Problem Context: Restoring Serial I/O on a Sinumerik System 3 CNC
The Sinumerik System 3 (sold by Siemens from the early 1980s through the early 1990s) used a hybrid architecture: an NC channel processor for motion and interpolation, plus a SIMATIC S5 PLC for I/O, M-codes, H-codes, and auxiliary functions. Programs were typically loaded through a paper tape reader (typically a Facit or Siemens-type punch reader) connected to the NC CPU's V.24 port labeled X103.
On a Chiron FZ16 high-speed machining center equipped with a Sinumerik 3 controller, the original tape reader may be present but non-functional, leaving the user with no path to load part programs. Field technicians frequently attempt to drop in an SIMATIC S5-115U CPU 511 (6ES5 511-5AA12) believing it adds a serial interface. It does not. The 511 is the PLC's central processor. The path forward lies in using the existing X103 V.24 port and confirming that the supporting PLC function blocks are loaded.
Module Identification: Decoding the Rack Population
Before any interface work, every module in the Sinumerik 3 subrack must be positively identified. The catalog numbers printed on the front labels follow Siemens' 6FX1xxx scheme for CNC-specific modules and 6ES5xxx scheme for S5 PLC modules. The order observed in this machine is documented in the table below.
| Slot | Catalog Number | Function Family | Role in System 3 |
|---|---|---|---|
| 1 | 6EV3054-0DC | Power Supply | 24 V DC / 5 V DC backplane converter for NC/PLC rack |
| 2 | — vacant — | — | Reserved slot |
| 3 | 6FX1117-8AA01 | Memory Expansion | Subroutine / UMS memory extension (bubble or RAM) |
| 4 | 6FX1118-1AA01 | NC CPU | Channel processor; front-panel X103 carries the V.24 (RS-232-C) port |
| 5 | 6FX1190-1AG00 | Operator Panel Interface | Coupling module between NC and operator panel (MDB, MDP buses) |
| 6 | 6FX1125-1AA01 | Position / Spindle Module | Positioning control and spindle management |
| 7 | 6FX1123-7AA01 | Feed Axis Module | Axis command value output, servo enable, follow-up |
| 8 | 6FX1115-0AA01 | PLC Interface / Serial Coupling | Couples PLC to NC, includes 20 mA current-loop I/O |
| 9 | — vacant — | — | Reserved for expansion |
| 10 | 6FX1191-0AG00 | Communication / Coupling | NC-to-PLC parallel coupling module |
| 11 | 6FX1118-4AA01 (x3) | PLC I/O Modules | Digital inputs/outputs for the S5-115U PLC |
The 511 Misconception: CPU vs. Serial Interface
The module in question, 6ES5 511-5AA12, is the central processing unit of an SIMATIC S5-115U PLC. Its block diagram is built around a SAB 8085 processor, an EPROM/RAM sub-system, and the standard S5 backplane bus driver. It exposes no serial port on the front panel and it cannot convert the NC's internal parallel coupling to RS-232 or 20 mA current loop.
| Parameter | 6ES5 511-5AA12 | 6FX1115-0AA01 (if fitted) |
|---|---|---|
| Family | SIMATIC S5-115U | SINUMERIK 3 NC coupling |
| Function | PLC CPU (Step 5) | NC-PLC coupling with 20 mA interface |
| Front-panel serial port | None (programming via PG via S5 bus) | 25-pin female X103 V.24 / 20 mA |
| Programming port | 15-pin PG connector on CPU front | n/a |
| Required for tape reader? | No | Yes (typical configuration) |
The "115 card" referenced in informal documentation is the 6FX1115 family — the 6FX1115-0AA01 is already present in this rack. This module is the correct place where the 20 mA current-loop / V.24 conversion occurs on a System 3. Slotting the 511 CPU in addition will not produce a new serial port; it will simply duplicate the existing PLC processor role and potentially conflict on the S5 backplane.
