cuttercn / isa-fdc

ISA Floppy Disk and Serial Controller

isa-fdc

ISA Floppy Disk and Serial Controller

Introduction

ISA Floppy Disk and Serial Controller card provides one floppy disk interface supporting up to two floppy drives, an RS-232 serial interface, and a BIOS extension ROM.

Specifications

• Floppy disk controller based on Intel 82077AA or National Semiconductor PC8477 FDC IC.
  • Supports IBM PC, AT, and PS/2 floppy types from 160 KB 5.25" single side disks to 2.88 MB 3.5" ED (Extended Density) disks.
  • Supports 2.88 MB IBM PS/2 drives (e.g. IBM FRU 64F4148 and IBM FRU 64F0204), that require +5V on pin 3. This feature is enabled using jumper JP1.
• Serial port.
  • Uses UART IC in PLCC package, supports newer UART versions such as 16C650 and 16C750 with extended FIFO.
  • Also compatible with 16550 and 16450 ICs in PLCC package.
  • UART I/O address and interrupt is selectable using a DIP switch. Supported I/O addresses: 3F8h, 2F8h, 3E8h, 2E8h. Supported interrupts: IRQ3, IRQ4, IRQ5, IRQ7.
• 8 KiB BIOS extension ROM.
  • Supports 28C64 EEPROMs and 27C64 UV erasable EPROMs.
  • The ROM is configured using a DIP switch: It can be enabled or disabled; the /WR line can be disabled; the address can be selected from 0C0000h to 0EE000h in 8 KiB increments.
• All functions above are optional (can be either installed or not)

Firmware / BIOS Extension

Multi-Floppy BIOS Extension

Hardware Documentation

Schematic and PCB Layout

Schematic - Version 1.0

PCB Layout - Version 1.0

Configuration Switches and Jumpers

SW1 - Serial Port (UART) Configuration

• SW1.1 - SW1.4 - Serial port IRQ selection
  • SW1.1: ON - IRQ3 (default setting for COM2 and COM4)
  • SW1.2: ON - IRQ4 (default setting for COM1 and COM3)
  • SW1.3: ON - IRQ5
  • SW1.4: ON - IRQ7

Note: Only one of SW1.1 - SW1.4 should be ON. All switches can be off if software doesn't use interrupt based I/O.

• SW1.5 - SW1.8 - Serial port base address selection
  • SW1.5: ON - 0x2E8 (COM4)
  • SW1.6: ON - 0x3E8 (COM3)
  • SW1.7: ON - 0x2F8 (COM2)
  • SW1.8: ON - 0x3F8 (COM1)

Note: One and only one of SW1.5 - SW1.8 switches should be ON.

SW2 - ROM Configuration

• SW2.1 - SW2.7 - ROM Configuration
  • SW2.1: ON - Enable ROM; OFF - Disable ROM
  • SW2.2: ON - Enable EEPROM write. This jumper is only relevant for EEPROM. It should be OFF if EPROM is used.
  • SW2.3 - SW2.7 - ROM address. Refer to the table below
• SW2.8 - Unused

