Measuring SPPA-T3000 CS275 Gateway Traffic: Bottleneck Analysis

Problem Statement

On hybrid SPPA-T3000 / Teleperm M installations in power plants, operators periodically observe that process value updates on T3000 graphics slow from the nominal sub-second refresh to cycles of 20 seconds or more during plant upsets and alarm storms. The refresh latency is not a single-segment phenomenon. Three independent subsystems sit between the field sensor and the operator screen, each with its own bandwidth budget, and the slowness can originate in any one of them:

1. The CS275 field bus of the Teleperm M underlayer, historically specified near 350 kbit/s
2. The CM-T3000 gateway modules (MPL-supplied) that bridge the CS275 bus to the T3000 Industrial Ethernet
3. The T3000 server / client Ethernet, configured at 100 Mbit/s for the operator network

Because the three segments differ in bitrate by almost three orders of magnitude, a one-size-fits-all guess is rarely sufficient. The diagnostic workflow below provides a measurement-based method to attribute the slowness to the correct segment, using widely available Ethernet capture tools and the Siemens BANY diagnostic suite.

Hybrid SPPA-T3000 / Teleperm M Architecture

The SPPA-T3000 control system is the Siemens (now Siemens Energy) distributed control system for the power generation industry, designed as the successor to the Teleperm M platform. A common migration strategy in brown-field power plants is to retain the proven Teleperm M automation and I/O layer in place while introducing SPPA-T3000 as the operator HMI, alarm and trending layer. The bridge between the two domains is a gateway module - historically a CM-T3000 unit from MPL, mechanically and logically compatible with the Teleperm M CS275 backplane.

From a networking perspective the topology is a three-tier chain:

The key asymmetry is bandwidth: 100 Mbit/s on the operator side versus roughly 350 kbit/s on the CS275 bus side. A single TCP segment flowing over the Industrial Ethernet can therefore represent tens of CS275 telegram equivalents. When the CS275 bus becomes saturated by an alarm burst, the gateway cannot magically increase the slow-side bandwidth; it can only queue messages, and the T3000 client simply sees long refresh delays.

Bottleneck Candidates and Throughput Budget

Before connecting a single probe, enumerate the three candidate bottlenecks and quantify the expected per-segment data rate. The CS275 bus, as originally specified for Teleperm M, runs at approximately 370 kbit/s gross. After frame overhead, the user payload per telegram is small, and a single process value typically consumes one or more telegrams depending on the value type (analog, digital, command, status).

Saturation on the slow side (CS275) is by far the most common cause of the 20-second refresh symptom. The Ethernet side, at 100 Mbit/s, is rarely the limiting factor for process values alone; it is usually a victim of DCOM chatter, antivirus scans, or backup windows that compete for the same physical medium.

CS275 Bus Characteristics Relevant to Diagnosis

The CS275 bus is a token-bus field bus used by Teleperm M automation stations. Telegrams are short, cyclic, and time-deterministic. The following characteristics matter when interpreting T3000-side measurements:

• Determinism. Each automation station owns the token for a fixed time slice. The gateway can only send one telegram per token pass per value type.
• Asymmetric rate. Process values are read more often than commands are written. The gateway produces a higher outbound rate from CS275 to T3000 than the reverse.
• Address-space. A single CS275 segment typically supports up to 252 stations (AS 220 / AS 230 / AS 235 family), and the gateway aggregates one or more of these onto the T3000 side.
• Error behaviour. Telegram errors (parity, timeout) cause retransmission, which doubles the load for the affected value type during the error window.
• Cyclic vs acyclic. Cyclic process-value telegrams are predictable; acyclic diagnostic and operator-command telegrams compete for the same token time and grow during upset conditions.

During a plant upset, alarm and event telegrams compete with normal process-value telegrams for the same token time. The T3000 picture refresh is starved because the gateway gives priority to fresh alarm state changes, leaving process values queued in the CS275 gateway buffer.

LAN Measurement Hardware

Measurement of T3000-side traffic requires either a passive tap or a SPAN port on a managed switch. Both are acceptable; the choice depends on whether a SPAN-capable managed switch is already present between the CM-T3000 gateway and the T3000 server.

Capture Point Matrix

Multiple capture points are needed only when the single-point capture does not pinpoint the source. The following matrix maps symptoms to the capture point that yields the most information:

Wireshark Capture Procedure

Wireshark is the standard open-source protocol analyser for Industrial Ethernet troubleshooting. The following procedure produces a capture file usable for offline analysis of the T3000 gateway segment.

1. Connect a managed capture host (a Windows or Linux laptop with a gigabit NIC) to the SPAN port of the switch that connects the CM-T3000 gateway. If no managed switch is available, insert a passive tap and connect the capture host to the monitor port of the tap.
2. Disable all offload features on the capture NIC to prevent the kernel from mangling timestamps and stripping padding bytes: ethtool -K eth0 rx off tx off tso off gso off gro off lro off sg off
3. Set the capture buffer to a ring buffer of 1-2 GB so that a 24-hour trend can be retained without filling the disk: dumpcap -i eth0 -b filesize:200000 -b files:10 -w t3000_capture.pcapng
4. Start a parallel text log of the operator-station refresh latency. A simple batch script that records time /t every 5 seconds and the operator's "values stale" flag gives a synchronised timeline.
5. Trigger the symptom. Either wait for the next plant upset or, in a controlled test, force a CS275 telegram burst by switching many outputs simultaneously from the Teleperm M engineering station. Mark the trigger time in the Wireshark capture with a comment packet (Tools > Packet Comment).
6. Stop the capture after the symptom has been reproduced and at least one full alarm burst has passed. Retain the .pcapng file for analysis.
7. Apply initial filters during capture to drop noise and reduce file size: capture filter: not broadcast and not multicast and host <gateway_ip>

For quick bandwidth checks, the I/O graph (Statistics > I/O Graph) shows bits-per-second per filter. Filter for the gateway IP to isolate the gateway-to-server conversation from background broadcast traffic. Save the graph as a PNG and annotate the alarm-burst start/end so it can be cross-referenced with the T3000 alarm log.

