Cromemco 4FDC
S-100 floppy controller with RDOS and RS-232 serial.
The Cromemco 4FDC is a floppy-disk controller board that connects both 5.25-inch and 8-inch drives to the S-100 bus, the standard used in Cromemco and other IEEE 696 computers. It also includes an RS-232 serial port whose baud rate can be set in software from 110 to 76,800. Onboard is a 1 KB 2708 ROM that holds Cromemco's RDOS, the resident disk operating system.
The 4FDC was built to work with Persci 277 8-inch single-density drives. Those drives had two unusual features: they used a fast voice coil actuator instead of a stepper motor to move the read/write head, and the data separator electronics were located on the drive itself. Because of that second feature, an unmodified 4FDC cannot be used with 8-inch drives that lack single-density data separators in their own electronics. Later Cromemco controllers, the 16FDC and 64FDC, included both single- and double-density data separators, and the 64FDC also added write pre-compensation.
An aftermarket add-on board called the FDCX4, made by JVB Electronics, provided double-density capability for the 4FDC. It was a daughterboard that replaced the original WD1771 single-density controller chip with either an FD1791 (early units) or a Fujitsu MB8876A (later units). The FDCX4 used an analog phase-locked-loop data separator in all modes and also applied write pre-compensation. These additions let a 4FDC equipped with the FDCX4 reliably use Persci 277 drives and other drives in double-density mode.
Four switches on the 4FDC card control its operation. Switch 1 is the RDOS disable switch: when ON, the 1 KB ROM cannot be accessed; when OFF, RDOS occupies memory addresses 0xC000 to 0xC3FF. Switch 2 is the RDOS disable after boot switch: when ON, the ROM is automatically disabled after CDOS is loaded from disk (by an output to port 40H), freeing that memory space; when OFF, RDOS stays in memory even after CDOS loads. RDOS contains two programs: a CDOS bootstrap loader and a console monitor. Switch 3 is the boot enable switch: when ON, the bootstrap runs immediately, loading CDOS without entering the monitor; when OFF, RDOS starts in monitor mode, allowing the bootstrap or other operations under console control. Switch 4 is the initialization inhibit switch: when ON, diskettes cannot be initialized under software control, protecting against accidental alteration; it must be OFF to initialize diskettes.
- Bus
- S-100 (IEEE 696)
- Rom
- 1 KB 2708 containing RDOS
- Serial baud rates
- 110 to 76,800
- Supported drives
- Persci 277 8-inch single-density floppy drives
Lore & Background
The 4FDC was designed to drive Persci 277 8-inch single-density floppy drives, which used a fast voice coil actuator instead of a stepper motor to position the read/write head, and had data separator electronics on the drive itself. Because the data separator was on the drive, an unmodified 4FDC cannot be used with 8-inch drives that lack single-density data separators on their electronics. Later Cromemco disk controllers such as the 16FDC and 64FDC contained both single- and double-density data separators, and the 64FDC also supplied write pre-compensation.
An aftermarket add-on board, the FDCX4 Double Density Upgrade Board for the 4FDC, was designed and marketed by JVB Electronics. This daughter board replaced the WD1771 single-density disk controller chip on the 4FDC with an FD1791 (early production) or Fujitsu MB8876A (later production) double-density controller chip. The FDCX4 used an analog phase-locked-loop data separator in all modes and write precompensation, allowing the equipped 4FDC to reliably use Persci 277 drives and others in double-density mode.
Four switches on the 4FDC interface card set its operation. Switch 1 disables the RDOS ROM; switch 2 disables RDOS after CDOS is bootstrapped; switch 3 enables automatic bootstrap; switch 4 inhibits diskette initialization under software control. The board includes a ripple priority circuit compatible with the Cromemco TU-ART.
Reader's Guide
The Cromemco 4FDC represents a transitional design in early microcomputer storage, bridging single-density 8-inch drives and the S-100 bus ecosystem. Its integration of an RS-232 serial channel and a resident disk operating system (RDOS) in a 1 KB ROM made it a self-contained controller for Cromemco systems. The controller's reliance on drives with onboard data separators, such as the Persci 277, limited its compatibility with other 8-inch drives, a constraint later addressed by Cromemco's own 16FDC and 64FDC controllers. The aftermarket FDCX4 upgrade extended the 4FDC's life by adding double-density capability and write precompensation, allowing it to work reliably with Persci 277 drives in double-density mode. The board's switch-configurable ROM behavior—including automatic disabling after CDOS boot—reflected a design philosophy that prioritized both flexibility and protection against accidental disk initialization. The 4FDC's priority chain and interrupt handling were designed to interoperate with other Cromemco peripherals like the TU-ART, cementing its role in a cohesive system architecture.
Did You Know?
- The 4FDC uses a voice coil actuator in Persci 277 drives rather than a stepper motor for head positioning.
- The FDCX4 upgrade board replaced the WD1771 chip with an FD1791 or Fujitsu MB8876A for double-density operation.
- Switch 4 on the 4FDC can inhibit diskette initialization to prevent a runaway program from altering diskettes.
- The 4FDC's RDOS ROM can be automatically disabled after CDOS is bootstrapped, freeing memory space from 0xC000 to 0xC3FF.
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