS511 is a passive-TTY, full-duplex, optically-isolated current loop operating at 20 mA. The PG cable includes an onboard 24 V supply and a current-source transmitter; the PLC side provides the loop return. Maximum reliable cable length is 16 m at 9 600 baud. The default transfer rate is 9 600 bit/s with 8 data bits, even parity, and 1 stop bit (8E1).

3.2 Protocol Stack

The AS511 frame uses Siemens proprietary framing with the following layers:

3.3 Pinout (PG Cable 6ES5 734-1BD20)

4. PG/PC Hardware Configuration

4.1 Native COM Port Setup (Windows XP/2000)

1. Open Control Panel → System → Hardware → Device Manager → Ports (COM & LPT).
2. Select the communications port (e.g., COM1) and confirm base I/O and IRQ do not conflict with the PG card.
3. Set the port to 9 600, 8, Even, 1, None in Properties → Port Settings → Advanced. Disable FIFO if available.

4.2 USB-to-TTY Adapter Setup (Windows 7/10)

1. Install the vendor driver (e.g., FTDI VCP driver for 6ES5 734-1CD10 or compatible).
2. Verify the virtual COM port number (Device Manager) and bind it to a stable COM port (e.g., COM3).
3. Set the same 9 600 8E1 framing. Keep FIFO buffers at the lowest setting to reduce latency.

5. STEP 5 Software Configuration

5.1 Selecting the Online Interface

1. Launch STEP 5 V7.x.
2. Open the project (or start with an empty container for upload).
3. Choose PLC → Online → Set Interface from the menu bar.
4. In the interface dialog, select AS511 as the active protocol.
5. Set the COM port to match the physical connection (COM1, COM2, or mapped virtual port).
6. Click Test. The status line should report Connection OK within 2 s.

5.2 CPU Type Selection

STEP 5 must match the CPU model of the target PLC to allocate address space and block memory correctly. Confirm the CPU type in PLC → Set CPU Type:

6. Download Procedure (PC → PLC)

6.1 Sequence

1. Place the PLC in STOP mode via the mode selector on the CPU front panel. The red STOP LED must be lit.
2. In STEP 5, open the source project that contains the program image.
3. Confirm CPU type matches (see 5.2).
4. Choose PLC → Download (or press the toolbar download icon).
5. The dialog Download Blocks lists every block present in the project. Use Select All for a complete transfer, or check only the changed blocks for an incremental update.
6. Click Start. STEP 5 will perform an erase → write → verify cycle for each block.
7. Monitor the status line for transfer progress. A 4 KB block typically completes in 6 s at 9 600 baud.
8. When the dialog reports Download finished, no errors, the transfer is complete.

6.2 What Happens Inside the PLC

During download the PLC:

1. Erases the existing block from battery-backed RAM or EPROM submodule.
2. Receives the new block in 32-byte segments.
3. Performs a 16-bit checksum (BCC) comparison on every segment.
4. On success, sets the block valid bit and updates the block address list (BAL).

If the PLC has an EPROM submodule (e.g., 6ES5 375-1LA15) for permanent storage, blocks must be written to EPROM and then copied into RAM at restart. STEP 5 will detect the EPROM memory type and prompt for the appropriate procedure.

7. Upload Procedure (PLC → PC)

7.1 Sequence

1. Open a new, empty STEP 5 project (or the target project).
2. Verify CPU type is set correctly to match the connected PLC.
3. Choose PLC → Upload.
4. STEP 5 reads the block address list (BAL) from the PLC and enumerates every valid block.
5. Confirm the block selection and press Start.
6. The status line will show each block name, type, and transfer progress.
7. On completion, STEP 5 writes the blocks to the local project directory as *.S5D archives.

7.2 Block-by-Block Upload (Selective)

For diagnostic or comparison purposes, individual blocks may be uploaded by:

1. PLC → Online → Block List
2. Highlight the desired block
3. Right-click → Read Block to PG

This is the safest method when only a specific FB or DB has been modified in the field and a comparison is needed against the master archive.

