ISO connection refused — ISO-on-TCP connection rejected on port 102

The TCP connection to port 102 was refused or the COTP handshake failed. You never reached the S7 layer.

Start free PROFINET / S7 → canonical

Diagnosis

When you hit this

Wrong rack or slot in the connection parameters. S7-300 is typically rack 0 slot 2; S7-1200 and S7-1500 are rack 0 slot 1.
PUT/GET communication not enabled on an S7-1200 or S7-1500, which blocks external S7 clients by default.
The CPU's connection resources are exhausted by other clients.

Resolution

How to fix it

Enable PUT/GET access
On S7-1200 and S7-1500 this is off by default. In TIA Portal: Protection & Security, permit access with PUT/GET communication. This single checkbox is the most common cause.
Get rack and slot right
Slot 1 for 1200/1500, slot 2 for most 300/400. A wrong slot fails at COTP, before any read is attempted.
Free a connection resource
CPUs have a fixed number of S7 connections. Disconnect idle clients to test.

Normalization

What Forge does about it

This is a transport failure, so there is nothing for Forge to normalize: the reading never arrives. What Forge does is refuse to let the gap look like good data. The read-time dropout check flags a field that fell to zero and recovered as a communication glitch rather than a real measurement, and the stuck-sensor check fires when a value repeats for 20 consecutive readings — which is what a cached or last-known value looks like after a link drops. Forge does not retry or reconnect for you; that is your client's or your gateway's job.

Connect

Normalize PROFINET / S7 telemetry

One MCP endpoint serves every protocol adapter. 16,908 curated mappings, 366 canonical fields.

# normalize telemetry from PROFINET / S7 — and 13 other protocols

claude mcp add --transport http forge https://mcp.foundrynet.io/mcp \

--header 'Authorization: Bearer YOUR_FORGE_KEY'

✓ forge connected — 32 tools available