The **FST193** (often associated with the prefix **ST FST193**) is a high-speed CMOS 10-bit bus switch. It is designed to provide high-speed switching and level shifting for digital signals in computing and telecommunications systems. --- ## 1. Core Technical Specifications The FST193 belongs to the "Bus Switch" family of integrated circuits. Its primary function is to connect or disconnect two buses without adding significant propagation delay. | Feature | Specification | | :--- | :--- | | **Logic Family** | FST (Fast CMOS) | | **Channel Count** | 10-Bit (10 individual switches) | | **Voltage Range** | 4.0V to 5.5V (Standard TTL Compatible) | | **Propagation Delay** | < 0.25 ns (Negligible) | | **On-Resistance ($R_{on}$)** | ~5 $\Omega$ (Typical) | | **Package Types** | SSOP, TSSOP, QSOP | --- ## 2. Pinout and Internal Logic The device is typically organized as a 10-bit bus switch with one or more output enable signals. ### Key Pins: * **$A_0 - A_9$**: Port A Inputs/Outputs. * **$B_0 - B_9$**: Port B Inputs/Outputs. * **$\overline{OE}$ (Output Enable)**: Active-low pin. When pulled low (0V), the switch is closed (A is connected to B). When high ($V_{CC}$), the switch is open (high impedance). --- ## 3. Key Electronic Characteristics ### A. Low Propagation Delay Unlike buffers or transceivers, the FST193 does not have a "drive" stage. It acts like a mechanical wire when the switch is closed. This results in near-zero delay, making it ideal for high-speed memory buses. ### B. Level Shifting Capability The FST193 can be used for voltage translation (e.g., 5V to 3.3V). By adding a diode in series with the $V_{CC}$ pin, the output voltage swing can be limited to protect lower-voltage downstream components. ### C. Hot-Plug Protection The device is designed to support "Hot-Swapping." Its high-impedance state during power-down prevents unwanted current backflow into the system bus. --- ## 4. Common Applications * **Memory Interleaving**: Switching between two different banks of RAM. * **Bus Isolation**: Separating a motherboard bus from a peripheral slot to prevent noise or loading. * **Voltage Translation**: Interfacing 5V legacy logic with 3.3V modern microcontrollers. * **Port Multiplexing**: Routing data from one source to multiple destinations. ---