Siemens S5 AS511 TTY Pin Assignment: 15-Pin to 25-Pin Wiring

Siemens S5 AS511 TTY Pin Assignment: 15-Pin to 25-Pin Adapter Wiring Reference

Connecting a Siemens S5 PLC family CPU (S5-90U, S5-95U, S5-100U, S5-115U, S5-135U, S5-150U) to a non-Siemens HMI or programming device across an AS511 link requires a transition between two common D-Sub connector formats: the 15-pin male Sub-D used on the S5 AG (interface) port and the 25-pin male Sub-D found on most legacy OP/text-panel devices, printers, and serial terminals. Because AS511 rides on top of a TTY/20 mA current loop physical layer rather than RS-232, the wiring is fundamentally different from a V.24 null-modem adaptation. This reference covers the protocol, both connector pinouts, the official Siemens adapter cable (6XV1440-2DE32), and the field rules for cable length and the 50-meter distance that operators routinely ask about.

1. AS511 Protocol and Physical Layer

AS511 is the Siemens proprietary point-to-point protocol used for online programming, OP and TD (text display) communication, and diagnostic services on the S5 family. It is functionally positioned between the application layer and the electrical/physical layer, and it is typically transported over:

• TTY (20 mA current loop) — the original and most common S5 port setting. Passive or active current source.
• V.24 (RS-232-C) — used where the HMI or programming device only provides RS-232.
• V.11 (RS-422-A) — used in some network configurations via the CP 521/523 modules.

For the 15-pin S5 AG port on most CPU/CP modules, AS511 over TTY is the default. The default line speed at commissioning is 9600 baud, with selectable options from 300 up to 19200 baud on modules with newer firmware.

2. Siemens S5 15-Pin AG (IF) Connector Pinout

The 15-pin male Sub-D connector on S5-115U / S5-135U / S5-150U CPU front panels and on most CP modules conforms to the Siemens AG-interface convention. The layout documented in the S5-135U manual (at the back of the binder, per the field source) and reused across S5-115U/150U is as follows. Pin numbering looks at the solder/cable side of the male connector:

Active vs. passive TTY on the S5 port

The S5 15-pin port can be wired in either:

• Active mode — S5 port supplies the 20 mA loop current on both transmit and receive pairs (pins 2/3 and 4/5).
• Passive mode — Far-end device supplies the 20 mA loop current; the S5 port merely switches the loop open/closed.

The Siemens selector configuration in STEP5 (under Configure PG/OP Interface) toggles a hardware strap or firmware bit that controls whether pins 2 and 4 are current sources or current sinks. A mismatch on a custom cable is one of the most common reasons an AS511 link fails to establish.

3. Standard 25-Pin D-Sub TTY / 20 mA Current Loop Pinout

The 25-pin DB-25 connector is the historical standard for serial terminals, printers, and many OP/text-panel devices. Most 25-pin serial devices follow either TIA-232 (V.24) or 20 mA current loop assignments. For TTY/20 mA current loop on DB-25, the historical convention (compatible with DEC, IBM, and Siemens OP-25/OP-35/OP-45 cables) is:

The exact pin assignments of the 25-pin connector at the non-Siemens end are determined by the manufacturer's pinout. Some OPs (notably Siemens OP-25 series) use a TTY-on-25-pin layout where pin 9 acts as a mode strap. A reliable build cannot proceed without the device's vendor pinout sheet.

General RS-232/TTY 9-pin to 25-pin reference

Where the non-Siemens HMI exposes only 9-pin RS-232 and you must convert through a 25-pin shell, the generic RS-232/423 mapping in the Oracle hardware connector reference applies. See the 9-pin to 25-pin adapter reference in the Oracle hardware documentation. Converting a TTY (15-pin Siemens S5) directly to a 9-pin RS-232 device is not possible with passive wires; an active V.24 ↔ TTY converter is required.

4. Siemens 6XV1440-2DE32 Reference Cable

Siemens publishes the formal connection diagram for the adapter cable used to interface S5 AG ports to V.24/TTY 25-pin D-sub systems. From the Siemens industry support portal entry "What is the pin assignment for the adapter cable 6XV1440-2DE32?":

• Connector 1: 15-pin Sub-D male (V.24 / TTY active)
• Connector 2: 25-pin Sub-D male (V.24 / TTY)
• Function: Cross-couples Siemens S5 TTY-AG port to a 25-pin V.24/TTY host while maintaining the TTY current-loop polarity and providing V.24 fallback through the same conductors.
• Mode: "Active" — meaning one side (S5 AG) sources the 20 mA loop current.

