The **FST3345WMX** is a high-speed CMOS 8-bit bus switch produced by Fairchild Semiconductor (now part of ON Semiconductor). It is primarily used for signal routing, isolation, and voltage level shifting in high-performance computing and telecommunications systems. --- ### ## Technical Specifications Below are the primary electrical and physical characteristics of the FST3345WMX: | Parameter | Specification | | :--- | :--- | | **Logic Family** | FST (Bus Switch) | | **Number of Bits** | 8-Bit (Octal) | | **Configuration** | 8:8 (Single Ended) | | **Supply Voltage (Vcc)** | 4.0V to 5.5V | | **On-Resistance (Ron)** | 4Ω (Typical) | | **Propagation Delay** | 0.25 ns (Max) | | **Package Type** | SOIC-20 (Wide) | | **Operating Temperature** | -40°C to +85°C | --- ### ### Key Features 1. **Near-Zero Propagation Delay:** Because the device uses a simple NMOS pass transistor, it does not add significant timing delays to the high-speed bus. 2. **Low On-Resistance:** The 4Ω typical resistance ensures minimal signal distortion and voltage drop across the switch. 3. **Undershoot Hardened:** The internal circuitry is designed to handle signals that might briefly drop below ground level without triggering latch-up. 4. **Level Shifting:** It can be used to interface between 5V systems and 3.3V systems by limiting the output voltage through the Vcc supply. --- ### ### Pin Configuration and Functionality The FST3345 utilizes an Output Enable (**OE**) pin to control the connection between the 'A' bus and the 'B' bus. * **A0–A7:** Input/Output ports for Bus A. * **B0–B7:** Input/Output ports for Bus B. * **OE (Active Low):** When OE is LOW, the A and B ports are connected. When OE is HIGH, the switch is open (High-Impedance state). --- ### ### Typical Application Circuit In a typical setup, the FST3345 acts as a bridge between two data buses. Below is a conceptual representation of how it is used for bus isolation: ```text Vcc (5V) | [FST3345] /-------\ A0--|S S|--B0 A1--|S S|--B1 <-- Low Ron Connection ... | | A7--|S S|--B7 \-------/ | / \ [OE] --- (Control Signal: Low = Connect, High = Isolate) ``` --- ### ### Usage Scenarios * **Hot-Plugging:** Protecting sensitive circuitry by isolating the bus during the insertion of peripheral cards. * **Bus Multiplexing:** Switching between different memory banks or I/O devices. * **Voltage Translation:** Reducing a 5V TTL signal to a level compatible with 3.3V CMOS logic.