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===== SED9420 Data Separator =====

The SED9420 chip (specifically the SED9420CAC) is a CMOS VFO (Variable Frequency Oscillator) type LSI data separator designed for floppy disk interfaces. In Amstrad CPC and ZX Spectrum +3 systems, this component worked in conjunction with the uPD765 floppy disk controller (or variants such as the Z7065A) to manage data reading from the disk drive. 

{{:hardware:imagenes:sed9420.JPG?340|}}

Below are its technical functions and its role in Amstrad equipment:

**Main Functions**

  * __Data Separation__: Its primary function is to separate the data signal from the mixed clock signal coming from the floppy drive, allowing the controller (uPD765) to process the information correctly.
  * __Synchronization Detection__: It includes integrated synchronization field (SYNC) detection and loop filtering, reducing the need for many external components.
  * __Density Support__: It is capable of handling single density (FM) and double density (MFM).
  * __Integrated Timer__: It features a timing circuit for functions such as head load time or motor start-up.

**Technical Specifications**

  * __Compatibility__: Designed to work with popular controllers of the era such as the uPD765, uPD765A, FD1791/93, and MB8876/77.
  * __Pins and Voltage__: It is a 24-pin chip powered by +5V.
  * __Output Clocks__: It generates a clock signal for the floppy disk controller (FDC clock), typically 1 MHz for 5.25" disks and 2 MHz for 8" disks.

**Use in Retro Hardware**

  * __Amstrad CPC__: Found in models with integrated disk drives such as the CPC 6128 or CPC 664, as well as in the DDI-1 external interface for the CPC 464.
  * __Sinclair/Amstrad Spectrum__: It was also used in the ZX Spectrum +3, manufactured by Amstrad after the acquisition of Sinclair.
  * __Rarity__: Currently, it is considered a difficult component to find for repairs, sometimes requiring extraction from other vintage equipment (such as the Spectrum +3) to restore an Amstrad CPC.

**Chip Details**

  * __Appearance__: It is a 24-pin integrated circuit with a DIP (Dual In-line Package) encapsulation. It usually bears the inscription SED9420 or SED9420CAC.
  * __Board Location__: In an Amstrad CPC 6128, PCW, or a ZX Spectrum +3, you will typically find it near the uPD765A floppy disk controller. It is a fundamental part because the disk controller alone cannot directly interpret the analog signal coming from the drive; the SED9420 acts as the "translator" that separates data pulses from clock pulses.
  * __Availability__: Since it was manufactured specifically for these systems in the 80s, it is a scarce component today. Units found are usually salvaged from old boards or New Old Stock (NOS) from specialized retro-computing stores. 

If you are performing a repair, it is important to verify that the chip is exactly the CAC model, which is the most common variant used by Amstrad to ensure compatibility with the uPD765.

__**Documentation**__

At the following link, you can view the (PDF) documentation regarding the chip.

  * {{:hardware:descargas:SED9420.pdf|SED9420 chip information}}