The official Siemens designation 6XV1440-2DE32 puts this cable in the 6XV1440 series (Siemens SIMATIC NET serial/peripheral cables). It is the closest off-the-shelf hardware equivalent to the custom 15-to-25 adapter that is often needed when marrying an S5 CPU to a non-Siemens HMI port.

Signal translation table derived from 6XV1440-2DE32 logic

The following mapping describes the logical cross-connection that the 6XV1440-2DE32 implements. Use it as a baseline for wiring any custom 15-pin to 25-pin adapter cable for AS511/TTY interconnections:

5. Building a Custom 15-Pin to 25-Pin TTY Adapter

When the 6XV1440-2DE32 cable or its current successor is not in stock, the conversion can be done with a passive adapter housing plus a short pigtail cable, provided the HMI side is TTY-capable. Use this procedure when you have confirmed the HMI is 20 mA current-loop on its 25-pin port.

Prerequisites

• Siemens S5-115U/135U/150U CPU (or compatible CP module) with functional AG port set to TTY active.
• Non-Siemens HMI with 20 mA current loop 25-pin connector. Pin-out sheet from the HMI vendor.
• DE9 / DA15 / DA25 D-sub housings and hoods, shielded.
• Twisted-pair cable, 2 pairs + shield minimum (e.g., LiYCY 2×2×0.34 mm²).
• Termination resistors only if line exceeds the standard TTY guidance for that baud rate (see §7).
• Constant-current source if HMI is passive-only (rare on HMIs).

Step-by-step wiring

1. Confirm both endpoints are in TTY active or matching active/passive mode. Document which side sources the 20 mA.
2. Identify HMI 25-pin TTY pins. For most third-party current-loop terminals, this is pins 4/5 for transmit and 17/20 for receive (per the historical convention in §3).
3. Wire the S5 15-pin transmit pair (pins 2 and 3) to the HMI 25-pin receive pins. Maintain polarity: pin 3 of S5 to current-source side of HMI Rx, pin 2 of S5 to current-return side of HMI Rx.
4. Wire the S5 15-pin receive pair (pins 4 and 5) to the HMI 25-pin transmit pins. Pin 4 to current-source side of HMI Tx, pin 5 to return.
5. Connect shields on pin 1 of the 15-pin connector to pin 1 of the 25-pin connector. Tie the shield to chassis ground at one end only — usually the S5 CPU side, since the S5-115U/135U backplane provides the reference.
6. Do NOT connect signal ground (pin 8 on the 15-pin S5) to any of the TTY conductors. Signal ground on TTY loops introduces ground loops and can defeat the noise rejection that makes 20 mA current loop useful in industrial environments.
7. Add a 0.1 µF ceramic across each pair at each connector end if the line exceeds 50 m, to suppress switching transients. Do not add large electrolytic capacitors; they will round off the loop edges.
8. Double-check that pin 9 / pin 15 mode strap conventions are honored if the HMI expects them (older Siemens OP family behavior, see §2).

6. Cable Type, Shielding, and Termination

For AS511/TTY interconnections on an industrial floor, the cable must be selected with the electrical environment in mind. 50 m in a cable tray with VFD power wiring inside is not the same as 50 m in an enclosed control cabinet.

For the 15-pin/25-pin transitions specifically, use metalized-metal hoods on both connectors. Plastic hoods with metalized paint look the same but break shielding continuity. The cable shield must clamp 360° at the hood entry.

7. Maximum Cable Length and Do We Need a Repeater?

The single most common field question on S5 AS511 is whether 50 m needs a repeater. The answer is no — and the underlying physics of TTY/20 mA current loop explains why.

The widely cited industry reference for 20 mA current-loop performance is the epanorama serial interface summary, which states:

The 20 mA current loop is suitable for distances to 2000 feet (650 meters) at data rates up to 19.2k baud with careful attention to interface design. It can be used at longer distances when data rates are as low as 300 baud.