ROM Addresses Configuration Table

SW2.3	SW2.4	SW2.5	SW2.6	SW2.7	Start Address	End Address	SW2.3	SW2.4	SW2.5	SW2.6	SW2.7	Start Address	End Address
ON	ON	ON	ON	ON	0xC0000*	0xC1FFF	OFF	ON	ON	ON	ON	0xE0000**	0xE1FFF
ON	ON	ON	ON	OFF	0xC2000*	0xC3FFF	OFF	ON	ON	ON	OFF	0xE2000**	0xE3FFF
ON	ON	ON	OFF	ON	0xC4000*	0xC5FFF	OFF	ON	ON	OFF	ON	0xE4000**	0xE5FFF
ON	ON	ON	OFF	OFF	0xC6000*	0xC7FFF	OFF	ON	ON	OFF	OFF	0xE6000**	0xE7FFF
ON	ON	OFF	ON	ON	0xC8000	0xC9FFF	OFF	ON	OFF	ON	ON	0xE8000**	0xE9FFF
ON	ON	OFF	ON	OFF	0xCA000	0xCBFFF	OFF	ON	OFF	ON	OFF	0xEA000**	0xEBFFF
ON	ON	OFF	OFF	ON	0xCC000	0xCDFFF	OFF	ON	OFF	OFF	ON	0xEC000**	0xEDFFF
ON	ON	OFF	OFF	OFF	0xCE000	0xCFFFF	OFF	ON	OFF	OFF	OFF	0xEE000**	0xEFFFF
ON	OFF	ON	ON	ON	0xD0000	0xD1FFF	OFF	OFF	ON	ON	ON	0xF0000***	0xF1FFF
ON	OFF	ON	ON	OFF	0xD2000	0xD3FFF	OFF	OFF	ON	ON	OFF	0xF2000***	0xF3FFF
ON	OFF	ON	OFF	ON	0xD4000	0xD5FFF	OFF	OFF	ON	OFF	ON	0xF4000***	0xF5FFF
ON	OFF	ON	OFF	OFF	0xD6000	0xD7FFF	OFF	OFF	ON	OFF	OFF	0xF6000***	0xF7FFF
ON	OFF	OFF	ON	ON	0xD8000	0xD9FFF	OFF	OFF	OFF	ON	ON	0xF8000***	0xF9FFF
ON	OFF	OFF	ON	OFF	0xDA000	0xDBFFF	OFF	OFF	OFF	ON	OFF	0xFA000***	0xFBFFF
ON	OFF	OFF	OFF	ON	0xDC000	0xDDFFF	OFF	OFF	OFF	OFF	ON	0xFC000***	0xFDFFF
ON	OFF	OFF	OFF	OFF	0xDE000	0xDFFFF	OFF	OFF	OFF	OFF	OFF	0xFE000***	0xFFFFF

Notes:

• * - Address range conflicts with EGA / VGA BIOS.
• ** - 0xE0000 - 0xEFFFF address range is not available on IBM AT - Reserved for on-board BIOS extension ROMs
• *** - 0xF0000 - 0xFFFFF address range is not available on IBM PC, IBM XT, and IBM AT - Used system BIOS or reserved for on-board BIOS extension ROMs

When setting ROM address make sure it doesn't conflict with system BIOS and with BIOS extension ROMs of other cards installed in the system.

JP1 - Connect +5V to pin 3 of floppy interface

This jumper is only required by some IBM PS/2 floppy drives that lack a separate power connector and use pin 3 for +5V. DO NOT install it in any other case as it will cause a short circuit.