8. Program File Management

8.1 STEP 5 File Types

8.2 Backing Up Before Download

Always upload the existing program before any planned download. Save the upload as a timestamped S5D file:

D:\S5_Archive\PressLine_115U_CPU944_20240617_1432.S5D

Keep at least three historical revisions per PLC in a controlled directory with read-only attributes.

9. Communication Diagnostics

9.1 Status Indicators on the CPU

9.2 STEP 5 Connection Test

The Test Connection dialog returns one of the following status codes:

9.3 ISTACK and BSTACK Inspection

For deeper diagnostics after a failed download, read the Interrupt Stack (ISTACK) and Block Stack (BSTACK) from the PLC:

1. Online → CPU Functions → Read ISTACK
2. Online → CPU Functions → Read BSTACK

The ISTACK indicates the instruction pointer at the time of the last fault, the cause code, and the accumulator contents. Common fault codes encountered after a botched download are QVZ (timeout, OB 26) and ADF (address error, OB 22).

10. Troubleshooting Matrix

11. Verification After Transfer

After every download or upload, perform the following verification cycle:

1. Block list comparison: Choose PLC → Online → Block List and compare the runtime BAL with the project BAL. Block counts and types must match exactly.
2. Checksum verification: Use PLC → Online → Verify Block for each block. STEP 5 reads the block back and computes a checksum. Any mismatch is flagged.
3. Status query: Use PLC → Online → Status to read CPU status, operating mode, and error word (USTACK / Fehlerwort). Address any error code before continuing.
4. Restart sequence: With the PLC still in STOP, perform a manual restart by switching the mode selector: STOP → RUN. Monitor the front LEDs for clean restart (no flashing red).
5. Functional test: Run the program in single-scan or restricted mode where supported. Verify process I/O behaves as expected before returning the PLC to automatic (RUN) mode.

12. Legacy Considerations and Modern Alternatives

12.1 When to Keep STEP 5

STEP 5 V7.23 remains the only environment that natively programs S5 CPUs. Where the S5 PLC drives a long-life production asset and the controller is functioning reliably, the AS511 workflow is the most direct and lowest-risk maintenance path.

12.2 Migration Options

Where migration is desired, the following options exist, listed from least to most invasive:

• Co-existence gateway: Use a PROFIBUS DP-to-ET 200 converter or an S5-to-PROFINET gateway (e.g., Helmholz DP/PN Coupler) to expose S5 I/O to a STEP 7 PLC. The S5 remains in service; the STEP 7 controller handles new logic.
• Code conversion: Tools such as S5 to S7 Converter translate STEP 5 sources to STEP 7 STL/FBD. Manual review is mandatory; standard FB conversions rarely work one-to-one.
• Full migration: Replace S5 with S7-300/400 or S7-1500 plus a S5 migration rack. This is the most expensive but the most future-proof option.

12.3 Driver Availability

STEP 5 V7.23 will not install on a 64-bit Windows version. Recommended platforms for field engineering:

• Windows XP SP3 32-bit (native, fully supported)
• Windows 7 32-bit (compatibility mode, signed driver required)
• Windows 7/10 64-bit with virtualized Windows XP (VirtualBox or VMWare Workstation, USB passthrough for the AS511 cable)

13. Field Commissioning Checklist

Use the following checklist for every S5 service visit that involves program transfer:

1. Confirm PLC type, CPU type, and firmware version (read from front plate).
2. Upload current program; save to dated S5D archive.
3. Verify the upload checksum against any local master (if present).
4. Place PLC in STOP, download revised project, verify each block.
5. Perform step-by-step restart with output inhibit engaged.
6. Run a controlled test sequence with operator supervision.
7. Return PLC to automatic (RUN) and hand over to operations.
8. Sign off on the engineering log with archive filenames and timestamps.

This procedure is deterministic, repeatable, and aligned with the documented STEP 5 AS511 link. The same workflow applies to S5-95U through S5-155U with the appropriate CPU type selection in STEP 5.

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