# FDC+ Enhanced Floppy Disk Controller for the Altair 8800 — User's Manual Source: FDC+ Manual.pdf > Manual version 1/26/26 v2.0. Board revisions B and C are covered; where they > differ, the difference is called out. This file summarizes the parts a programmer > needs; cabling part numbers, mechanical mounting and marketing text are omitted. --- ## 1. Overview The Altair FDC+ (FarmTek, PC038) is a single-board, 100% compatible drop-in replacement for the original **two-board** MITS Altair 8" floppy disk controller (88-DCDD) and, in a different configuration, for the original two-board Altair Minidisk controller (88-MDS). From the perspective of the Altair CPU and software it is register- and timing-compatible with the originals. Key facts: - The controller registers occupy **4 consecutive I/O ports** (default `08h`–`0Bh`). - It emulates both the 8" hard-sectored drive (32 sectors) and the 5.25" Minidisk (16 sectors), selectable by a latched **Drive Type** setting. - It can synthesize **virtual hard-sector pulses** from soft-sectored media by syncing to the disk's rotational rate — transparent to software. - It carries up to **64K of on-board RAM** (1K increments) and **8K of PROM** (256-byte increments) that can be independently enabled and address-mapped. - It adds a **high-speed serial port** that can act as the controller for a PC-hosted "serial drive" (no rotating media), and also hosts an on-board monitor for firmware/config. - The internal interrupt fires at the **start of each sector** and can be routed to any Altair vectored interrupt line V0–V6 or to PINT. ### Enhancements beyond the original MITS controller - Drop-in replacement for the two-board Minidisk controller (incl. 8800bt "Foley" configuration). - Direct connection to Shugart SA-80x 8" drives made to look like Altair/Pertec 8" drives (media interchangeable with aligned Altair/Pertec drives). - Direct connection to Shugart SA-400 5.25" drives made to look like an Altair Minidisk (media interchangeable with a 48-TPI Altair Minidisk). - Accepts **soft-sectored** media as a substitute for hard-sectored media. - 5.25" high-density drive (e.g., Teac 55-GFR) usable as an Altair 8" drive. - 1.5 Mb drive support (HD 5.25" or DSDD 8") under a patched CP/M 2.2. - 8" IBM-3740 SSSD soft-sector support (iCOM/Pertec FD3712 emulation). - PC-hosted serial drive, including an 8 Mb virtual 8" drive. --- ## 2. I/O Port Map (register interface) By default the FDC+ responds to I/O addresses **08h–0Bh**. Cutting two traces and adding two jumpers (cross-wire A7↔A3) relocates it to the alternate block **80h–83h**. These are the SAME registers as the original 88-DCDD / 88-MDS. | Port (default) | Port (alt) | Read | Write | |----------------|-----------|----------------|----------------| | `08h` | `80h` | Drive Status | Drive Select | | `09h` | `81h` | Sector Position| Drive Command | | `0Ah` | `82h` | Read Data | Write Data | | `0Bh` | `83h` | Reserved | Reserved | > The FDC+ manual documents the port assignments but does NOT re-print the > individual bit fields of the Drive Status / Drive Select / Sector Position / > Drive Command / data registers — those are identical to the original MITS > 88-DCDD (8") and 88-MDS (Minidisk) controllers and must be taken from those > manuals / the existing `dcdd` and `mds` card implementations in this repo. > The FDC+ additions relevant to those registers are: > - The internal (sector) interrupt is **de-asserted as soon as the 8080 reads > any FDC+ register** (see §2.4.1), which differs from the original. > - Sector pulses are synthesized on the fly for soft-sectored media, so the > Sector Position register always presents the expected 32 (8") or 16 (Minidisk) > sectors plus index regardless of the physical media. --- ## 2.1 Configuration switches (four DIP banks + jumpers) Four DIP switch banks configure the board: 1. **Drive Type (S3)** — selects the emulated drive type and enables the monitor. 