ISA Floppy Disk and Serial Controller

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

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.

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: 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: ON - ROM selected on A17 == 0; OFF - ROM selected on A17 == 1

  • SW2.4: ON - ROM selected on A16 == 0; OFF - ROM selected on A16 == 1

  • SW2.5: ON - ROM selected on A15 == 0; OFF - ROM selected on A15 == 1

  • SW2.6: ON - ROM selected on A14 == 0; OFF - ROM selected on A14 == 1

  • SW2.7: ON - ROM selected on A13 == 0; OFF - ROM selected on A13 == 1

  • SW2.8 - Unused

ROM addresses configuration table

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.

Known Issues

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

PCB Versions

Version 1.0 (Current)

• • Fixed all the errata from the prototype version.

  • Changed description of SW2.3 - SW2.7 on the back silk screen. Now it lists ROM start addresses instead of address bits.

  • Errata (to be fixed in the next version):

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

Version 0.9 (Prototype Version)

• • Fully functional

  • Errata (to be fixed in the next version):

    • 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).