Siemens BANY Diagnostic Tool

BANY is a Siemens diagnostic tool historically shipped with Teleperm M installations for bus-level troubleshooting of the CS275 bus, and is now also available in a PROFINET IO variant (BANY_PNIO) for modern Industrial Ethernet networks. BANY operates independently of T3000 and can be installed on a service laptop connected directly to the CS275 segment or to the gateway's CS275-side diagnostic port.

For CS275 diagnosis, BANY provides:

• Per-station telegram counters (send / receive / error)
• Bus-load measurement as a percentage of the nominal 370 kbit/s bandwidth
• Error and retransmission statistics per station
• Live telegram decode with time stamping
• Token-pass duration histogram and station-on-bus timeline

The bus-load percentage reported by BANY is the single most useful number for the symptom described in this article. If BANY reports >80 % bus load during the alarm storm, the CS275 side is the bottleneck; the T3000-side Ethernet capture will simply confirm the consequence. BANY_PNIO exposes equivalent metrics for the PROFINET side of the gateway, useful when the CM-T3000 module is replaced by a PROFINET-capable successor.

T3000 Built-in Diagnostic Blocks

The T3000 function-block library contains diagnostic blocks for the most common Siemens communication modules: S7-400 CPUs, ET200M stations, OSM/ESM switches, and the operator server itself. These blocks expose real-time diagnostic data that can be subscribed to from the T3000 engineering tool and trended over time in the same trend database used for process values.

Typical diagnostic signals exposed by the gateway block are:

Archive these signals in the T3000 trend database and correlate them with the operator's "values stale" log. A spike in telegram queue length concurrent with the symptom is positive proof of gateway-side saturation. The diagnostic block addresses and tag names are project-specific; consult the T3000 engineering documentation for the installed project. For T3000 projects under Siemens Energy support, the diagnostic block library is delivered with the engineering toolset and is documented in the project's control narrative.

Interpreting Captured Data

Once a capture has been recorded, apply a layered analysis:

1. Overall bandwidth. In Wireshark, Statistics > Conversations > TCP shows the top talkers. The T3000 server to CM-T3000 gateway conversation should dominate. If the top talker is a different IP (e.g. an engineering station, a backup server, an antivirus server), the Ethernet side is the problem.
2. OPC/DCOM chatter. Apply the display filter dcom || opc || port 135 || portrange 49152-65535. Healthy T3000 traffic uses a small number of long-lived TCP connections for OPC subscriptions; a flood of short-lived connections indicates a client reconnect storm.
3. TCP retransmissions. Statistics > TCP Stream Graphs > Time-Sequence (Stevens) reveals retransmissions. A retransmission rate above 1 % of the gateway conversation is suspicious and points to either switch congestion or a duplex mismatch.
4. BANY bus load. Compare the BANY bus-load trend to the operator's refresh timeline. If the bus load crosses 80 % exactly when the refresh freezes, the CS275 side is the bottleneck.
5. TCP window size. Statistics > TCP Stream Graphs > Window Scaling shows if the gateway is advertising a small receive window. A window stuck at 16 KB or below indicates that the gateway cannot keep up with the OPC subscription rate.
6. TCP round-trip time. Statistics > TCP Stream Graphs > Round Trip Time. A sudden increase in RTT during the alarm burst indicates queuing in either the gateway or the server stack.

For long-term trending, the .pcapng files can be processed with tshark -r capture.pcapng -qz io,stat,10,"AVG(bits/s)" to produce per-interval bandwidth reports. Combine the per-interval reports with the BANY log and the T3000 diagnostic trend to obtain a complete bottleneck map. Import the resulting CSV into the T3000 trend database or an external trending tool (Grafana, Excel) for cross-correlation with the plant timeline.

Optimization Strategies

Once the bottleneck segment is identified, the mitigation differs by segment.

As a low-risk first step, raise the T3000 picture refresh interval from 1 s to 2 s on operator screens that do not require second-by-second updates. This halves the subscription rate to the gateway and frequently restores normal refresh even under heavy alarm load, without any hardware changes. A second low-risk action is to enable the T3000 "deadband" filter on analogue values, so that values within the configured deadband are not transmitted on every cycle.

Quick-Reference Troubleshooting Matrix

Verification Checklist

After applying any mitigation, repeat the measurement to confirm improvement:

• [ ] Wireshark capture re-recorded under the same plant condition
• [ ] BANY bus load below 60 % during the alarm burst
• [ ] T3000 diagnostic block: telegram queue length below 10
• [ ] Operator picture refresh latency below 5 s at all times
• [ ] No new T3000 gateway-timeout alarms in the alarm log
• [ ] 24-hour trend archived for the next plant upset
• [ ] Mitigation changes documented in the T3000 change log
• [ ] Baseline performance recorded for future comparison

Frequently Asked Questions