Siemens SIMATIC S7 System Manual page 474

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9.2 Overview of Modbus communication

Mapping Modbus addresses to CPU addresses

All Modbus addresses are one-based.

MBUS_INIT parameters that limit slave accessibility

The Modbus slave/protocol allows you to limit the number of inputs, outputs, analog inputs,

and holding registers (V memory) that are accessible to a Modbus master.

● MaxIQ assigns the maximum number of discrete inputs or outputs (Is or Qs) a Modbus

master is allowed to access.

● MaxAI assigns the maximum number of input registers (AIWs) a Modbus master is

allowed to access.

● MaxHold assigns the maximum number of holding registers (V memory words) a Modbus

master is allowed to access.

See the description of the MBUS_INIT (Page 491) instruction for more information on setting

up the memory restrictions for the Modbus slave.

Mapping Modbus addresses to CPU addresses

Modbus address

Vx (Holding reg. start)

Vx+2 =(Hold reg. start+2)

Vx+4 =(Hold reg. start+4)

Vx+2(yyyy-1) or Vx+2(zzzzz-1)

System Manual, V2.3, 07/2017, A5E03822230-AF

S7-200 SMART

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