diff --git a/apps/secs_server.cpp b/apps/secs_server.cpp
index 3d982fe..c5cb37c 100644
--- a/apps/secs_server.cpp
+++ b/apps/secs_server.cpp
@@ -16,6 +16,7 @@
 #include "secsgem/endpoint.hpp"
 #include "secsgem/gem/control_state.hpp"
 #include "secsgem/gem/data_model.hpp"
+#include "secsgem/gem/e90_constants.hpp"
 #include "secsgem/gem/messages.hpp"
 #include "secsgem/gem/router.hpp"
 #include "secsgem/secs2/message.hpp"
@@ -284,6 +285,61 @@ int main(int argc, char** argv) {
         }
       });
 
+  // ---- E90 substrate state-change emitters -----------------------------
+  // Map SubstrateState / SubstrateProcessingState transitions to the
+  // standard E90 CEIDs.  Only the post-transition state matters; the
+  // event-report machinery already gates on enabled CEIDs so emitting
+  // for every transition is safe and gives the host a complete trace.
+  model->substrates.set_location_handler(
+      [logfn, emit_event](const std::string& substid,
+                          gem::SubstrateState from,
+                          gem::SubstrateState to,
+                          gem::SubstrateEvent ev) {
+        logfn(std::string("SUB ") + substid + ": " +
+              gem::substrate_state_name(from) + " -> " +
+              gem::substrate_state_name(to) + "  (" +
+              gem::substrate_event_name(ev) + ")");
+        switch (to) {
+          case gem::SubstrateState::AtSource:
+            emit_event(gem::e90::kCeidSubstrateAtSource); break;
+          case gem::SubstrateState::AtWork:
+            emit_event(gem::e90::kCeidSubstrateAtWork); break;
+          case gem::SubstrateState::AtDestination:
+            emit_event(gem::e90::kCeidSubstrateAtDestination); break;
+          case gem::SubstrateState::NoState: break;
+        }
+      });
+
+  model->substrates.set_processing_handler(
+      [logfn, emit_event](const std::string& substid,
+                          gem::SubstrateProcessingState from,
+                          gem::SubstrateProcessingState to,
+                          gem::SubstrateProcessingEvent ev) {
+        logfn(std::string("SUB ") + substid + " proc: " +
+              gem::substrate_processing_state_name(from) + " -> " +
+              gem::substrate_processing_state_name(to) + "  (" +
+              gem::substrate_processing_event_name(ev) + ")");
+        switch (to) {
+          case gem::SubstrateProcessingState::NeedsProcessing:
+            emit_event(gem::e90::kCeidSubstrateNeedsProcessing); break;
+          case gem::SubstrateProcessingState::InProcess:
+            emit_event(gem::e90::kCeidSubstrateInProcess); break;
+          case gem::SubstrateProcessingState::Processed:
+            emit_event(gem::e90::kCeidSubstrateProcessed); break;
+          case gem::SubstrateProcessingState::Aborted:
+            emit_event(gem::e90::kCeidSubstrateAborted); break;
+          case gem::SubstrateProcessingState::Stopped:
+            emit_event(gem::e90::kCeidSubstrateStopped); break;
+          case gem::SubstrateProcessingState::Rejected:
+            emit_event(gem::e90::kCeidSubstrateRejected); break;
+          case gem::SubstrateProcessingState::Lost:
+            emit_event(gem::e90::kCeidSubstrateLost); break;
+          case gem::SubstrateProcessingState::Skipped:
+            emit_event(gem::e90::kCeidSubstrateSkipped); break;
+          case gem::SubstrateProcessingState::NoState: break;
+        }
+      });
+
   // ---- Build the SECS dispatch table once -------------------------------
   gem::Router router;
 
diff --git a/include/secsgem/gem/e90_constants.hpp b/include/secsgem/gem/e90_constants.hpp
new file mode 100644
index 0000000..541e79a
--- /dev/null
+++ b/include/secsgem/gem/e90_constants.hpp
@@ -0,0 +1,27 @@
+#pragma once
+
+#include <cstdint>
+
+// E90 §6 standard collection event identifiers.  Equipment vendors
+// frequently register additional CEIDs around the base set; the
+// constants below are the canonical ones called out by E90-0716 §6.5,
+// keyed off the SubstrateState transitions so application code can
+// emit them directly from a SubstrateStore change handler.
+//
+// Values use a high block (901+) to avoid collisions with the demo
+// CEIDs in data/equipment.yaml (which uses 100s/200s/400s).
+namespace secsgem::gem::e90 {
+
+inline constexpr uint32_t kCeidSubstrateAtSource         = 901;
+inline constexpr uint32_t kCeidSubstrateAtWork           = 902;
+inline constexpr uint32_t kCeidSubstrateAtDestination    = 903;
+inline constexpr uint32_t kCeidSubstrateNeedsProcessing  = 910;
+inline constexpr uint32_t kCeidSubstrateInProcess        = 911;
+inline constexpr uint32_t kCeidSubstrateProcessed        = 912;
+inline constexpr uint32_t kCeidSubstrateAborted          = 913;
+inline constexpr uint32_t kCeidSubstrateStopped          = 914;
+inline constexpr uint32_t kCeidSubstrateRejected         = 915;
+inline constexpr uint32_t kCeidSubstrateLost             = 916;
+inline constexpr uint32_t kCeidSubstrateSkipped          = 917;
+
+}  // namespace secsgem::gem::e90