Layer: L7 Application · Build flags: PC_ENABLE_SIMATIC

What this example teaches

Before PROFINET, Siemens controls talked to computers over a serial point-to-point link: 3964R (the byte-oriented link protocol on the S5/S7 PtP CP modules - CP 341 / CP 441 / CP 524 / CP 525) carrying RK512 computer-link telegrams. services/simatic is a zero-heap codec for both; the RS-232 / RS-485 UART is normally the application's.

3964R frames a block as STX <payload, DLE bytes doubled> DLE ETX [BCC] with an interactive handshake - the sender emits STX and waits for the receiver's connect DLE, sends the block, waits for the end DLE/NAK, and retries on timeout; the receiver arbitrates a simultaneous-STX collision by station priority. The "R" variant's BCC is the longitudinal XOR of the block (a doubled DLE pair cancels). RK512 rides inside as a fixed-header SEND (write words) / FETCH (read words) telegram addressing a DB / flag / I-O area, big-endian words, answered by a reaction telegram.

// 3964R block framing (pure)
size_t n = pc_3964r_build_block(buf, sizeof(buf), data, len, /*with_bcc=*/true);
pc_3964r_parse_block(buf, n, true, out, sizeof(out), &olen);   // un-stuff + verify BCC
 
// the interactive link state machine (drives STX/DLE + retries via a tx sink)
pc_3964r_init(&ctx, /*high_priority=*/true, /*with_bcc=*/true, tx_sink, on_rx, user);
pc_3964r_send(&ctx, telegram, tn, pc_millis());
pc_3964r_rx_byte(&ctx, incoming, pc_millis());   // feed UART bytes
pc_3964r_tick(&ctx, pc_millis());                // drive QVZ/ZVZ timeouts
 
// RK512 telegrams (big-endian words)
pc_rk512_build_fetch(t, sizeof(t), Rk512Area::DB, 5, 0x0000, 2);  // read DB5.DBW0, 2 words
pc_rk512_build_send(t, sizeof(t), Rk512Area::MB, 0, 0x0010, words, 4);

Run it (no second device needed)

The sketch runs two link stations on one board - A (high priority) and B (low priority) - and cross-wires their byte sinks, so it drives the whole 3964R handshake + an RK512 FETCH round-trip against itself. A completed round increments a counter served at GET /simatic:

pio ci --board=esp32dev --project-option="framework=arduino" \
  --project-option="build_flags=-DPC_ENABLE_SIMATIC=1" \
  --lib="." examples/L7-Application/SimaticSerial/SimaticSerial.ino

$ curl http://192.168.1.42/simatic

{"rounds":37,"lastFetchTelegram":"0100010500000002","lastReactionTelegram":"020000"}

On a real installation station A is this device and station B a Siemens PtP CP over an RS-232/RS-485 UART - feed the UART's received bytes to pc_3964r_rx_byte and let the state machine's tx sink write the UART.

Annotated source

The complete sketch is SimaticSerial.ino. The codec is in src/services/fieldbus/simatic/simatic.h; the 3964R handshake, XOR-BCC, QVZ/ZVZ timeouts and the RK512 header follow the Siemens "3964(R) transmission protocol" and "RK 512 computer link" CP-module manuals, cross-checked byte-for-byte against an independent python reference peer (test/servers/peers/simatic_peer.py). Pairs with the HaasMdc machine-tool example. The MELSEC and FANUC FOCAS codecs ship as services (src/services/fieldbus/melsec, src/services/machine_tool/focas) but have no example yet.