The **FST3126QSC** is a high-speed CMOS TTL-compatible 4-bit bus switch manufactured by Fairchild (now part of ON Semiconductor). It is designed to connect or isolate two different nodes on a data bus with minimal propagation delay. --- ## 1. Device Overview The FST3126 consists of four independent high-speed CMOS FET switches. Each switch has its own independent active-HIGH Enable (**OE**) input. When the enable pin is HIGH, the switch is closed (ON), allowing signals to flow between Port A and Port B. When LOW, the switch is open (OFF), and a high-impedance state exists between the ports. ### Key Specifications | Parameter | Value | | :--- | :--- | | **Logic Family** | FST (Bus Switch) | | **Number of Channels** | 4-Bit | | **Supply Voltage (Vcc)** | 4.0V to 5.5V | | **On-Resistance (R_on)** | 4Ω (Typical) | | **Propagation Delay** | 0.25 ns (Max) | | **Package Type** | QSOP-16 (QSC) | --- ## 2. Pin Configuration (QSOP-16) The "QSC" suffix refers specifically to the **QSOP** (Quarter Size Outline Package) with 16 pins. | Pin Number | Name | Description | | :--- | :--- | :--- | | 1, 4, 10, 13 | **OE 1-4** | Output Enable (Active HIGH) | | 2, 5, 9, 12 | **A 1-4** | Port A (Input/Output) | | 3, 6, 8, 11 | **B 1-4** | Port B (Input/Output) | | 7 | **GND** | Ground | | 16 | **Vcc** | Power Supply | --- ## 3. Core Features & Advantages * **Zero Propagation Delay:** Because it uses an NMOS pass transistor, it does not add significant timing delay to the circuit, making it faster than standard logic buffers. * **Low On-Resistance:** The 4Ω resistance ensures minimal voltage drop across the switch. * **Bidirectional Flow:** Signals can pass from A to B or B to A with no directional control pin required. * **High Impedance in Power-Off:** When $V_{cc} = 0V$, the device is effectively isolated, preventing backflow of current. --- ## 4. Typical Applications 1. **Bus Isolation:** Separating a heavily loaded bus into smaller segments to improve signal integrity. 2. **Voltage Level Translation:** When used with a diode or specific biasing, it can translate 5V TTL levels down to 3.3V LVTTL levels. 3. **Hot-Plugging:** Protecting sensitive components during live insertion of cards or modules. 4. **Signal Gating:** Turning on or off specific data paths in complex micro-controller environments. ---