Siemens S7-300/400PLC addressing mode - Database & Sql Blog Articles

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The direct addressing method of Siemens S7-300/400 PLC is similar to that of S7-200, but the indirect addressing methods differ significantly. There are two types of indirect addressing: "memory indirect addressing" and "register indirect addressing." The S7-300/400 features dedicated pointer registers AR1 and AR2, which make indirect addressing more flexible and powerful compared to the S7-200. 1. **Memory Indirect Addressing** In S7-300/400, memory indirect addressing allows you to directly use the “[]” symbol before the address to indicate it's indirect. Unlike S7-200, there’s no need to pre-establish a pointer. Additionally, this method supports binary bit addresses, offering greater flexibility. *Example 1:* Read input I22.2 into local variable LD10, then perform an AND operation with I0.0 and output the result to Q0.1: ``` LP#22.2 TLD10 AI[LD10] A I0.0 = Q0.1 ``` This is equivalent to: ``` AI22.2 A I0.0 = Q0.1 ``` 2. **Register Indirect Addressing** This method uses pointer registers AR1 or AR2 for offset addressing. The format is [AR1,m] or [AR2,m]. The address is determined by the content of the pointer register plus the offset m. These registers can store addresses, bytes, and bits, enabling access across different memory areas like internal flags (M), inputs (I), outputs (Q), etc. The pointer register includes several key fields: - **Address bit (bit31):** Indicates whether the register contains an address. - **Address character (bits 24–26):** Specifies the type of address (e.g., I, Q, M, DBX). - **Byte number (bits 18–3):** Defines the byte address. - **Bit number (bits 2–0):** Defines the bit within the byte. *Example 2:* Shift binary bit 22.2 using AR1, read I32.3, AND with I0.0, and output to Q0.1: ``` LP#22.2 LAR1 AI[AR1, P#10.1] A I0.0 = Q0.1 ``` Equivalent to: ``` AI32.3 A I0.0 = Q0.1 ``` When working with binary bits, remember that the unit is in bytes, so offsets should be calculated in octal. *Example 3:* Use AR1 to shift bit 10.5, read I21.4, AND with I0.0, and output to Q0.1: ``` LP#10.5 LAR1 AI[AR1, P#10.7] A I0.0 = Q0.1 ``` Equivalent to: ``` AI21.4 A I0.0 = Q0.1 ``` If the pointer register contains an address, you don't need to specify it again in the instruction. *Example 4:* Use M6.0 as a pointer to write data from IW10 to MW56: ``` LP#M6.0 LAR1 LIW10 TW[AR1, P#50.0] ``` Equivalent to: ``` LIW10 TMW56 ``` This method simplifies programming and expands the range of accessible memory locations, making it ideal for complex control systems.