Teoria
Teoria
ECR17 separates a framed physical transport layer from application messages. The package models that split through PacketCodec, Ecr17Protocol, Ecr17Session, and typed response parsers.
Frame model
flowchart TD
A[Application payload] --> B[STX payload ETX LRC]
B --> C[TCP send]
C --> D{POS physical reply}
D -->|ACK| E[Wait for application result]
D -->|NAK or timeout| F[Retransmit within retry policy]
E --> G{Valid LRC}
G -->|yes| H[ACK and parse response]
G -->|no| I[NAK and keep waiting]
LRC intuition
For a byte sequence b_i, LRC is an XOR-style checksum over the configured scope:
The lrc_mode config changes which control bytes are included.
Gotcha
Different terminal fleets may expect different LRC scopes. If every message is NAKed, validate ECR17_LRC_MODE against the POS configuration before changing business code.
Deep contract
The session owns ordering. Callers submit one command and receive one parsed result; concurrent use of the same session for multiple financial exchanges is outside the design contract.
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