Addressing S7-300 Modules

Slot Numbers

The slot numbers in the rack of an S7-300 simplify addressing in the

S7-300  environment. The position of the module in the rack determine the first

address on a module.

Slot 1

Power supply. This is the first slot by default.

A power supply module is not absolutely essential. An S7-300  can also be

supplied with 24V directly.

Slot 2

Slot for the CPU.

Slot 3

Logically reserved for an interface module (IM) for multi-tier configurations using

expansion racks. Even if no IM is installed, it must be included for addressing

purposes.

You can physically reserve the slot (such as for installing an IM at a later date) if

you insert a DM370 dummy module.

Slots 4-11

Slot 4 is the first slot that can be used for I/O modules, communications

processors (CP) or function modules (FM).

Addressing examples:

• A DI module in slot 4 begins with the byte address 0 .

• The top LED of a DO module in slot 6 is called Q8.0 .

Note

Four byte addresses are reserved for each slot. When 16-channel DI/DO

modules are used, two byte addresses are lost in every slot!

Multi-Tier Configurations

The slots also have fixed addresses in a multi-tier configuration.

Examples:

• Q7.7 is the last bit of a 32-channel DO module plugged into slot 5 of rack 0.

• IB105 is the second byte of a DI module in slot 6 of rack 3.

• QW60 is the first two bytes of a DO module in slot 11 of rack 1.

• ID80 is all four bytes of a 32-channel DI module in slot 8 in rack 2.

Address  Overview

Views the I/O addresses of the station configured. Select: View -> Address Overview …

Abbreviations:

R Rack number

S Slot number of the relevant module

DPRelevant only when Distributed Peripherals (I/O) are used

IF Interface module ID when programming the M7 system (in C++).

Slot dependent Addressing 

The modules are assigned fixed slot-dependent addresses with the S7-300  (CPUs without DP interface) and S7-400 (without hardware configuration).

Variable Addressing

With the S7-300  (CPUs with integrated DP interface) and with the S7-400 you can assign parameters to the starting addresses of the modules.

What to Do

When you double-click a digital or an analog module, the parameter assignment

screen is opened. After you choose the "Addresses" tab, you can cancel

"System selection". You can now define the starting address in the "Start" box. If

the address is already used, an error message is triggered.

Part process images can be defined only in the S7-400 . That way, specific

inputs and outputs (such as time-critical signals) can be combined into one

group. A system function triggers the updating of a part process image in the

user program.

Note

After a CPU memory reset, the parameters, and therefore also the addresses

are lost. This means that the slot-dependent addresses of the S7-300  or the

default addresses of the S7-400  are valid once more.