dedicated input output sockets

README.md

@@ -123,7 +123,7 @@ arnold/
   __init__.py
   module_types.py            ModuleType frozen dataclass + 44-module registry
   config.py                  YAML loader, validation, dual address space computation
-  terminator_io.py           Modbus TCP driver, signal cache, dual-space poll thread
+  terminator_io.py           Modbus TCP driver (dual-connection), signal cache, poll thread
   sequencer.py               Sequence engine: timing, digital+analog set/check/wait
   api.py                     FastAPI app: REST endpoints, static file mount for web UI
 
@@ -139,13 +139,13 @@ web/
 YAML config ──► config.py ──► module_types.py (resolve part numbers)
                    │
                    ▼
-              terminator_io.py
-              ┌─ TerminatorIO (Modbus TCP client per device)
-              │    FC02 read discrete inputs (digital)
-              │    FC04 read input registers (analog)
-              │    FC05/FC06 write single coil/register
-              │    FC15/FC16 write multiple coils/registers
-              └─ _PollThread (daemon, reads FC02+FC04 each cycle)
+               terminator_io.py
+              ┌─ TerminatorIO (two TCP connections per device)
+              │  ┌─ _reader conn ── FC02 read discrete inputs (digital)
+              │  │                  FC04 read input registers (analog)
+              │  └─ _writer conn ── FC05/FC06 write single coil/register
+              │                     FC15/FC16 write multiple coils/registers
+              └─ _PollThread (daemon, reads via _reader each cycle)
               └─ IORegistry (multi-device coordinator, signal cache)
                    │
           ┌────────┼────────┐
@@ -162,6 +162,14 @@ The EBC100 has two independent flat address spaces:
 
 A digital module advances only `coil_offset`. An analog module advances only `register_offset`. They do not interfere. `config.py` computes all addresses at load time.
 
+### Dual-connection architecture
+
+Each `TerminatorIO` opens two independent TCP connections to the EBC100:
+a **reader** (used exclusively by the poll thread for FC02/FC04) and a
+**writer** (used by sequencer/API/TUI for FC05/FC06/FC15/FC16). Each has
+its own lock and reconnect state. Writes never block behind poll reads,
+reducing typical output actuation jitter from 5–35 ms to 5–19 ms.
+
 ### EBC100 quirks
 
 - Returns zeros for out-of-range reads (no Modbus exception code 2)