Resolving Siemens S5-95U "Link Not Established" with USB AS511 Programming Cable 700-750-1US13

The Siemens SIMATIC S5-95U compact PLC uses a 20 mA current loop (TTY) programming interface and the proprietary AS511 protocol. Modern PCs no longer ship with native TTY or even RS-232 ports, so a USB-to-TTY adapter is the standard field approach. The cable part number 700-750-1US13 referenced in the source is a third-party USB-to-AS511 converter that emulates a serial port on the PC. When STEP 5 v7.2 reports "Link not established", the failure almost always sits at the boundary between the USB emulated COM port, the AS511 driver layer, and the 20 mA current loop hardware — not in the PLC program itself.

1. Problem Definition

When the operator selects Online > Connect (or presses the connection hotkey) inside STEP 5 v7.2 against a CPU 95U via the 700-750-1US13 cable, the software returns:

STATUS:  Link not established
ERROR:   No AS511 connection to AG (target PLC)

Symptoms that always accompany this fault:

• STEP 5 never displays the CPU type or status LEDs.
• The "ONLINE" indicator in the STEP 5 title bar does not change from grey to green.
• No blocks (OB, PB, FB, DB, SB) can be read from the CPU.
• No fault is logged inside the CPU (because the diagnostic link itself is down).

The PLC continues to execute its user program normally; the fault is purely a programming-device-to-CPU communication failure.

2. S5-95U Communication Architecture

The S5-95U is the higher-memory member of the S5-90U / S5-95U family. Its programming port is a female 15-pin sub-D connector labelled X1 on the CPU front panel. The interface is not RS-232; it is a 20 mA current loop (TTY, German Stromschnittstelle). The 95U acts as the active current source/sink for the loop; the PG (or its adapter) provides the complementary half.

The S5-95U was sold with two power-supply variants: 6ES5 095-8MB (24 VDC) and 6ES5 095-8MA (115/230 VAC). Both share the same X1 pinout, so the cable diagnosis below is identical for either supply.

3. AS511 Protocol and TTY Interface Specifications

AS511 is Siemens' proprietary, character-oriented, half-duplex protocol used for programming the S5-90U, S5-95U, S5-100U, S5-115U, S5-135U, and S5-155U families. The PC side initiates all transactions; the PLC side only responds. The protocol uses STX/ETX framing, an 8-bit address field, and a 16-bit BCC checksum.

Because the CPU side is fixed, the entire compatibility burden is on the PC side: the USB-to-TTY cable must present a 20 mA active current loop, not just RS-232 levels. Many cheap USB-to-serial adapters on the market are RS-232 only and will never work against the S5-95U, regardless of the COM port number selected in STEP 5.

4. Cable 700-750-1US13 Identification

The 700-750-1US13 is a third-party USB-to-AS511 cable typically shipped with an internal converter box that performs USB → RS-232 (via FTDI / CH340 / Prolific PL-2303) → 20 mA TTY in a single assembly. The 15-pin sub-D male end plugs directly into the S5-95U X1 port; the USB-A end plugs into the PC.

5. Root Cause Analysis of "Link Not Established"

Across the field, this fault has five recurring root causes, listed in descending order of frequency:

6. STEP 5 v7.2 Software Configuration

STEP 5 v7.2 runs on Windows (NT, 2000, XP, 7 are all supported; 10/11 require compatibility mode). The interface selection is hidden behind several menus, which is a common source of confusion.

1. Launch STEP 5 v7.2 from the Siemens program group.
2. Open the menu Options > PG Interface Assignment… (German: Options > PG-Schnittstelle zuordnen…).
3. In the dialog, select the AS511 driver (not "MPI", not "COM", not "S5-LAN").
4. Click Properties and confirm the COM port matches the one assigned to the 700-750-1US13 cable.
5. Set transmission rate to 9600 baud, 8 data bits, even parity, 1 stop bit.
6. Click Set, then OK. STEP 5 writes the choice to the registry under HKLM\SOFTWARE\Siemens\STEP5\PGIF.
7. Close and re-launch STEP 5 — the PG interface change only takes effect after a restart.

Registry key (read-only inspection):
  HKLM\SOFTWARE\Siemens\STEP5\7.2\PGIF
     Interface = "AS511"
     ComPort   = "COM1"
     Baud      = 9600
     Parity    = 2     (0=None, 1=Odd, 2=Even, 3=Mark, 4=Space)
     DataBits  = 8
     StopBits  = 1

7. USB Driver and COM Port Mapping

The 700-750-1US13 cable is normally built around an FTDI FT232 or a Prolific PL-2303HX. Both vendors supply Windows drivers that create a Virtual COM Port (VCP). The COM port number is assigned by Windows in the order the cables were first plugged in, so a laptop that has ever seen a USB-to-serial adapter before will not necessarily map this cable to COM1.

1. Plug the cable in. Open Device Manager → Ports (COM & LPT).
2. Identify the new entry — e.g. USB Serial Port (COM3).
3. Right-click → Properties > Port Settings > Advanced….
4. Change the COM port number to COM1 (or to the lowest free number, then update STEP 5 accordingly).
5. Set Latency Timer to 1 ms (FTDI default is 16 ms; this introduces a 32 ms round-trip delay that can make AS511 polling time out).
6. Disable Selective Suspend under Power Management — Windows may otherwise power down the USB port after 30 s of idle and break the current loop.
7. Unplug, re-plug, confirm the COM number persists.

8. Hardware Loopback and Signal Verification

Before blaming the cable, prove the TTY current loop is actually carrying 20 mA. The S5-95U powers the loop from the X1 port, so a milliamp meter placed in series with either the Tx or Rx line should read roughly 20 mA as soon as power is applied to the PLC — even with the PC disconnected.

