Sinumerik 840C PLC Interface: STEP5 RS232 Connection Guide

Sinumerik 840C PLC Interface to PC with STEP5 V7.23 over RS232

The Sinumerik 840C is a CNC controller whose integrated PLC is a derivative of the Simatic S5 family. The CPU mounted inside the 840C is functionally identical to a standard S5 PLC (commonly the CPU 130W or CPU 150 family), with the addition of NC-specific data blocks and I/O mappings. Because the PLC portion of the 840C is a stock Simatic S5, it can be programmed, monitored, and diagnosed with the original Siemens STEP5 package — no NC-specific offline editor is required for pure PLC work.

Online communication between a PC and the 840C PLC is performed over the AS511 protocol carried on a serial physical layer. This guide covers hardware selection, the correct 25-pin cable pinout, STEP5 V7.23 commissioning, and the recovery of the common "Undefined Character from AS" fault reported when the wrong converter or the wrong pinout is in use.

1. System Overview and Architecture

The 840C control consists of three logical subsystems:

• NC kernel (residing in the NC-CPU).
• PLC subsystem (residing on the S5 CPU board, slot-based I/O backplane).
• HMI / MMC panel.

The PLC subsystem communicates with the NC kernel over a dual-port RAM. For external engineering access, the PLC side exposes a serial interface on the back of the 840C housing. That interface is wired as RS232 (V.24) on a 25-pin D-sub female connector, not as a 20 mA TTY current loop. This is the single most important fact when troubleshooting connectivity: a TTY converter is not required for the 840C PLC port — the port is already a true RS232 (V.24) interface.

2. Prerequisites

3. Hardware Identification — Locating the PLC Port on the 840C

The PLC programming interface is a 25-pin female D-sub connector located on the rear of the 840C chassis, distinct from the operator panel connector and the NC service port. In most installations it is labelled X1 or X121 on the backplane silk-screen. The pin numbering is the standard TIA-574 / ISO 2110 layout:

4. Cable Wiring (25-pin 840C ↔ 9-pin PC)

The cable is a straight-through 3-wire RS232 link with hardware handshaking loops. The PC's DTR/DSR and RTS/CTS pairs are looped back at the PC end to satisfy the AS511 driver's expectation of a fully wired modem interface.

4.1 Recommended 25-pin (840C) to 9-pin (PC) pinout

Loops at the PC end (solder bridges inside the 9-pin hood):

• Pin 4 (DTR) ↔ Pin 6 (DSR) — local handshake loop.
• Pin 7 (RTS) ↔ Pin 8 (CTS) — local handshake loop.

4.2 25-pin (840C) ↔ 25-pin (older PC or Siemens PG) pinout

This is a true crossed-cable layout (TX/RX and RTS/CTS/DSR/DTR swaps). It corresponds to the wiring used by the original Siemens PG685/PG720 programming consoles and works with any PC that has a 25-pin COM port.

5. STEP5 V7.23 COM-Port Configuration

STEP5 reads its serial parameters from S5CONV.S5P or from the Online dialog. The default AS511 parameters for an S5-CPU are fixed; on the PC side you only need to set the baud rate, parity, and COM number.

The baud rate is not adjustable on the 840C PLC side — it is fixed by the CPU firmware. Do not attempt to change it; STEP5 will not negotiate. The PLC sends its 11-bit frame (8E1) at 9600 bit/s on cold connection, and STEP5 auto-synchronises.

