Troubleshooting 'Transmission Buffer (FIFO) for RC Server Is Full' in SIMATIC PCS 7 Route Control

In SIMATIC PCS 7 Route Control (RC v8.1 and v8.2), the system warning 'Transmission buffer (FIFO) for the RC server is full' indicates a communication bottleneck where the Automation Station (AS) is generating route status and material transport telegrams faster than the redundant Route Control Servers can ingest them across the Industrial Ethernet connection. This occurs when high-frequency material transfer routes flood the send FIFO buffer, network latency spikes on the Terminal/Plant bus, or the RC communication interface block (RC_IF_CFG / RC_IF_ROU) is configured with an excessively small send queue. To resolve this, increase the FIFO buffer capacity in the RC block parameters and optimize communication cycle times.

SIMATIC Route Control (RC) Communication Architecture and Telegram Buffering

SIMATIC PCS 7 Route Control manages automated material transport routes (piping, conveyor networks, silos):
* AS-RC Server Handshake: The AS controller executes Route Control interface blocks (RC_IF_CFG, RC_IF_ROU, RC_IF_VALV). When route elements request allocation or change state, the blocks assemble binary telegrams.
* The Send FIFO Buffer: Because route events occur in burst batches (e.g. 50 valves opening simultaneously during route initiation), the AS maintains a First-In-First-Out (FIFO) transmission buffer in DB memory.
* Redundant Ingestion: The telegrams are transmitted via Named Connections across the Plant Bus to redundant Route Control Servers. If the servers are busy logging archive data, the AS FIFO buffer accumulates pending messages.

Root Causes of Transmission Buffer (FIFO) Overflow

When the warning Transmission buffer (FIFO) for the RC server is full is logged:
1. Burst Route Allocation Overload: Multiple simultaneous routes requested concurrently overwhelm the default FIFO buffer size (typically 50 to 100 entries).
2. Plant Bus Network Latency or Packet Drops: Intermittent packet loss on the CP 1623 / CP 443-1 communication link delays TCP acknowledgments, stalling telegram draining.
3. RC Server Disk I/O Saturation: If the Route Control Server is executing heavy historical database queries or shared SQL transactions, the RC communication service (RCC_Server.exe) slows down telegram processing.
4. Driver Execution Frequency Too Fast: Running RC interface blocks in an aggressive cyclic OB (e.g. OB35 at 50 ms) without matching server reception rates.

Remediation Procedure - Expanding FIFO Queues and Tuning Bus Parameters

Follow this procedure to eliminate buffer overflows and stabilize Route Control:

[Phase 1: Increase AS Interface FIFO Buffer Size]
   │  • In SIMATIC Manager: Open the CFC chart containing RC_IF_CFG (FB800)
   │  • Open block Object Properties -> Parameters:
   │      - Locate parameter: "FIFO_Len" or "Queue_Size"
   │      - Increase buffer capacity (e.g. from 50 to 200 entries)
   │  • Verify that instance DB memory allocation has sufficient spare capacity
   │
[Phase 2: Optimize Execution Cycle of RC Driver Blocks]
   │  • In CFC Runtime Editor (Ctrl + R):
   │      - Ensure RC interface blocks are scheduled in OB34 (200 ms) or OB35 (100 ms)
   │      - Do not execute RC blocks faster than required for mechanical valve opening
   │
[Phase 3: Verify RC Server Communication and Disk Performance]
   │  • On the Route Control Server:
   │      - Open Windows Task Manager -> Performance -> Disk:
   │          Verify disk queue length is < 2 (Ensure fast enterprise SSDs)
   │      - In NetPro: Check S7 connection parameters to the AS controller
   │  • Compile CFC and perform download changes to the AS controller

Route Control Buffer Capacity and Timing Parameter Matrix

Evaluate FIFO configuration options and performance parameters:

Parameter / MetricFactory Default SettingHigh-Load Recommended SettingImpact of Insufficient CapacityFIFO Buffer Length50 telegrams200 to 500 telegramsBuffer overflow warning; lost route feedback.Execution TaskOB35 (100 ms)OB34 (200 ms)High CPU jitter; burst packet generation.Connection TypeNamed ConnectionsFault-Tolerant Named ConnectionsCommunication timeout during server failover.RC Server Heartbeat1000 ms1000 ms (Steady)False disconnect alarms if heartbeat stalls.

Verification

To confirm that the Route Control transmission buffer is stable:
1. In the Route Control Center (RCC), initiate multiple simultaneous complex routes across the plant.
2. Monitor the WinCC Alarm Logging view; confirm that no 'Transmission buffer (FIFO) is full' warnings appear.
3. In CFC Test Mode, inspect RC_IF_CFG.FIFO_FillLevel; verify that buffer utilization remains below 60% during peak route allocation bursts.
4. Verify that route element feedback icons (valves, pumps) update dynamically without delay.

Limitations and Operational Precautions

Increasing the FIFO buffer length allocates additional data block memory in the AS controller. In older S7-400 CPUs with constrained RAM, ensure that expanding the buffer does not exhaust work memory. Never disable Route Control communication diagnostics, as undetected buffer overflows can lead to route desynchronization where valves are commanded open but the server assumes the route is aborted.

Engineer FAQ

Does a FIFO buffer full warning abort active routes?

Not immediately. Active routes continue executing, but status updates and feedback messages may be delayed or dropped, leading to route timeouts if the condition persists.

Why does this warning occur with only one AS controller?

Even with a single AS, if multiple complex routes containing dozens of elements are commanded simultaneously, the burst of state transitions easily fills a small default buffer.

Where is the Route Control server diagnostic log saved?

On the RC Server under C:\Program Files (x86)\Siemens\SIMATIC PCS 7\Route Control\Log\ in file RCC_Diag.log.

Can Route Control operate with redundant servers?

Yes. Route Control supports redundant server pairs running in hot-standby, synchronizing route reservations and material tracking continuously.

Technical References

Related Tags

Route Control
FIFO Buffer Full
RC_IF_CFG
Material Transport
Plant Bus Communication

Plant Bus Communication

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Troubleshooting 'Transmission Buffer (FIFO) for RC Server Is Full' in SIMATIC PCS 7 Route Control