2. **RAM (S1)** — enable/disable on-board RAM and its 1K start address. 3. **PROM (S2)** — enable/disable PROM, sets RAM end address and PROM start. 4. **Interrupt Select (S4)** — routes the internal interrupt to a bus line. Additionally, PCB traces can be cut/jumpered to change the I/O address block and to disable RAM/PROM when *Phantom is asserted. Switch convention: actuator toward the **top** of the PCB = **1**, toward the **bottom** = **0**. On every bank the silkscreen labels (not the numbers printed on the switch body) identify each switch. ### Drive Type switches (S3) The right-most four switches (silkscreen "3 2 1 0") select the drive type; the far-left switch is "Mon" (monitor). Switch positions between "Mon" and drive-type bit 3 are unused. **The drive type is latched at power-on**; changing switches afterward has no effect until the next power cycle. **Table 2.1 — Drive Types (switch bits 3 2 1 0)** | 3 2 1 0 | Value | Drive Type | |---------|-------|-------------------------------------------------------------| | 0 0 0 0 | 0 | Original Altair 8" drive | | 0 0 0 1 | 1 | Direct connect to Shugart 8" drive as Altair 8" drive | | 0 0 1 0 | 2 | Original Altair Minidisk | | 0 0 1 1 | 3 | Direct connect to 5.25" drive as Altair Minidisk | | 0 1 0 0 | 4 | 5.25" HD drive as Altair 8" drive | | 0 1 0 1 | 5 | 1.5 Mb 5.25" HD drive or 1.5 Mb 8" DSDD drive | | 0 1 1 0 | 6 | Serial drive as Altair Minidisk | | 0 1 1 1 | 7 | Serial drive as Altair 8" drive | | 1 0 0 0 | 8 | 8" SSSD Soft-Sectored (iCOM/Pertec FD3712, IBM-3740) | | 1001–1111 | 9–15| Not assigned | **On-board monitor ("Mon" switch):** far-left switch on S3. Set to 1 at power-on to enter the monitor (used for firmware updates and settings such as serial baud rate). Accessed over the built-in serial port (J2, IDC-14 → DB-25), **9600 baud, 8N1**, DB-25 wired as DCE (TX/RX/GND only). If the FDC+ appears non-functional, suspect the monitor switch being left on — turn it off and power-cycle. ### Interrupt Select switches (S4) and interrupt clearing Like the original FDC, the FDC+ generates an interrupt **at the start of each sector**. S4 routes the internal interrupt to exactly one of the bus vectored interrupt lines **V0–V6** or to **PINT**. Actuate one switch (silkscreened V0–V6 / PINT) toward the top; only one at a time. **§2.4.1 Interrupt clearing (a change from the original FDC):** On the FDC+, the interrupt is **de-asserted as soon as the 8080 interrupt routine reads ANY register on the FDC+**. On the original FDC, the interrupt was cleared only by the 8080 interrupt-acknowledge cycle (which could belong to a different device). This "bug fix" works with all known Altair disk software because the first thing a disk interrupt routine typically does is read the sector position register. ### Phantom (§2.6) The FDC+ can be write-through or full Phantom: - **Write-through Phantom:** when *Phantom is asserted, the FDC+ ignores memory **reads** but still services memory **writes** to its RAM. - **Full Phantom:** both memory reads and writes are ignored when *Phantom is asserted. The FDC+ **cannot drive** the Phantom signal, only respond to it. (Revision B and C enable these modes via different board mods.) --- ## 2.2 On-board RAM and PROM address mapping ### RAM (S1) - **`*EN` switch** (active-low): `0` = RAM enabled, `1` = RAM disabled. (Opposite polarity from the PROM enable switch.) - Switches **"15"–"10"** map to address lines **A15–A10** and set the **1K** block at which RAM starts. E.g. A15–A10 = `000000` → RAM