# The Backplane and Modules

All I/O on the SLM-RP4 is provided by modules clipped to the rail beside the base. The modules connect to the base through a SPI backplane that runs through the edge connectors of each module. The base controller is the bus master, and each module's position on the rail determines its slot address. Up to 8 modules are supported.

```text
   ┌──────────┐ ┌────────┐ ┌────────┐ ┌────────┐
   │ SLM-RP4  │ │ Slot 1 │ │ Slot 2 │ │ Slot 3 │  ...  up to slot 8
   │  (base)  │ │        │ │        │ │        │
   └──────────┘ └────────┘ └────────┘ └────────┘
        └────────────┴──────────┴──────────┘
                     backplane
```

Slots are numbered **from 1**, starting with the module immediately to the
right of the base.

```{image} ../../images/hardware/backplane-stacking.jpg
:alt: An SLM-RP4 with eight SLM modules stacked to its right on DIN rail
:width: 700px
:align: center
```

To add a module, clip it to the rail and slide it left until its connector
mates with its neighbour and the latch engages.

```{admonition} Never install or remove modules with power applied
:class: danger

The backplane is **not hot-swappable**. Modules can be permanently damaged if
they are installed in, or removed from, the stack while the SLM-RP4 is powered.

Shut the device down and disconnect 24 VDC before adding, removing or replacing
any module. See {ref}`clean-shutdown`.
```

## Matching the configuration to the stack

At startup each module reports its identity to the base controller, but this
does not configure your project. The Backplane Configuration in OpenPLC Editor
is written by hand, and it **must match the modules physically installed, slot
by slot**.

A mismatch is not always obvious. For example, if slot 2 is declared as an
**SLM-AI-8-V** (0-10 V analog input) but an **SLM-AI-8-mA** (0-20 mA analog input) is
installed, the program addresses the wrong data and every reading from that
slot is wrong.

Whenever the physical stack changes, update the
Backplane Configuration to match (see
{doc}`../getting-started/configuring-the-backplane`) and upload the
project again. If the PLC Logs show different modules than you expect, see
{ref}`sign-on-wrong-modules`.