Bill of Materials

Component Type	Reference	Description	Quantity	Possible sources and notes
PCB		ISA FDC and Serial PCB	1	Refer to the RetroBrew Computers Board Inventory page for ordering information, or order from a PCB manufacturer of your choice using provided Gerber or KiCad files
Integrated Circuit	U1	Intel N82077AA-1 or National Semiconductor PC8477BV-1 FDC	1	eBay. National Semiconductor PC8477BV-1 (recommended); Intel N82077AA (recommended, no tape support); Intel N82077AA-1 (tape support, FM broken); Intel N82077AA-5 (doesn't support 1 Mbps rate / ED disks); National Semiconductor PC8477AV-1 (older version of PC8277BV-1)
Integrated Circuit	U2	16550 or compatible UART in PLCC-44 package	1	16550/16C550 (16-byte FIFO, AT standard): Mouser 771-SC16C550BIA44-T, 701-ST16C550CJ44-F, 701-ST16C550IJ44-F, 595-TL16C550CFNR, 595-TL16C550CIFN, 595-TL16C550CFN, 926-PC16550DV; 16C650 (32-byte FIFO): Mouser 701-ST16C650ACJ44-F; 16C750 (64-byte FIFO): Mouser 771-SC16C750BIA44518, 595-TL16C750FN; 16C450 (no FIFO, not recommended): Mouser 595-TL16C450FN, 595-TL16C450FNG4, 701-ST16C450CJ44-F
Integrated Circuit	U3	GD75232, SN75185, or SN75C185 RS-232 Drivers/Receivers	1	GD75232: Mouser 595-GD75232N; SN75185: Mouser 595-SN75185N, 595-SN75185NE4; SN75C185 (recommended): Mouser 595-SN75C185NE4, 595-SN75C185N
Integrated Circuit	U4	28C64 EEPROM or 27C64 UV erasable EPROM	1	Mouser 556-AT28C64B15PU; Note: Atmel AT28C64B is the recommended part
Integrated Circuit	U5	74LS688 magnitude comparator	1	Mouser 595-CD74HCT688E, 771-74HCT688N, 595-SN74LS688N, 595-SN74LS688NE4
Integrated Circuit	U6, U7	74LS138 1-of-8 decoder	2	Mouser 595-SN74AHCT138NE4, 595-SN74AHCT138N, 771-74HCT138N, 595-SN74HCT138N, 595-CD74HCT138E, 863-MC74HCT138ANG, 512-MM74HCT138N, 595-SN74ALS138AN, 595-SN74ALS138ANE4, 595-SN74LS138N
Crystal Oscillator	U8	1.8432 MHz full DIP oscillator	1	Mouser 520-TCF184-X. Note: This oscillator is only required for the serial port function
Capacitor	C1 - C10	0.1 uF ceramic, 5.08 mm pitch	10	Mouser 810-FK28X7R1H104K, 80-C323C104K5R
Capacitor	C11 - C14	10 uF, 16V, multilayer ceramic or tantalum, 5.08 mm pitch	4	Mouser 810-FK24X5R1C106K
Capacitor	C15, C16	22 pF ceramic, 5.08 mm pitch	2	Mouser 581-SR215A220KARTR1
Capacitor	C17	4.7 nF ceramic, 5.08 mm pitch	1	Mouser 581-SR215C472KAA. Note: This capacitor is only required for Intel 82077AA FDC (U1)
Resistor Array	RR1	10k, 10 pin SIP, 9 resistors	1	Mouser 266-10K-RC. Note: This resistor array is only required for BIOS extension ROM function
Resistor Array	RR2	1k, 6 pin SIP, 5 resistors	1	Mouser 264-1.0K-RC. Note: This resistor array is only required for FDC function
Switch	SW1, SW2	16 pin DIP switch	2	Mouser 571-54356405. Note: SW1 is required for the serial port function; SW2 is required for BIOS extension ROM functon
Connector	P1	17x2 pin header	1	Mouser 737-BHR-34-VUA. Note: This connector is only required for FDC function
Connector	P2	DE9M, PCB mount, right angle	1	Mouser 806-K22X-E9P-NJ15-99. Note: This connector is only required for the serial port function
Connector	JP1	2 pin header	1	Mouser 649-68002-102HLF. Note: This connector is only required for IBM PS/2 floppy disk drives that get +5V on pin 3 of floppy disk connector. DO NOT INSTALL it in any other case
IC Socket	U1	68 pin PLCC through hole socket	1	Mouser 517-8468-11B1-RK-TP, 737-PLCC-68-AT. Note: This socket is only required for FDC function
IC Socket	U2	44 pin PLCC through hole socket	1	Mouser 517-8444-11B1-RK-TP, 737-PLCC-44-AT. Note: This socket is only required for the serial port function
IC Socket	U3, U5	20 pin 300 mil (narrow) DIP socket	2	Mouser 517-4820-3000-CP. Note: U3 socket is only required for the serial port function; U5 socket is only required for BIOS extension ROM function
IC Socket	U4	28 pin 600 mil (wide) DIP socket	1	Mouser 517-4828-6000-CP. Note: U4 socket is only required for BIOS extension ROM function
IC Socket	U6, U7	16 pin 300 mil DIP socket	2	Mouser 517-4816-3000-CP. Note: U6 and U7 sockets are required for the serial port and FDC functions
IC Socket	U8	4 pin 300 mil full size oscillator socket	1	Mouser 535-1107741. Note: U8 socket is only required for the serial port function
Crystal	X1	24 MHz, 12 pF crystal	1	Mouser 774-ATS240C. Note: X1 is only required for FDC function
Bracket		ISA card bracket - Keystone 9200-1 (with DE9 cut out) for cards with serial port function, Keystone 9202 (with ears mounting), for cards without serial port function	1	Mouser 534-9200-1 - For cards with serial port function / with P2 connector, or a 3D printed bracket; Mouser 534-9202 - For cards without serial port function / without P2 connector
Screw		Screw, 4-40 thread, 1/4" length	2	Mouser 534-9900

