Siemens 6ES5900-7AA21 CPU Module | 8KB 8 Inputs 6 Outputs

The Siemens 6ES5900-7AA21 CPU Module serves as the central processor for SIMATIC S5-100U systems. This compact unit executes user logic and manages I/O scanning. You will find it in small to medium-sized automation applications. It includes integrated digital I/O for basic control tasks. Therefore, this unit suits conveyor systems, packaging machines, and assembly lines.

The Siemens 6ES5900-7AA21 CPU Module measures 140 mm in height and 90 mm in width. Its depth reaches 55 mm for standard mounting. The unit weighs approximately 0.48 kilograms. It contains one 8-bit processor and 8 KB of user memory. Eight digital inputs and six digital outputs sit on the front panel. Additionally, you get one programming port for software uploads.

Key Technical Specifications

Parameter	Value
Manufacturer	Siemens AG
Model	6ES5900-7AA21
Product Type	CPU Module
Processor	8-bit @ 10 MHz
User Memory	8 KB RAM (battery-backed)
Integrated Inputs	8 (24 VDC)
Integrated Outputs	6 (24 VDC, 0.5A)
I/O Capacity	64 digital points max
Scan Time	5 ms per 1K logic
Height	140 mm
Width	90 mm
Depth	55 mm
Weight	0.48 kg
Programming Port	15-pin D-sub
Status LEDs	8 (PWR, RUN, ERR, I/O)
Energy Storage	≈ 300 µF (capacitive)
Battery	Lithium (memory backup)
Power Consumption	150 mA @ 5 VDC
Operating Temperature	0°C to +60°C

Internal Architecture and Energy Storage

The Siemens 6ES5900-7AA21 CPU Module stores minimal energy in its power filter. Small electrolytic capacitors hold approximately 300 microfarads total capacitance. An 8-bit processor runs user logic at 10 MHz. This processor includes a real-time clock for time-stamped events. Eight KB of battery-backed RAM stores user programs and data. A lithium battery maintains memory during power-down for up to 2 years. Eight optical isolators protect the inputs from electrical noise. Six transistor outputs drive small relays and indicators directly.

Connection Interfaces and Wiring Details

You will find two 10-pin terminal blocks on the bottom of the module. One block accepts eight 24 VDC input wires and two commons. The other block connects six 24 VDC output wires and two returns. Connect the 24 VDC supply positive to the input common terminals. Connect field sensors to the individual input channel terminals. The Siemens 6ES5900-7AA21 CPU Module includes a 15-pin D-sub programming port. Use a standard programmer cable to connect to a PC. Eight LEDs show power, run, error, and I/O status. A green RUN LED lights up when the CPU executes logic normally. A red ERR LED indicates a hardware or program fault.

Configuration and Installation Steps

You mount this module onto a standard DIN rail or panel. First, hook the top clip onto the 35 mm DIN rail securely. Then, press the bottom downward until it locks in place. Connect the 24 VDC power supply to the designated terminals. The Siemens 6ES5900-7AA21 CPU Module consumes 150 mA from the supply. Use STEP 5 software to write and download your control program. Connect the programmer to the 15-pin D-sub port. Set the CPU to RUN mode using the front panel switch. Verify that the green RUN LED remains steady after startup. Test all inputs and outputs before connecting to field equipment.