Resolving SINUMERIK 810 CPU 6FX1132-8BB01 Serial Port Failure

Problem Overview

The SINUMERIK 810 CNC control is a late-1980s Siemens architecture still found on thousands of lathes, milling machines, and grinding cells worldwide. The CPU module carrying Siemens part number 6FX1132-8BB01 is the central processor board for the standard (T version) and GA1/GA2 variants, and it hosts the two V.24 (RS-232-C) channels used for data upload, download, and DNC operation. Field reports describe a failure mode in which:

• The control enters Initial Clear after battery discharge and the backup of part programs, tool offsets, and machine data can no longer be loaded back.
• Both COM1 and COM2 channels on the CPU card refuse to communicate with the host PC, even with a known-good cable and the correct PCIN transfer sequence.
• A red status LED on the 6FX1132-8BB01 remains lit while the control fails to complete boot, or the LED is normal but the V.24 drivers are dead.
• Pin-level probing reveals a broken DSR/DTR loop on both serial channels, while RTS/CTS toggles correctly.

Because the card is no longer in serial production and Siemens has moved most 810-series spares into the spare-parts-on-request channel, repairing the existing board is often the only economically viable path.

SINUMERIK 810 Architecture and the 6FX1132-8BB01 CPU

The 810 control is built around a 16-bit multiprocessor design. The CPU card 6FX1132-8BB01 contains:

The two serial channels are routed through small ceramic hybrid modules labeled Siemens V24, which integrate the +12 V/-12 V charge-pump, the 1488 line driver, the 1489 line receiver, and the charge-pump capacitors in a single DIP-style package. When both ports fail simultaneously, the failure is almost always upstream of the hybrids (UART, address decoder, or buffer) rather than in the hybrids themselves.

Serial Port Pinout and Handshaking on SINUMERIK 810

The CPU card exposes two 25-pin D-Sub male connectors (X121 and X122 in most 810 wiring diagrams) wired to the Siemens V.24 standard. The relevant control signals for diagnosing a dead port are:

The 810 control only enables the DTR → DSR local loop when its own UART is initialized and the channel is selected. PCIN observes the four status rows in the lower half of its main window. On a healthy control you read:

DSR  STOP    DTR  <-- DSR  OK    DTR
DTR  OK      DSR

On a control with the symptom described above you read:

DSR  STOP    DTR  -- (no transition)
DTR  OK      DSR

That is, the host PC drives DTR high but the SINUMERIK never echoes it back to its DSR line. RTS/CTS is unaffected. This pattern is the diagnostic fingerprint of a failed UART, dead address-latch, or blown output driver on the 1488 side, and it appears on both ports because both UARTs share the same bus interface and reset line.

Relevant Machine Data for the Serial Interface

Before assuming a hardware failure, verify that the control side of the protocol is configured. The two serial interfaces of SINUMERIK 810 are governed by the following machine data words:

If the battery-backed RAM has been lost (typical after a multi-week power-down), all of the above revert to default. The defaults vary by software version but usually disable port 2 and run port 1 at 9600/8/N/1 with RTS/CTS. To restore operation after a data loss:

1. Enter Initial Clear by holding the appropriate keys at power-up, then load the standard machine data block from the BTSS / disk image supplied with the machine.
2. Edit MD 5015 bit 7 to 1 to expose port 2.
3. Set MD 5019, 5020, 5021, 5022, 5024 to the values that match the PCIN profile on the host PC (9600 8 N 1 RTS/CTS is the de-facto standard for 810 → PC).
4. Save with the SETUP / SAVE soft-key, then perform an NCK reset.

PCIN Transfer Procedure (Verification Baseline)

PCIN is the legacy Siemens transfer utility for 810/820/840 controls. Use the following baseline to confirm a known-good cable and host before declaring the CPU card faulty:

1. Connect the PC's COM port to the SINUMERIK 810 CPU card directly via a 1:1 (no crossover) shielded cable, 3 m or shorter. Pin 7 must be wired through; pins 1, 4, and 6 must be bridged on the PC side (DB9 pins 4-6-1 bridge on a USB-serial adapter).
2. Launch PCIN, set the protocol to FE (Siemens transparent), baud 9600, 8 data bits, no parity, 1 stop bit, RTS/CTS handshake.
3. On the 810 side, navigate to Data In / Data Out → V.24 → select the channel (1 or 2) → Send or Receive.
4. Issue the transfer from the PCIN side. If the four status fields in the PCIN window do not flip as described in the previous section, the handshake loop on the SINUMERIK side is broken.

Try a second PC, a different USB-to-serial adapter with FTDI chipset (avoid prolific clones on legacy software), and a second cable before opening the cabinet. Rule out ESD on the host port by testing on a Windows XP / Windows 7 machine with a real COM1 if available.

