4FDC Floppy Disk Controller
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The Cromemco 4FDC Disk controller is designed to interface both 5 inch and 8 inch floppy disk drives to the S-100 computer bus used in Cromemco and other IEEE 696 computers.
In addition the 4FDC contains an RS232 Serial Io channel with software selectable baud rates from 110 to 76,800 baud.
The 4FDC controller also has a 1K resident 2708 ROM containing Cromemcos RDOS that is the Resident Disk Operating System.
The 4FDC was designed to drive Persci 8 inch floppy drives. These drives were interesting in 2 respects
- They used a fast voice coil actuator and not a stepper motor to position the drive read write head
- The data separator electronics was on the drive itself
Due to the second fact the 4FDC could not easily drive other types of drive which assumed the data separator was on the disk controller itself. In practical terms this means that should you want to an S100 card to control an 8 inch diskette drive you would be better off using the 16FDC or later Cromemco controller.
[edit] Technical Notes
Four switches on the 4FDC interface card are used to set the operation of the card. Switch 1 is the RDOS DISABLE switch. When this switch is ON the lK ROM containing RDOS cannot be accessed by the computer. When this switch is OFF the RDOS program resides in the computer memory space from address COOO to C3FF.
Switch 2 is the RDOS DISABLE AFTER BOOT switch. If this switch is ON the lK ROM containing RDOS will automatically be disabled after CDOS is bootstrapped in from a disk thus clearing memory space from COOO to C3FF for system use. (In this mode the ROM is actually disabled by an output to port 40H which is done automatically by CDOS). If switch 2 is OFF, RDOS remains in memory space even after CDOS is loaded.
RDOS contains two programs; 1) the CDOS bootstrap program and 2) the console monitor program. Switch 3 is the BOOT ENABLE switch. When this switch is ON the bootstrap program will execute (thus loading CDOS) without first entering the monitor program. If this switch is off, RDOS begins in the console monitor mode permitting the bootstrap operation or other operations to be performed under console control.
Switch 4 is the INITIALIZATION INHIBIT switch. When this switch is ON, diskettes cannot be initialized under software control thus preventing a "runaway," program from unintentionally altering the diskette initialization. This switch must be OFF when initializing diskettes.
- Western Digital FD1771-1 Interfaces
All signals from the drives are TTL .buffered. and have 150 ohm pullups. Maxi and mini signals are wired and at the pullup side of the buffers. Signals which. do not apply to the mini (i.e. ,.READY and SEP CLOCK), are disabled and pulled high when the mini is selected. Signals to the drives from the 1771 are TTL buffered with separate buffers for mini and. maxi connectors. The STEP output is stretched by IC37 to about 16 microseconds before going to the .drives.. The HLD (head load) output does not go directly to the drives but rather enables the drive select lines through IC10 Pin1. Thus, the actual drive select signal to the drive is the coincidence of a latched drive selection (done at port 34H) and HLD from the 1771. Head loading time is determined by counters IC36. and 27. timeout is controlled by the count loaded into lC3'S by IC53. Signals DRQ, HLD, and INTRQ (or EOJ) are available at input port 34H (rC9). Various control signals are assigned to output port 34H and are latched by rcs 24 and 41.
- Board Priority Chain
The 4FDC includes a ripple priority circuit which will defeat the interrupt acknowledge cycle of Priority IN/ is held low. If the 4FDC is allowed to perform the interrupt acknowledge, it will pull down its Priority Out/ line to signal others in the chain not to respond. This chain is compatible with the Cromemco TU-ART.
