Expansion of Inputs and Outputs in the S7-200 Series PLC

Expansion of Inputs/Outputs in the S7-200 Series PLC

Core Concept: Modular Expansion

The S7-200 PLC adopts amodular structure for expansion. Its basic unit (CPU module) comes with a fixed number of I/O points (referred to asbase I/O). When these I/O points are insufficient, dedicatedexpansion modules can be connected through the expansion interface on the right side of the CPU to increase the number of I/O points or add special functions.

1. Types of Expansion Modules

The expansion modules for the S7-200 are mainly divided into the following categories:

1. Digital Expansion Modules (DI/DO)

Used to increase the number of digital input/output points.

• EM 221: Pure digitalinput module. For example: EM 221 8x24V DC input.
• EM 222: Pure digitaloutput module. For example: EM 222 8x24V DC output (transistor). For example: EM 222 4 relay outputs.
• EM 223: Digitalmixed module (both input and output). For example: EM 223 4 inputs/4 outputs (DC/relay), EM 223 8 inputs/8 outputs (DC/relay), etc. The combinations are very rich.

2. Analog Expansion Modules (AI/AO)

Used to connect analog sensors (such as temperature, pressure, flow) or control analog actuators (such as frequency converters, control valves).

• EM 231: Pure analoginput module. For example: EM 231 4×12-bit analog input (can accept voltage or current signals). For example: EM 231 4x thermocouple input (dedicated temperature module). For example: EM 231 2x RTD input (RTD dedicated module).
• EM 232: Pure analogoutput module. For example: EM 232 2×12-bit analog output.
• EM 235: Analogmixed module. For example: EM 235 4 inputs/1 output (most commonly used).

3. Communication Expansion Modules (CP)

Used to increase communication interfaces for network connections.

• EM 277: PROFIBUS-DP slave module. Allows the S7-200 to connect as a slave to the PROFIBUS-DP network, communicating with the master (such as Siemens S7-300/400, WinCC) or other masters.
• CP 243-2: AS-Interface master module. Used to connect AS-i bus devices.
• CP 243-1: Industrial Ethernet module. Allows the S7-200 to connect to industrial networks via Ethernet, supporting S7 communication, etc.

2. Expansion Rules and Capabilities (Very Important!)

Expansion is not unlimited and must adhere to hardware limitations.These limitations vary by CPU model.

1. Expansion Capabilities of S7-200 (Classic Series):

• Maximum Number of Modules: Up to7 expansion modules can be connected (including signal boards and signal modules, but signal boards do not occupy expansion slot positions).
• Maximum Digital I/O: The size of the digital image area limits the maximum number of I/O points, typically256 (128 inputs/128 outputs).
• Maximum Analog I/O: The size of the analog image area limits the maximum number of channels, typically64 (32 inputs/32 outputs).
• 5VDC Power Load: This isthe most critical hard limit! Each expansion module requires power from the CPU’s +5V DC supply. The +5V DC current that each CPU can provide is limited (for example, CPU 224 is about 660mA). You must ensure that the total 5V current consumption of all connected expansion modulesdoes not exceed the CPU’s power capacity.

Example: A CPU 224 (base 14 inputs/10 outputs) can be expanded to:

• 1 EM223 (8DI/8DO) + 1 EM235 (4AI/1AO) + 1 EM221 (8DI) + 1 EM277 (communication module).
• Check the manual to verify the 5V current consumption of each module to ensure the total does not exceed the limit.

2. Expansion Capabilities of S7-200 SMART (Next Generation Series):

The expansion capabilities of S7-200 SMART are stronger, and the design is more intuitive.

• Maximum Number of Modules: Up to6 expansion modules can be connected (excluding signal boards).
• CPU Model Determines Capability:Standard CPUs (e.g., SR20/ST20, SR30/ST30, SR40/ST40, SR60/ST60): Support all types of expansion modules (digital, analog, temperature, communication).Economical CPUs (e.g., CR20s, CR30s, CR40s, CR60s): Cannot connect expansion modules, only use base I/O and signal boards.
• Power Calculation: Power calculations are also required, including +5V DC (for expansion modules) and +24V DC (for sensor loads).

3. Address Allocation for Expansion Modules

The PLC system automatically allocates addresses for expansion modules, and the rules are very clear:

1. Sequential Allocation: Addresses areallocated consecutively, according to the physical connection order of the modules on the rail (from left to right: CPU -> first expansion module -> second…).
2. Independent Allocation: The address areas for digital I/O and analog I/O are separate.
3. Allocated in Bytes: Digital I/O is allocated in8 bits (one byte). Even if a module has only 4 points, it will occupy one byte of address.
4. Analog Allocated in Words: Analog I/O is allocated in16 bits (one word), with each channel occupying one word.

Example (S7-200 SMART):

Assuming the system configuration is:CPU ST30 (base: 18 inputs/12 outputs) -> EM DE08 (8DI) -> EM AQ04 (4AO) -> EM DR08 (8 relay outputs)

• CPU ST30 Base I/O Address: Inputs: I0.0 ~ I2.1 (total 18 points, occupying 3 bytes) Outputs: Q0.0 ~ Q1.5 (total 14 points, occupying 2 bytes)
• First Module EM DE08 (8DI): It is allocated addresses immediately after the base inputs. Inputs: I3.0 ~ I3.7 (occupying 1 byte)
• Second Module EM AQ04 (4AO): Analog output addresses start from AQW0. Outputs: AQW0, AQW2, AQW4, AQW6 (4 channels, each occupying 1 word)
• Third Module EM DR08 (8DO): It is allocated addresses immediately after the base outputs. Outputs: Q2.0 ~ Q2.7 (occupying 1 byte)

When programming, you only need to use these system-automatically allocated addresses.

4. Summary of Expansion Configuration Steps

1. Assess Requirements: Count how many digital inputs/outputs, analog inputs/outputs are needed, and whether special communication is required.
2. Select CPU: Choose a CPU based on control complexity and basic I/O requirements.
3. Select Expansion Modules: Choose suitable expansion modules from the product catalog based on the number and type of missing I/O points.
4. Verify Hardware Limitations: Be sure to consult the technical specification manual of the selected CPU and expansion modules, perform5VDC power verification, and ensure the total current consumption is within the CPU’s capacity.
5. Allocate Addresses: Determine the connection order of the modules and list the final I/O address allocation table for programming.

Practical Recommendations

• Consult Manuals: Siemens’ official website provides very comprehensive manuals, which are the most authoritative source of information.
• Use Selection Tools: Siemens provides online or offline selection tools (such as theS7-200 SMART Product Selection Guide), which can automatically verify power and configuration.
• Leave a Margin: When selecting, it is recommended to leave a 10%-20% margin for I/O points and power for future project modifications or upgrades.