at `0000h`; `110000` → RAM at `C000h`. - RAM's **end** address is set by the PROM start address (even if PROM is disabled). Max PROM start is `FF00h`, so RAM normally cannot extend above `FF00h` — the **top 256 bytes are inaccessible** by default. - Revision B: RAM can stop no lower than `E000h`. - Revision C: RAM can stop as low as `8000h`. - The leftmost S1 switch is unused. **Full 64K mod (§2.2.1):** cut the trace U9p13↔U8p19, connect U9p13↔U9p14. After the mod the PROM-switch page value becomes (start page − 1) of EPROM and (last page) of RAM. Setting PROM switches to page `FF` then gives RAM `FF00–FFFF` (full 64K); EPROM becomes inaccessible and PROM enable should be off. ### PROM (S2) - Socket holds a **27C64 (8K)** EPROM. - **`EN` switch:** `1` = PROM enabled, `0` = disabled. (Opposite polarity from the RAM `*EN` switch.) - Switches **"12"–"8"** map to address lines **A12–A8**, selecting the 256-byte page within the 8K PROM at which the PROM begins to respond. - **Revision B:** 8K PROM fixed at `E000h`–`FFFFh`; A15–A13 permanently `1`. The two leftmost S2 switches are unused. - **Revision C:** A15 permanently `1`; switches A14 and A13 (typically both `1` for standard Altair) allow the 8K PROM to start at `8000h`, `A000h`, `C000h`, or `E000h`. Below `E000h`, PROM content repeats through `FFFFh`. Examples: - Only the 256-byte **DBL** (disk boot loader) at `FF00h`: program DBL into the top 256 bytes of the 27C64 (`1F00h–1FFFh`); set A12–A8 = `11111` (page 1Fh); with A15–A13 = all ones → responds at `FF00h`. - **MBL + TURMON + hex-loader + DBL**: burn to `1C00h–1FFFh` of the 27C64; set A12–A8 = `11100` (page 1Ch) → responds at `FC00h`. --- ## 3. Drive types (geometry, timing, media) ### 8" drive (types 0, 1) — Altair 88-DCDD compatible - **32 hard sectors** + index (33 pulses total). The Altair FDC and FDC+ expect all 33 pulses and perform sector/index separation on the board. - 77 tracks (standard Altair 8" geometry). - 8" **disk image file** size: **330K**, `.dsk` extension. - Shugart 800 (soft-sectored) and 801 (hard-sectored) both usable; an 801 must be jumpered to behave like an 800 (disable its sector separator) because the FDC+ does the separation. A "soft-sectored" Shugart 800 still asserts all 32 sector holes plus the index hole when a hard-sectored floppy is inserted. - **Index-alignment quirk:** a calibrated Shugart 80x and a calibrated Altair/Pertec drive differ in index alignment by **~360 µs**, enough to make media incompatible. The FDC+ **automatically compensates** for this 360 µs offset in firmware. Align Shugart 80x with a Shugart SA120 / Dysan 360A disk — NOT the Pertec/Dysan 500 disk. ### Minidisk (types 2, 3) — Altair 88-MDS compatible - **16 hard sectors** (5.25" Altair Minidisk format). - Minidisk **disk image file** size: **75K**, `.dsk` extension. - Type 2 = original Altair Minidisk hardware (incl. 8800bt "Foley" internal cabling variant). Type 3 = direct-connect Shugart SA-400 (or any 48-TPI 5.25" drive) presented as an Altair Minidisk. - 48-TPI media is interchangeable with an aligned Altair Minidisk. 96-TPI drives work and their media interchanges between 96-TPI drives, but 96-TPI-written media will NOT read in a 48-TPI drive and vice-versa. - Use SD or DD media for Minidisk applications. ### 5.25" HD drive as 8" drive (type 4) - A 96-TPI 5.25" HD drive (e.g., **Teac 55-GFR**) presents as an Altair 8" drive. - Configured to spin at **360 RPM** to match 8" rotation. - The drive has **80 tracks** (vs 77 on real 8"); firmware generates **32 virtual hard sectors** for soft-sectored HD media. Media is NOT interchangeable with a real 8" drive, but interchanges between Teac drives. - Use soft-sectored **HD** floppies. The FDC+ creates virtual hard-sector