The X103 V.24 Connector on the NC CPU
The NC channel processor (6FX1118-1AA01 in this rack) provides the X103 connector — a 25-pin female sub-D receptacle wired to V.24 (RS-232-C) levels. This is the same physical connector that the through-panel bulkhead at the tape-reader location is wired to. The bulkhead is typically a 25-pin male sub-D on the cubicle wall, with the cable routed through the cabinet wall to a 25-pin female on the reader.
Because Sinumerik 3 documentation is rarely bundled with fielded machines after three decades of service, the X103 pinout is best cross-referenced against:
- Siemens Industry Online Support (SIOS) — legacy SINUMERIK section
- The SINUMERIK 3 installation and start-up guide (Siemens ordering number 6FX1191-1AA series)
- The "SINUMERIK System 3 — Serial Interface for Data In/Output" application sheet (System-38 reference)
The functional assignments for X103 V.24 follow CCITT V.24 / EIA RS-232-C. Confirmed signal roles on SINUMERIK System 3 NC CPUs are summarized below.
| Pin (25-pin D-sub) | CCITT V.24 Circuit | Direction (DTE = NC) | Signal |
|---|---|---|---|
| 1 | 101 | — | Protective / chassis ground (PGND) |
| 2 | 103 | Output | TXD — Transmitted Data |
| 3 | 104 | Input | RXD — Received Data |
| 4 | 105 | Output | RTS — Request To Send |
| 5 | 106 | Input | CTS — Clear To Send |
| 6 | 107 | Input | DSR — Data Set Ready |
| 7 | 102 | — | Signal ground (SGND) |
| 8 | 109 | Input | DCD — Data Carrier Detect |
| 15 | 114 | Input | Transmit clock (when configured for synchronous mode) |
| 17 | 115 | Input | Receive clock (when configured for synchronous mode) |
| 20 | 108/2 | Output | DTR — Data Terminal Ready |
| 22 | 125 | Input | Ring Indicator (rarely used) |
| 25 | — | — | Reserved / unused on System 3 NC |
Three-Conductor Minimum Cable to a Modern PC
A modern PC's COM port (DE-9 male) only needs three signals to exchange data with a System 3 in asynchronous, no-handshake mode:
- NC TXD (X103 pin 2) → PC RXD (DE-9 pin 2)
- NC RXD (X103 pin 3) → PC TXD (DE-9 pin 3)
- Signal ground (X103 pin 7) → PC SGND (DE-9 pin 5)
Recommended wiring for an X103 to DE-9 adapter is given in the table below. Pins not listed are not connected.
| Function | X103 (25-pin female at NC) | PC DE-9 (male) |
|---|---|---|
| TXD → RXD | 2 | 2 |
| RXD ← TXD | 3 | 3 |
| RTS → CTS (loopback, optional) | 4 | 7 |
| CTS ← RTS (loopback, optional) | 5 | 8 |
| DSR ← DTR (loopback, optional) | 6 | 4 |
| SGND | 7 | 5 |
| DCD ← DTR (optional) | 8 | 4 |
| DTR → DSR (optional) | 20 | 6 |
| PGND (chassis) | 1 | Shell |
For a null-modem style 25-pin-to-9-pin cable intended for the through-panel bulkhead (where the bulkhead mirrors the X103 pin-for-pin), use a standard 25-pin-to-DE-9 converter wired straight-through on pins 2/3/7 and leave the control lines floating. Hardware handshaking is not required because the System 3 NC CPU drives TXD only while the tape reader is selected and active.
20 mA Current Loop vs. V.24 — Why It Matters
The 6FX1115-0AA01 coupling module exposes both a V.24 (RS-232-C voltage) interface and a 20 mA current loop. The two are mutually exclusive on a given pin pair. The machine's reader is wired for current loop if the reader itself uses passive optical or mechanical sensors that switch a current source; it is wired for V.24 if the reader contains active RS-232 drivers.
To choose the correct mode, measure the reader's transmit pair with the reader powered but idle:
- If you read ~5–12 V DC across the pair, it is V.24 (RS-232-C).
- If you read near 0 V DC with a current source present (~20 mA flowing when the LED is on), it is a 20 mA current loop.
