Home

Alarm 087 / SR0087 and Buffer Overflow Guide

A practical workshop guide to resolving data transmission spillover errors during file transfers and DNC drip-feeding on Fanuc and compatible machine controls.

The Symptoms: Are you seeing one of these error messages?

Alarm 087 (BUFFER OVERFLOW): The standard error message on classic Fanuc controls during serial transmission.
Alarm SR0087 (BUFFER OVERFLOW): The modern equivalent layout displayed on newer open-architecture Fanuc controls.
Data Corruption / Mid-program Freeze: The CNC suddenly stops mid-sentence during a drip-feed, or drops text characters, causing execution faults.

🛠️ The Quick Fixes: Stop the Data Spillover

If you see an 087 alarm, the PC is pushing data down the serial wire after the CNC has already yelled "STOP!" (XOFF). To fix this immediately, apply these three rules:

Lower Windows FIFO Buffers: Restrict Windows from caching data behind the software's back (steps below).
Drop the Baud Rate in EasyDNC: Slow down the raw transmission speed (e.g., lower it from 9600 to 4800 or 2400) to give the CNC processor breathing room.
Inject a Block Delay in EasyDNC Setup: Add a tiny pause (e.g., 10 to 50 milliseconds) at the end of every line of G-code to let the machine digest the blocks sequentially.

Step-by-Step: How to Disable Windows FIFO Buffers

Windows naturally tries to speed up data transmission to modern office peripherals by pre-loading chunks of data into an internal hardware transmit buffer. On a vintage CNC machine, this often causes a buffer overflow because the Computer's buffer is bigger than that of the CNC. The solution is to disable that buffer in the computer

Press the Windows Key + X and select Device Manager.
Expand the Ports (COM & LPT) category.
Right-click your active communications port (e.g., COM3) and choose Properties.
Navigate to the Port Settings tab and click the Advanced button.
Locate the FIFO Buffers checkboxes or sliders. Uncheck "Use FIFO buffers" entirely, or drag the Transmit (TX) Buffer slider all the way down to 1. Click OK to save.

⚠️ The USB-to-RS232 Adapter "Ghost Buffer" Trap

Be aware that many generic USB-to-Serial conversion adapters use low-grade clone chips and simple device drivers which are often incorrectly detected by MS Windows. Where possible it's recommended that you install device drivers supplied by the device manufacturer.

Even though Windows Device Manager will often display a 16byte FIFO setup window and allow you to uncheck the FIFO box or slide the buffer down to 0, the adapter's internal hardware driver often ignores your setting. It continues to force its own internal multi-byte data queue, making a smooth DNC drip-feed difficult using standard settings.

If you have modified your Windows settings and are still plagued by intermittent Alarm 087 messages, you are dealing with a ghost buffer hardware issue.

💡 Bypassing Bad Hardware Using EasyDNC Settings

If your USB adapter is ignoring the Windows FIFO disable command, you do not necessarily need to throw it away. You can compensate for the hardware lag directly within the EasyDNC Setup Profile for that machine:

Step down the Baud Rate: If you are running at 9600, pull the machine parameters and EasyDNC down to 4800 or 2400. The slower transmission pace stops the clone chip's buffer from piling up data ahead of the CNC.
Use Block Delays: Inside EasyDNC's transmission settings, locate the **Block Delay** option. Injecting a nominal delay forces the PC to wait a fraction of a second before hitting the CNC with the next line of code, allowing the control to completely empty its buffer before the next block arrives.
It's sometimes a balance between baud rate and block delay. Perhaps set your machine up to 'cut air' while you try the different options.

Technical Deep Dive: Why does a Buffer Overflow happen?

For those who want to understand the tracking logic: Fanuc controls have an extremely compact input memory buffer. When executing complex, high-speed 3D profiling programs via drip-feed (DNC), the control fills its data container rapidly.

Once the buffer reaches capacity, the CNC transmits a standard software flow control marker called an XOFF character (ASCII 19 / DC3) back to the PC, signaling it to freeze the transmission line. The Fanuc engineering rule states that the PC must stop sending immediately. If the PC pushes more than 10 additional characters past the wire after the XOFF was sent, the data spills out, and the control triggers an immediate 087 Buffer Overflow Emergency Stop to protect the program structure from breaking.