Root-Cause Matrix

Step-by-Step Hardware Diagnosis on the 6FX1132-8BB01

1. Power off and ESD-ground. Discharge the main DC bus, remove the CPU card from the backplane, and place it on an anti-static mat with the card supported at all four corners.
2. Visual inspection. Look for:
   • Leaked SMD capacitors around the 1488/1489 area (white or brown residue).
   • Tarnished edge-connector fingers — clean with isopropyl alcohol and a fibreglass pen.
   • Cracked ceramic hybrids (the white Siemens V24 packages) — often invisible to the naked eye, but a tap test with a wooden probe can identify them.
   • Cold solder joints on the 25-pin D-Sub posts; reflow if necessary.
3. Verify supply rails. Power the card on a bench supply (5 V ± 5 %, ±12 V for the V.24 hybrids). Confirm 5.0 V at the 80186 Vcc pins and at the UART Vcc pins.
4. Clock and reset. Probe the oscillator (typically 16 MHz × 2 or 24 MHz divided down to 8 MHz for the 80186). Confirm the reset line goes high within 200 ms of power-up and that the red status LED blinks at least once during POST.
5. UART probe. On both UARTs, observe the chip-select line (CS) toggling at 2 Hz intervals while the control is in the data-transfer menu. If CS toggles but the line drivers stay dead, the UART is alive and the hybrids or 1488/1489 stage is suspect. If CS is stuck, the address decoder or the 80186 bus is suspect.
6. 1488/1489 substitution. Power down and inject a 5 Hz square wave into the UART TxD. Watch for ±9 V to ±12 V swings on the DB-25 pin 2 of the matching channel. No swing → replace the 1488 or the entire V.24 hybrid.
7. PROM and ROM check. Read each EPROM (27C256 / 27C512) in a programmer and verify against a known-good dump. Siemens FW for the 810 is not generally redistributable; request an official copy via the regional Siemens office if the existing EPROM is corrupted.

Field-Proven Repair: UART Swap

Multiple repair technicians report that the 8250 / 16450 / 16550 UART on the 6FX1132-8BB01 can fail in a way that takes down only the modem-control outputs (DTR, RTS) while the data lines continue to switch. This produces exactly the DSR/DTR pattern shown in PCIN. The repair procedure is:

1. Identify the two UART packages on the CPU card. On a 6FX1132-8BB01 these are usually 16-pin or 20-pin DIP devices near the two DB-25 connectors.
2. Desolder both UARTs and the surrounding 74LS188 / 74S188 / 74S189 PROM/RAM buffers if present. The 74S188 is a 32×8 bipolar PROM, 74S189 is its open-collector counterpart. Both are socketed on most revisions.
3. Fit machined-pin DIP sockets and drop in new 16550A or 16450 UARTs (16550A recommended for the higher baud rates on the 810 GA2 / GA3 software versions).
4. Power up, re-run POST, and re-test both ports with PCIN.

Sourcing a Spare 6FX1132-8BB01

The CPU card is still listed in the Siemens SiePortal spares catalog under the SINUMERIK 810 / 820 family, but availability is restricted to legacy stock. Options in priority order:

1. Regional Siemens office (spare-parts desk). Provide the MLFB (machine-readable product designation) 6FX1132-8BB01, the serial number of the affected control, and the FS (functional state) of the firmware. Siemens can confirm whether the part is still in active stock or in the repair-exchange program.
2. Siemens SRA / repair-exchange. Submit the faulty card for repair; Siemens will return a refurbished exchange unit with a six-month warranty when stock allows.
3. Industrial surplus dealers. Suppliers that specialise in decommissioned CNC cards occasionally have working pulls. Always request a photograph of the EPROM firmware label and a bench-test report before purchase.
4. Donor card from a scrap control. A 6FX1132-8BB04 or 6FX1132-8BB02 can sometimes be downgraded by swapping the EPROMs, but the backplane and connector pin-out must match exactly.

Pricing on the open market is in the mid-three-figure to low-four-figure EUR range depending on firmware. Compare this against the cost of a board-level repair (a UART, a hybrid, and two hours of bench time) before ordering a replacement.

Verification After Repair

Once the UART, hybrid, or full card swap is complete, validate the control end-to-end:

1. With the control in Data In mode and PCIN in FE receive mode, send a known-good 1 KB part program from the PC. Confirm the file lands in the NC memory and that the checksum dialog reports OK.
2. Send a tool table and a zero-point table from the PCIN side. Reload each into the running NC program to confirm non-volatile storage.
3. Run an Upload all (Datenliste / Softkey) and confirm that the tool list, zero offsets, and R parameters round-trip identically.
4. On a separate PC, run the same transfer to rule out a host-side fault.
5. Leave the control powered on for at least 72 hours, then repeat the upload. This catches solder-joint and hybrid failures that are thermally intermittent.

Preventive Maintenance Notes

The root cause of the parameter loss that triggered the original incident is the on-board lithium backup battery. Recommended preventive actions once the control is back online:

• Replace the 810's battery pack every 3 years regardless of measured voltage, using the original Siemens part where possible (e.g. 6FX1 132-8BB compatible holder with three AA lithium cells).
• Schedule a quarterly upload of the full data backup to a network share via PCIN scripted in a host scheduler.
• Mount the operator panel and CPU card area away from sources of vibration; ceramic hybrids on early 6FX1132-8BB01 cards have shown sensitivity to long-term mechanical shock.
• Inspect the 25-pin D-Sub mounting screws and shield bonding; a loose shield raises the ESD risk on the hybrids and stresses the 1489 receivers.

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