pulses. **Table 3.5.1 — Teac 55-GFR jumper settings for 8" compatibility** | Jumper | In/Out | Function | |----------|-------------------|-----------------------------------------------------| | LG | Out | HD mode (pin 2 high / not driven) | | I | Out | 360 RPM | | E2 | In | Index pulses continue while seeking | | DS0–DS3 | Install for drive#| 0–2 = Shugart 1–3; DS3 on P6 → jumper to IDC-50 P32 | | IU | Out | Ignore "In Use" on P4 | | U0,U1 | In | Light on with drive select and motor spinning | | RY, DC | Out | Neither Ready nor Disk Change on drive P34 | ### 1.5 Mb drive (type 5) - Introduced in FDC+ firmware **v1.3**. Provides **1.5 Mb** formatted on a 5.25" HD floppy or an 8" DSDD drive. - Disk format and controller access differ from a normal Altair drive, so **original Altair software (Disk BASIC, Altair DOS, CP/M for Altair) is NOT compatible**. A special CP/M 2.2 (from the FDC+ "Altair Software" link) supports up to four 1.5 Mb floppies. - Drive wiring: HD 5.25" as in §3.5.1 (Teac config); 8" DSDD as in §3.4.1 (Shugart config). ### Serial drive (types 6, 7) — no rotating media - The FDC+ high-speed serial port connects to a PC running a disk-image server; software sees a normal Altair drive. Performance ≈ real Altair floppy. - Type 6 = serial drive as **Minidisk**; type 7 = serial drive as **8" drive**. - Serial port = same J2 header/IDC-14→DB-25 cable used by the monitor; DCE, standard DB-25→DB-9 cable (no null modem). - Baud rates (set in the monitor): **403.2K** (default/first choice), **460.8K** (460.K), or **230.4K**. At 230.4K, 8" operation is slightly slower than a real 8" drive; Minidisk still runs full speed. Fast rates usually require a USB-to-serial adapter rather than a native PC serial port. - Server: "Altair Server.exe" (Windows), no install. **8 Mb serial 8" drive (§3.7.4):** introduced with FDC+ firmware **v1.2** / Serial Drive Server **v1.3**. Looks like an Altair 8" drive with **2048 tracks instead of 77**. Set drive type to **7**. CP/M for the 8 Mb drive expects an 8 Mb image on drives **A and B (0,1)** and normal 77-track Altair images on **C and D (2,3)**. The 8 Mb CP/M is patched to show the current user number in the prompt and will run a command from drive A user 0 if not found on the current drive/user. ### iCOM/Pertec FD3712 (type 8) — 8" SSSD soft-sectored, IBM-3740 - Presents as an iCOM/Pertec FD3712 dual-drive cabinet/controller with its S-100 interface card, implementing the **8" SSSD soft-sector IBM-3740** format common in the 1970s. - Connects to up to four Shugart 800 drives (config as §3.2.1). 5.25" HD drives (e.g., 1.2 Mb Teac 55-GFR) also work, wired/configured as §3.5. - Runs vintage FD3712 software (FDOS, CP/M) and can read/write 8" SSSD disks written by other systems. ### Soft-sectored media as substitute for hard-sectored (general) The original Altair FDC required hard-sectored media (32-sector 8", 16-sector 5.25"). The FDC+ also accepts soft-sectored media: if it detects no sector holes as the disk spins up, it **syncs to the rotational rate and generates virtual sector pulses** on the fly, fully transparent to the Altair. Soft-sectored media written on one aligned FDC+ drive works in another aligned FDC+ drive, but NOT in an original two-board FDC. Belt-driven 8" drives (Shugart 80x) are unreliable with soft-sector media due to intra-revolution speed variation; belt-driven 5.25" drives tolerate it better. Use SD/DD media for Minidisk, HD media when running a 5.25" as an 8" drive. --- ## 4. PROM / boot (Appendix B) — as shipped The 27C64 ships pre-programmed with (each entry is a 256-byte page): | Address | Contents | |---------|---------------------------------------------------------------------| | `FF00` | **CDBL** — Combined