Component Selection and Substitution

• FDC IC (U1) - Either National Semiconductor PC8477 or Intel 82077AA FDC ICs can be used. These ICs come in several versions:
  • National Semiconductor PC8477BV-1 - is the later version of PC8477 FDC and it is the recommended IC
  • National Semiconductor PC8477AV-1 - earlier version of PC8477 (a couple of subtle differences are listed in PC8477B datasheet)
  • Intel N82077AA - original version of 82077AA
  • Intel N82077AA-1 - later version of 82077AA it includes tape support, but FM support is broken (Note: FM is not used by any IBM PC disk formats, and only required for some older disk formats)
  • Intel N82077AA-5 - Similar to Intel N82077AA-1 but lacks 1 Mbps transfer rate support, which is required for ED (extended density, 3.5" / 2.88 MB) disks.
• UART IC (U2) - Any 16450 / 16550 compatible UART in PLCC package can be used.
  • 16C550 UART is recommended.
  • UARTs with extended FIFO such as 16C650 and 16C750 can be used. Note that software support is required to take advantage of the extended FIFO. If software lacks such support, these UARTs will function in 16550/16450 compatible mode.
  • 16450/16C450 lacks FIFO and not recommended.
• ROM (U4) - Either EEPROM, or UV erasable EPROM can be used.
  • Atmel AT28C64B is recommended part (and it likely will be supported by built-in BIOS configuration utility)
  • Other 28C64 parts can be used, but they don't provide software controlled write protection
  • 27C64 UV erasable EPROM can be used. In this case BIOS needs to be configured prior to programming the EPROM.
• 74-series logic ICs (U5, U6, U7). Any TTL-LS or TTL compatible CMOS ICs can be used.
  • TTL compatible CMOS 74AHCT/74HCT recommended for lower power consumption and better performance.
  • Advanced TTL such as 74ALS and 74F are preferred TTL families.
  • 74LS will function as well.
• RS232 drivers/receivers IC (U3) - GD75232 or compatible chips should be used
  • SN75C185 recommended for lower power consumption
• Crystal Oscillator U8 - TTL or TTL compatible 5V CMOS in full can package.
  • If an oscillator with enable / tri state function is used it might be needed to connect enable pin (pin 1) to VCC. Please consult with oscillator's datasheet.
  • A half can oscillator can be used, in this case connect pin 11 to pin 14 with a piece of wire
• Power filtering capacitors (C11 - C14)
  • Multilayer ceramic, tantalum, or electrolytic capacitors can be used. Please observe polarity when installing tantalum and polarized electrolytic capacitors.

Construction Notes

Function Configuration

The ISA Floppy Disk and Serial Controller card implements three independent functions: Floppy Disk Controller (FDC), Serial Port Controller (UART), and BIOS extension ROM (ROM). A builder can choose to build the card with any combination of these functions. For example if the card will be used in an Xi 8088 or an AT compatible system that has extended floppy support, the BIOS extension ROM is not required.

Shared Components

• Only power filtering capacitor C11 and the PCB is shared by all functions.
• U6 and U7 ICs are shared by FDC and UART functions, and so are their bypass capacitors - C6 and C7.
• ISA bracket type depends on presence of P2 (serial) connector:
  • If Serial Port is assembled (and P2 is present), the Keystone 9200-1 bracket should be used. This bracket has a cut out for DE9 connector and mounted on the P2.
  • If Serial Port is not assembled, the Keystone 9202 bracket should be used. This bracket has two mounting ears and it is mounted on the PCB.

Floppy Disk Controller Components

• U1, X1, C1, C12, C15, C16, C17, C7, RR2, P1, JP1
  • C17 can be omitted if National Semiconductor PC8477 FDC IC is used.
  • JP1 is only required for IBM PS/2 drives that don't have a separate power connector and use pin 3 of the floppy interface connector for power. It is recommended NOT to install this header if card is to be used with standard floppy drives (installing a jumper at this header with regular floppy drives will cause short circuit of +5V to GND).

Serial Port Components

• U2, U3, U8, C2, C3, C8, C9, C10, C13, C14, P2, SW1

BIOS extension ROM Components

• U4, U5, C4, C5, RR1, SW2

Changes

Version 1.0

• Fixed all the errata from the 0.9 prototype version
• Changed description of SW2.3 - SW2.7 on the back silk screen. Now it lists ROM start addresses instead of address bits

Version 0.9

• Prototype version
• Fully functional

Errata

All versions

• IBM AT BIOS can throw error 601 during POST if the original IBM Fixed Disk and Diskette Drive Adapter is not installed or replaced with another floppy disk controller. The system will boot and work normally after pressing F1

Version 1.0

• Cut and paste error in 0E0000h-0E6000h switch description. SW2.3 should be OFF (0) instead of ON (1).

Version 0.9

• Connector P1, pin 1 diameter needs to be increased
• Description for SW1 on the silk screen needs to be updated. SW1.1-SW1.4 description needs to be switched with SW1.5-SW1.8 description.
• Update board name to "ISA Floppy Disk and Serial Controller"
• Make ground traces wider (where ground plane connects to the ISA connector).