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S7 Write (s7-write)

Writes variables to a SIEMENS S7 PLC over RFC1006 / ISO-on-TCP. Three modes mirror s7-read: a fixed list with values from config / messages, fully msg-driven, or raw byte block.

Inputs Outputs
1 0 (silent), 1 (ack / passthrough)

By default the node is a pure sink — no output, no ACK. Enabling emitAck or passthrough exposes an output port for results.

Modes

static (default) writes the configured list on every input message. Per-row valueSource decides whether the value comes from a baked-in constant or from a message field (valuePath). This is the right mode for the most common pattern: fixed addresses, values flowing in via messages.

dynamic ignores the sidebar list entirely — the variable list comes only from the message via msg.variables (full form) or the convenience triple msg.address + msg.dataType + msg.payload (single write). Use this when the address itself varies per message.

block writes raw bytes from msg.<inputProperty> (default payload) in one AGWriteArea call. Pair with s7-parser action encode upstream to build the buffer from a structured object.

Configuration

Field Default Description
plc — ID of the referenced s7-plc config node.
mode static static / dynamic / block.
variables [] Static-mode list, see Variables. Hidden in dynamic mode (the runtime ignores it there).
block — Block-mode descriptor, see Block mode.
emitAck false Emit a fresh ACK message after every write attempt. Mutually exclusive with passthrough.
passthrough false Forward the input message with msg.s7Write enriched. Mutually exclusive with emitAck.

Variables

Field Required Description
address yes Siemens-style address — see s7-read address syntax.
dataType yes TIA-Portal type. Counter / Timer cannot be written (rejected at deploy).
valueSource yes static (baked-in) or msg (read from message).
value when static Literal value, typed against dataType.
valuePath when msg Dotted path on the incoming message, e.g. payload, payload.setpoint.
scale optional Inverse scaling on write: raw = (value − offset) / scale. Default 1.
offset optional Inverse offset, applied before division. Default 0.

Block mode

Field Default Description
area DB DB / M / Q. I (Inputs / PE) is not writable — rejected at deploy.
db — DB number. Required when area=DB.
start 0 Start byte offset within the area.
length 0 Optional fixed length. 0 = use the incoming buffer's length. When set, the buffer length must match exactly.
inputProperty payload Message field carrying the byte slice ([]byte, []int, or []any of numerics).

In block mode msg.s7.area / msg.s7.db / msg.s7.start override the configured destination per message — useful when one writer node serves multiple targets driven by an upstream router.

Outgoing message (when emitAck or passthrough enabled)

{
  "payload": true,
  "s7Write": {
    "plc":   "Press Line 2",
    "results": [
      { "address": "DB10.DBD0",       "dataType": "real",   "ok": true },
      { "address": "M0.0",            "dataType": "bool",   "ok": true },
      { "address": "DB1.STRING50.20", "dataType": "string", "ok": false, "error": "Address out of range" }
    ],
    "allOk": false
  }
}

msg.payload is true if every variable wrote successfully, false if at least one failed. msg.s7Write.results always lists every attempted variable in original order, with per-item ok and (on failure) error.

passthrough=true keeps the input message and only adds msg.s7Write — the original payload and other fields survive unchanged. Mutually exclusive with emitAck (which emits a fresh message with payload=allOk).

Error semantics

The write surfaces three distinct failure classes:

  • Per-variable encoding errors (out-of-range, type mismatch, missing value at valuePath): the offending row goes into results[i].error and is excluded from the wire request — the surviving items still get written. The ACK message reflects the partial success via allOk: false.
  • Per-variable PLC errors (address out of range, DB doesn't exist): the wire round-trip succeeds but individual items report a Snap7 error code; mapped to results[i].error with the same partial-success semantics as above.
  • Whole-transaction failures (transport timeout, lost connection): the function returns an error to the engine, which routes it to the Catch node (if any) and does not emit an ACK. The flow's status pill goes red until the next reconnect.

For "fire and forget" writes (no ACK / passthrough), per-variable failures are silently logged at WARN level and the engine routes whole- transaction failures to the Catch path. The status pill always reflects the underlying PLC connection state.

Examples

Static write, value from message

The most common pattern — a setpoint that an upstream Function or Inject computes:

mode: static
variables:
  - address:    DB100.DBW12
    dataType:   int
    valueSource: msg
    valuePath:   payload.setpoint
emitAck: true

Every input message writes msg.payload.setpoint to DB100.DBW12 as a 16-bit signed int.

Dynamic single write (convenience form)

mode: dynamic

Upstream sends:

{ "address": "DB100.DBW12", "dataType": "int", "payload": 1500 }

No sidebar config needed — the address, type, and value all come from the message.

Block write of pre-encoded bytes

mode:  block
block:
  area:           DB
  db:             1
  start:          12
  length:         0          # use the incoming buffer length
  inputProperty:  payload

Pair with s7-parser (action encode) upstream to build the buffer from a structured object. The block is sent in one AGWriteArea call (auto-split on PDU).

Counters and Timers

counter and timer are intentionally read-only. Siemens treats them as CPU-internal state, the wire encoding is asymmetric / fragile, and writing to them from a client is rare in industrial practice. The encoder rejects with a clear message so misuse is obvious at deploy rather than silent corruption.

Status

  • Green — ready (when the PLC is connected).
  • Yellow / Red — propagated from the underlying PLC config.

See also