Disk Boot Loader (Martin Eberhard; boots either 8" or Minidisk) | | `FE00` | **MBL** — Altair Multi-Boot Loader (boots cassette tapes, paper tapes) | | `FD00` | **TURMON** — Altair Turnkey Monitor (stack at `F800`; needs RAM just below `F800`) | | `FC00` | **Intel hex file loader** | | `F800` | **ALTMON** — Altair Monitor (full-featured) | Notes: - MITS shipped **separate** disk boot loaders for 8" and Minidisk, both at `FF00`, so both couldn't coexist. CDBL replaces both and boots either type. - MBL and TURMON are original Altair PROMs. - **Intel hex loader:** jump to `FC00`; prompts to send a hex file over the first port of a 2SIO board; displays load-progress addresses; idles in an endless loop when done. Stack at `8000h` → needs RAM just below `8000h`. - ALTMON: memory display/modify/search/compare/move, Intel hex loader, I/O port in/out, disk boot, etc. - Any subset of PROM content can be enabled (as little as the top 256-byte DBL). --- ## 5. Disk-image / file-transfer utilities (Appendix A) - **Image files:** `.dsk` extension. 8" = **330K**, Minidisk = **75K**, 8 Mb serial 8" = large (2048 tracks). Recognizable by file size. - **PC2FLOP** — Altair-side utility that writes a full floppy from a disk image received over a **2SIO** serial port via **XMODEM**. Runs stand-alone or under CP/M; can write any disk type. Separate builds exist per drive type (8"/330K, Minidisk/75K, 1.5 Mb). Can be bootstrapped via the loader technique or the PROM's Intel hex loader. - **PCGET / PCPUT** — per-file transfer to/from an Altair running CP/M over a 2SIO port via XMODEM (PCGET = PC→Altair, PCPUT = Altair→PC). Pre-installed on most CP/M disk images. - The 2SIO can be modified to run at **19,200 or 38,400 baud** to speed transfers. - TeraTerm is the recommended PC terminal emulator. --- ## 6. Summary - Default register block **08h–0Bh** (alt 80h–83h): Drive Status/Select (0), Sector Position/Command (1), Read/Write Data (2), Reserved (3). - Register bit semantics = original 88-DCDD (8", 32 sectors) or 88-MDS (Minidisk, 16 sectors) depending on latched drive type. See existing `dcdd`/`mds` cards. - Drive type is **latched at power-on**; ignore changes afterward. - Sector interrupt fires at the **start of each sector**; it clears on **any** FDC+ register read (not on interrupt-acknowledge). Route to V0–V6 or PINT. - 8" = 32 sectors + index (33 pulses), 77 tracks, 330K image; Minidisk = 16 sectors, 75K image. Virtual sector pulses are synthesized for soft-sector media. - Optional on-board 64K RAM (1K granularity) and 8K PROM (256-byte granularity) with the address rules in §2.2. Shipped PROM map: CDBL/MBL/TURMON/hex/ALTMON at FF00/FE00/FD00/FC00/F800. - Serial-drive variants (types 6/7) and iCOM FD3712 (type 8) are host/serial-side features rather than S-100 register behavior. --- ## 7. Operation with a Z80 at 4 MHz (separate note) Source: *FDC+ Operation with a Z80 at 4MHz*, 1 p, M. Douglas, from deramp.com (`.../altair/hardware/fdc+/Operation with 4MHz Z80.pdf`). Fetched 2026-09-26. Whether a 4 MHz CPU works depends on the **software's transfer loop**, not on the FDC+: | Setup | 2 MHz | 4 MHz | Why | |---|---|---|---| | Serial disk server (types 6/7) | yes | yes | — | | Real 5.25" drives, Altair Minidisk software | yes | yes | The minidisk transfer loop is not timing dependent. | | Real drives, original Altair **8"** software (Disk BASIC, Altair DOS, CP/M for Altair, …) | yes | **no** | Every Altair 8" loop reads or writes **two bytes per pass**, and the timing of the second byte depends on a 2 MHz CPU clock. | | 1.5 MB CP/M (type 5) | yes | **no** | The data transfer loop is timing dependent. | *"This has nothing to do with the FDC+ hardware"* — a 4 MHz 88-DCDD fails the same way.