For a current-loop reader driving into a PC, an opto-isolated active converter (e.g., a current loop to RS-232 converter module) is required. A passive resistor network is not sufficient because the System 3 NC side does not source loop current in V.24 mode.
PLC Software Requirement — Function Blocks
Hardware presence is necessary but not sufficient. The Sinumerik 3 firmware on the NC CPU only enables the V.24 data-channel handshake when the S5-115U PLC carries the appropriate function blocks (FBs) for tape / DNC handling. The relevant blocks are part of the SINUMERIK System 3 toolbox and include:
| Block | Typical Name | Function |
|---|---|---|
| FB 38 | ANR / Tape Handler | Tape reader control, character-by-character handshake |
| FB 39 | DNC / Send | RS-232 output of NC data |
| FB 40 | DNC / Receive | RS-232 input of NC data |
| Data Block | ||
| DB 38 | Tape parameters | Baud rate, parity, stop bits, XON/XOFF or hardware handshake |
| DB 39 | Status / buffer | Runtime status of tape channel |
If these blocks were lost during a battery failure or EPROM swap, the V.24 port will remain electrically present but functionally inert. Verify with a STEP 5 programming device (PG 675, PG 685, PG 710, or modern USB-to-S5 adapter) by listing blocks in the AG (automation device) memory map.
Configuration Parameters Stored in DB 38
DB 38 holds the tape channel configuration. The default System 3 values are listed below. Change them only with an authorized start-up tool.
| Offset (DW) | Parameter | Typical Value | Notes |
|---|---|---|---|
| DW 0 | Baud rate code | KH 0007 (= 9600 baud) | Codes: 0004=1200, 0005=2400, 0006=4800, 0007=9600 |
| DW 1 | Parity / framing | KH 0000 (= 8N1) | 8 data bits, no parity, 1 stop bit is the System 3 default |
| DW 2 | Handshake mode | KH 0001 (XON/XOFF) | 0=none, 1=XON/XOFF, 2=RTS/CTS |
| DW 3 | End-of-block character | KH 000A (LF) | Defaults to ISO 840 NC dialect |
| DW 4 | Timeout (s) | KF +30 | Reader must deliver a char within 30 s |
Commissioning Procedure
- Power down the NC and PLC. Confirm all DC bus voltages are at zero before opening the cubicle.
- Inspect the X103 connector at the NC CPU front panel for bent pins, oxidation, or a missing strain-relief screw. Clean with isopropyl alcohol and a non-abrasive swab.
- Ring out the harness from X103 to the through-panel bulkhead. Verify continuity on pins 2, 3, and 7 (the minimum data path). Verify no shorts between adjacent pins.
- Identify whether the existing reader is V.24 or 20 mA current loop using the voltage test above.
- Fabricate the appropriate cable per the table in §"Three-Conductor Minimum Cable to a Modern PC".
- Connect a PC running a terminal program (e.g., Tera Term, PuTTY, or Siemens' legacy PROG_ID tool) to COM1. Set 9600 baud, 8N1, XON/XOFF flow control.
- Apply power. Enter the System 3 Data In mode (key sequence typically
MENU → DATA IN/OUT → TAPE; verify against operator panel label). - Send a short test program containing only a safe motion (e.g.,
G91 G1 X0.001 F100) and confirm the NC reads it without a parity or framing alarm. - Once tape read works, verify tape punch (output) by sending a tool list to the PC and checking the character stream against expected ISO 840 output.
Verification Matrix
| Test | Expected Result | Pass Criterion |
|---|---|---|
| PC receives echo of test string | Full string, 8-bit-clean | No replacement of high-bit (>0x7F) chars |
| NC acknowledges LF | Cursor advances to next block | No "tape format error" alarm |
| NC processes G-code line | Block is stored in part-program buffer | Block number increments |
| Bidirectional: NC → PC | NC outputs tool list on operator-panel request | PC receives ISO 840 stream at 9600 baud |
| Failure Mode | Probable Cause | Remedy |
| Garbage characters | Baud mismatch or 7-bit strip | Set 9600 8N1 on PC; check reader dip switches |
| No data, LEDs dark | Open ground or missing handshake blocks | Ring out SGND; reload FB 38/39/40 |
| Reader clicks but no input | V.24 / current loop mode mismatch | Swap reader interface card or install converter |
| Partial blocks then timeout | XON/XOFF disabled but reader stops on buffer-full | Enable software flow control in DB 38 DW 2 |
When the PLC CPU 511 Has a Role
The 6ES5 511-5AA12 is the right CPU to retrofit if the S5-115U section of the Sinumerik 3 has suffered a CPU failure and the original is unobtainable. The replacement procedure is:
- Transfer the EPROM/RAM submodules from the failed CPU to the replacement, matching the physical orientation.
