When an Allen-Bradley 842E EtherNet/IP absolute encoder goes silent, the instinctive capture request is “stop the trace after N unanswered ENIP requests.” That mental model maps cleanly to ladder logic, but it does not map to Wireshark, tcpdump, or most appliance capture engines. Those tools evaluate inclusion filters on frames that exist. A silent encoder produces no UDP/2222 Class 1 frames for the filter to match, so there is nothing to stop on. The diagnostic problem is absence detection, not signature matching.

Capture continuously on UDP 2222, then compare observed encoder frames against the RPI math—do not wait for a negative trigger that the analyzer cannot express.

Scope and ports

This workflow covers 842E-SIP, 842E-MIP, 842E-DIP, and M12 variants documented in 842E-UM001. Confirm firmware from the encoder web page under Diagnostics → Device Information. Runtime I/O uses CIP Class 1 on UDP port 2222; explicit messaging uses EtherNet/IP encapsulation on TCP/UDP 44818. Capture profiles should enable both, but silence investigations focus on 2222 produced/consumed assemblies.

Why absence triggers fail

Display filters are Boolean predicates applied per frame. A filter such as (udp.port == 2222) && (eth.src == <842E_MAC>) retains encoder frames when they arrive. When they do not arrive, the predicate never becomes true. Maintaining “four requests with zero responses” requires cross-frame state that standard capture utilities do not keep. Module diagnostic tabs in Studio 5000, FactoryTalk Linx device statistics, and post-capture analysis are the tools that can express that state.

Workaround 1: rolling buffer then count

1. Mirror the encoder switch port to an analyzer NIC.
2. Capture UDP 2222 into a ring buffer (for example five 40–50 MB files).
3. Open the pcap and filter on the encoder MAC plus UDP 2222.
4. Compare frame count to observation_time / RPI. Deviations beyond about 5% warrant investigation.

tcpdump -i eth0 -w capture.pcap udp port 2222
# Wireshark display: eth.addr == <842E_MAC> && udp.port == 2222

Workaround 2: RPI-aligned windows

Align capture windows to integer multiples of the encoder RPI so the buffer contains complete I/O exchanges. At 10 ms RPI, a 10-second window should contain about 1000 Class 1 frames if the connection is healthy. Gaps larger than three RPI intervals in the “seconds since previous frame” column are the practical stop condition during review.

Workaround 3: CIP status and switch counters

Issue periodic Get_Attribute_Single reads against CIP Identity Object class 0x01, instance 1, attribute 6 (Status). Changes in Owned or Configured bits indicate the Class 1 connection dropped—commonly after three missed RPIs. Independently, poll managed-switch ifOutUcastPkts on the encoder port; a flat counter for more than 3 × RPI confirms silence without a pcap.

Field procedure and pitfalls

Verify IP, RPI, and assembly instances (produced 0x04 / consumed 0x01 defaults) against the Logix module configuration. Confirm link LEDs, capture sixty seconds, count frames, then attempt a CIP reset if the count is zero. Traffic that resumes after reset but continues to miss RPI points to cable noise, M12 power droop, or duplicate IP. Persistent silence after power cycle implicates the Ethernet PHY—replace the encoder. Do not waste time crafting a “stop on no response” trigger; the analyzer cannot express it. Keep encoder and controller spares aligned with plant PLC and PAC standards when a unit fails in the field.