6. Step-by-Step Connection Procedure

1. Power down the 840C. Locate the 25-pin PLC port on the rear of the unit (typically labelled X1 / X121).
2. Verify the connector is 25-pin female on the 840C side. If a 15-pin female is present, that is the NC service port, not the PLC port.
3. Connect the cable described in Section 4 between the 840C and the PC COM port. Do not insert the RS232-to-TTY (15-pin) converter; the 840C PLC port is V.24.
4. Power up the 840C. Wait for the PLC to complete its cold-restart sequence (the "Run" LED on the PLC-CPU card stays solid when ready).
5. Boot the PC into Windows XP, log in as Administrator.
6. Verify the COM port in Control Panel → System → Device Manager → Ports (COM & LPT). Note the COM number.
7. Start STEP5 V7.23.
8. Select Project → Set → Interface (or the equivalent in your localized STEP5 build). Choose AS511 and the COM port number from step 6. Leave baud at 9600, parity Even, 8 data, 1 stop.
9. Select Online → Connect to PLC (or press the corresponding function key).
10. STEP5 sends the AS511 init telegram; the PLC replies with its system message including the CPU type, firmware version, and total I/O count.
11. If the handshake is successful, STEP5 displays the connected CPU in the status bar and the online blocks can be browsed.

7. Troubleshooting: "Undefined Character from AS"

The error "Undefined Character from AS" (or its localized equivalent) indicates that the PC received one or more bytes from the PLC's serial line that did not match the AS511 frame format. The fault is almost always at the physical layer; PLC firmware and STEP5 code are correct by construction.

8. Why the TTY Converter Fails on the 840C

Many S5 CPUs of the same generation expose a 15-pin TTY (20 mA current loop) connector on their programming port. To connect them to a PC, an active or passive RS232-to-TTY converter is required. The TTY converter inserts an opto-isolated current loop driver between the PC and the S5.

The Sinumerik 840C breaks this rule. Its PLC port was engineered as a V.24 (RS232 level) port specifically so that an engineering laptop with a native COM port could be plugged in without converter hardware. When the user inserts a TTY converter between the 840C and the PC, two issues arise:

1. The TTY converter expects to source the 20 mA current loop. The 840C's RS232 driver does not generate a current loop — it drives ±12 V levels. The converter therefore idles with no signal, and any noise on the line is interpreted as an undefined character by the STEP5 AS511 driver.
2. The handshake pairs on a TTY converter (pin 4 RTS / pin 20 DTR on the 25-pin side) are usually looped. But the absence of a clean carrier from the 840C means STEP5 never receives the AS511 init response, and the driver eventually times out and reports the undefined-character error.

Conclusion: bypass the 15-pin TTY converter entirely on the 840C.

9. Verification Checklist

After a successful online connection, the following items should be confirmed before starting any online editing or download:

• STEP5 status bar shows "Online – CPU 130W" (or the actual CPU type string returned by the PLC).
• Block list populated: at least the OB1, OB21, OB22, FB/FC/DX/DB directories appear.
• Force table accessible: Online → Force/Set returns a "VAR" header without error.
• Diagnostic buffer readable: Online → Module Information → Diagnostic Buffer lists the most recent restart events.
• Scan time visible: Online → Module Information → Scan Cycle Time returns a numeric value in ms.

10. Common Edge Cases and Field Notes

ESD / Backplane Lock-up: Connecting a serial cable to a powered 840C occasionally stalls the PLC scan if the PC ground and the cabinet ground differ by more than a few volts. If the PLC goes to STOP immediately after plugging the cable, lift the cable shield bond at the PC end and re-test.

NC Key Switch in the Wrong Position: Some 840C builds lock the PLC programming port when the NC key switch is in position 0. Turn the key to position 1 or higher before attempting online access.

STEP5 Project Mismatch: STEP5 V7.23 will refuse to write blocks whose number is already in the offline project but does not match the online content. The error is not "Undefined Character" but rather a block-comparison warning. Use Online → Block Comparison to identify differences before downloading.

Hot Plug During Cold Restart: If the PLC is mid cold-restart (the S5 "OB21 / OB22" phase) when the PC connects, the AS511 driver may issue a connection timeout. Wait 30–60 s after the "Run" LED is solid before starting STEP5 online.

Multiple COM Ports on Laptops: Many business laptops have COM1 disabled in the BIOS to free IRQ 4 for an IR port. Enable the COM port in BIOS before expecting STEP5 to find it.

11. Quick Reference