- Set the S5 backplane address jumpers identically. On a System 3 the S5 is usually addressed as rack 0, slot 0 (PG can confirm with "STATUS AG").
- Power up cold. Use the PG to verify the AG cycle is running and FB 38/39/40 are present.
Note that other 115U CPU variants (e.g., 6ES5 941, 6ES5 942, 6ES5 943, 6ES5 944, 6ES5 945) carry different memory capacities and execution speeds. Cross-reference with the SINUMERIK 3 release notes before swapping a different CPU type.
Alternative Path: Bypassing the Tape Reader with a USB → RS-232 Adapter
If the goal is to feed programs from a modern PC without preserving the paper-tape reader, the simplest path is:
- Use a USB-to-RS-232 adapter with the FTDI FT232 or equivalent chipset (avoid clones with poor slew-rate control).
- Add a 25-pin-to-DE-9 adapter wired per §"Three-Conductor Minimum Cable to a Modern PC".
- Run a terminal emulator set to 9600 8N1 with software flow control.
- Send a small part program first to confirm the NC's tape channel is alive; once it is, save the program to a one-click send file for production use.
Long-Term Recommendations
- Document every wire on the through-panel bulkhead against X103 before disconnecting anything. Take photographs of the existing reader wiring with the harness still attached.
- Archive a STEP 5 listing of FB 38/39/40, DB 38, and the system data blocks to a modern media before the S5 battery is replaced. Lithium batteries have a 10–15 year service life and the memory is volatile.
- Verify the SINUMERIK System 3 firmware EPROMs are socketed (most are) and label them with the checksum and version before any swap.
- Maintain the original paper-tape reader as a tertiary fallback. Modern systems degrade; mechanical readers do not.
FAQ
Will a SIMATIC S5 CPU 511 (6ES5 511-5AA12) add an RS-232 port to my Sinumerik System 3?
No. The 6ES5 511-5AA12 is the central processing unit of an S5-115U PLC. It has no serial interface function for the NC. The serial port you need already exists at the NC CPU's X103 connector (V.24, 25-pin D-sub) and is conditioned by the 6FX1115-0AA01 coupling module.
What is the X103 connector on the Sinumerik System 3 NC CPU?
X103 is the V.24 (RS-232-C) 25-pin female D-sub on the front of the NC channel processor (6FX1118-1AA01 family). It carries the tape reader interface and is the same physical connector pinned out through the cabinet bulkhead.
Which pins carry data on X103?
Minimum three-conductor operation uses X103 pin 2 (TXD), pin 3 (RXD), and pin 7 (signal ground). For full hardware handshake add pins 4/5 (RTS/CTS) and 6/20 (DSR/DTR). Pin 1 is chassis ground and should be bonded to the cable shield.
Why does the tape reader fail to load programs even though the cable is good?
Most Sinumerik 3 tape channels require function blocks FB 38, FB 39, and FB 40 plus data block DB 38 to be loaded in the S5-115U PLC. After a battery or EPROM loss these blocks are gone and the port becomes electrically present but functionally dead. Reload from a STEP 5 archive using a Siemens PG.
What baud rate and framing does Sinumerik System 3 expect?
Default is 9600 baud, 8 data bits, no parity, 1 stop bit (8N1) with XON/XOFF software flow control. Lower baud rates (1200, 2400, 4800) are supported for legacy readers but may need DB 38 changes and a STEP 5 PG to commit them.