For a 50 m AS511 link at 9600 baud on a well-installed cable, the calculation is straightforward:

Loop headroom calculation

The Siemens 20 mA transmitter IC powering the loop typically produces a compliance voltage of 12 to 24 V across the pair. Available headroom after cable and receiver dropouts:

V_headroom = V_compliance − (2 × V_{receiver_diode}) − V_{transmitter_sense} − I × R_{cable_total}

For two-conductor 0.34 mm² cable, R_{per_m} ≈ 0.06 Ω/m; at 50 m round-trip path of ~100 m, R_{cable_total} ≈ 6 Ω. With 0.02 A loop current, the IR drop is 0.12 V — negligible vs. the 12 V minimum compliance. The 50 m run is therefore well inside the compliance window.

Using the empirical rule of 650 m at 19.2 kbaud (e.g., 19,200 baud), the conservative derating for AS511 nominal 9600 baud is approximately 1.0 km; the 50 m distance is at less than 5% of the documented design ceiling. No repeater is needed.

8. AS511 Specific Baud Rate Constraints

AS511 does not support flow control in the same way as RS-232 modems; the protocol uses fixed frames and short acknowledgments. At longer distances or with marginal cables, lower baud improves link stability because TTY is a relatively slow-edged current source.

For the 50 m case in question, running at 9,600 baud is a reasonable default. If the HMI cannot reach full AS511 throughput — for instance, a text-only OP running screen updates — 4,800 baud is a safe and well-supported fallback.

9. Commissioning and Verification

Once the cable is built and the HMI is connected, perform these steps to verify the 15-to-25 AS511 TTY link is healthy:

1. Verify cable type and shield termination. Use an insulation tester between the shield and pin 1 at the far end — should show only chassis-end continuity.
2. Measure the loop current. With PLC in AG mode, place a milliammeter (or clamp-on with mA range) in series with each TTY pair. Expected reading: 18–22 mA.
3. Confirm mode strapping. From STEP5, navigate Configure > PG/OP Interface > AG-Select and verify the port is set to "TTY active" for that CPU module.
4. Loopback test at near end. Disconnect from the field 25-pin HMI and short pins 2–3 to pins 4–5 on the 15-pin connector at the CPU; run a STEP5 PG connection — if it establishes, the S5 side is good.
5. Loopback at the far end. On the HMI side (after successful near-end loopback), short its Tx and Rx pairs together (whip-out-bypass test). Try to bring up the link.
6. Launch STEP5 / COM AS511 link. From the programming device, initiate Online > Connect to AG. Successful AG connect confirms the full link.
7. Check OP screen refresh or TD text panel. If the HMI is an OP or TD, an OP-level handshake will be visible in the OP diagnostics, plus the S5 CPU RUN/STOP LED should not be flashing a communication fault.

Acceptance criteria

• Loop current measured between 18–22 mA on both transmit and receive pairs.
• STEP5 PG connection succeeds within 5 seconds of attempting.
• No cyclic retries logged in the S5 diagnostic buffer relating to AG port.
• HMI messages received within 500 ms of operator action at default 9,600 baud.

10. Troubleshooting Matrix for AS511 TTY Link Failures

The table below maps the most common failure modes observed when interposing a 15-to-25 adapter or a 6XV1440-2DE32-class cable between an S5 CPU and a non-Siemens HMI.

11. When to Migrate Away from AS511/TTY

While the 15-to-25 TTY adapter works well for legacy retrofits, AS511/TTY should be considered legacy at this point. Migration considerations:

• Firmware: STEP5 ≤ 6.x supports the S5 family officially; the AG port firmware has been frozen at legacy features since the S5-150U EOL. There's no security patch path.
• Vendor support: S5 was officially discontinued at the end of production; spare availability is the only supply path.
• HMI migration: Newer HMI panels typically use MPI/PROFIBUS or PROFINET, not AS511. A gateway device (e.g., Helmholz or Siemens NetLink) converts AS511/TYY to Ethernet on installed S5 systems as a stopgap.
• EMC environment: Modern variable-frequency drives produce RF noise that can cause intermittent TTY errors at distances above ~30 m. Migrating to a 24 V RS-485 differential or fiber-converted system is more robust in that environment.

For new installations on existing S5 hardware, the 6XV1440-2DE32 or its successor combined with a current Siemens-class gateway is the recommended path. For pure connectivity questions like Baron's, this reference covers the 15-to-25 problem end-to-end.