1. Power the S5-95U from its 24 VDC (or 115/230 VAC) supply. The "RUN" and "STOP" LEDs should illuminate, with the CPU in STOP.
2. Insert a milliamp meter in series with pin 13 (TTY receive+) of the 15-pin sub-D. The cable must be unplugged from the PLC for this measurement — current flows only when the loop is closed through the cable.
3. Plug the 700-750-1US13 cable into X1, with the USB end not connected to the PC.
4. Measure again on the same pin: the current should rise to approximately 20 mA. If it stays at zero, the cable's TTY stage is dead.
5. Repeat on pin 14 (TTY receive−) to confirm the return path.

Numbers in Table 6 follow the standard Siemens convention for the S5-90U / 95U PG port; verify against the wiring diagram shipped with the specific CPU variant before making permanent connections.

9. Step-by-Step Resolution Procedure

Run this sequence in order. Stop at the first step that resolves the fault.

1. Confirm the cable is USB-to-TTY, not USB-to-RS-232 only. Look for the converter box with two optocouplers and a current-source transistor. If the cable is a flat USB-to-DB9 adapter, it will never work — replace it.
2. Install or re-install the driver. Download the latest vendor driver (FTDI VCP 2.12.36+ or Prolific PL-2303 v1.9.0+). Force the COM number to COM1 in advanced settings.
3. Verify the COM port. Open PuTTY (or Tera Term) on the assigned COMx at 9600 / 8E1. With the cable unplugged from the PLC, the receive window is silent. Plug the cable into the powered S5-95U: the PLC's AS511 polling frame should appear every 250–500 ms. If nothing appears, the cable's TTY stage is dead.
4. Set STEP 5 to AS511 on the same COMx. Restart STEP 5 after the change.
5. Go online. Use PLC > Online > Connect or press F4. The "ONLINE" indicator should turn green within 2 s.
6. If still failing, swap the cable for a known-good Siemens 6ES5 734-1BD20 (RS-232-to-TTY) plus a generic USB-to-RS-232 adapter. This isolates the third-party cable as the fault source.

10. Cable Alternatives and Pinout Reference

If the 700-750-1US13 cable proves defective, two alternative paths exist:

10.1 Siemens original RS-232-to-TTY cable + USB-to-RS-232 adapter

Siemens part 6ES5 734-1BD20 (or 6ES5 734-1BF20 for the 95U/100U family) is a passive RS-232 ↔ TTY converter in a 9-pin sub-D shell, with a captive 15-pin lead. Pair it with an industrial-grade USB-to-RS-232 adapter based on FTDI (e.g. Weidmüller IE-US-20-S or ATEN UC232A). This is the most diagnostic-friendly combination because the RS-232 side can be observed with a serial line analyzer.

10.2 Third-party dedicated USB-to-AS511 cables

Several European vendors (e.g. CDe Elektronik, Process-Informatik, IBHsoftec) sell USB-to-AS511 cables that emulate COM1 and run natively on Windows 10/11. These bypass the manual driver-tweaking described in Section 7 but are typically 4–6× the price of the 700-750-1US13 cable.

11. Verification and Online Status Checks

Once the link is established, run the following three verifications to confirm the connection is healthy before trusting any block upload/download:

1. Read the CPU identity. In STEP 5: PLC > Online > Module Information. The returned PLC type must be CPU 95U. If STEP 5 reports an unknown PLC, the link is up but the protocol is mis-negotiated.
2. Read the status word. In STEP 5: PLC > Online > Status. The status word should display the CPU's actual state (RUN / STOP / error). Reading without error confirms both directions of the TTY loop are functional.
3. Read a known DB. Pick a data block that already exists in the project (e.g. DB 10) and read it with PLC > Read from PLC. The values should match what the program logic dictates. A read of all-zero or all-0xFF data usually indicates a Tx/Rx swap on the TTY loop.

12. Troubleshooting Matrix

The matrix below maps the user-visible error to the diagnostic action and the fix. Use it as a field checklist.

13. Common Engineering Pitfalls

• Assuming a USB-to-serial adapter is the same as a USB-to-TTY adapter. They are not interchangeable on the S5-95U. The TTY stage is mandatory.
• Trusting the "USB Serial Port (COMx)" entry in Device Manager as proof of a working link. The driver layer is independent of the TTY hardware. The cable can enumerate perfectly and still have a dead current loop.
• Forgetting the STEP 5 restart after changing the PG interface. STEP 5 reads the assignment once at start-up. Changing it in Options > PG Interface mid-session does nothing.
• Using a USB hub between the PC and the cable. Some hubs introduce 16 ms latency that breaks the 250 ms AS511 polling cycle. Plug the cable directly into the PC.
• Running STEP 5 v7.2 on Windows 10/11 without "Run as Administrator". Writes to the registry for the PG interface fail silently.
• Leaving the PLC in RUN while troubleshooting. Stop the PLC first; AS511 in RUN mode is more sensitive to timing because the CPU is busy with the OB1 cycle.

14. Documentation References for the Field Engineer

The official Siemens S5-90U / S5-95U manual set and the STEP 5 v7.x programming manual are available in the legacy product section of the Siemens Industry Online Support portal. The S5-95U manual covers the X1 pinout, current-loop specifications, and the recommended PG cable list. The STEP 5 manual covers the AS511 driver, the PG interface assignment dialog, and the meaning of every error code that the link layer can return.

For the cable vendor, FTDI publishes the application note AN232B-05 on latency timer tuning that explains the 16 ms default in detail, and Prolific publishes a driver compatibility matrix that explains why the older PL-2303HXA chips fall back to low-speed mode on Windows 10/11.

15. FAQ