The engineering notebook
The machine is on the main page. This is the evidence under it: what MITS's own documents say, what was measured, which sources disagree, and which questions stayed open. Nothing here is needed to work the switches. Everything here is why the switches can be trusted.
What MITS bought, which is how the panel is checked
The panel's layout was measured off a photograph, which is one source and could be one mistake. MITS also published a parts list, effective 1 March 1975, and the Display/Control board line items settle three things a photograph can only suggest.
| 36 × RL-21 LED | at 60 cents each. This page draws 36 lamps: two flags, eight status and eight data on the top row, two flags and sixteen address on the bottom. The count is not an interpretation of a photograph, it is what was ordered. The part number had been an open question here since the beginning. RL- is Litronix's own prefix for discrete red lamps, which their 1982 catalogue confirms: RL-2, RL-50, RL-54, RL-209, RL-2000 and more, all red indicator lamps. RL-21 is not itself in the 1982 edition, seven years after the Altair shipped, but a 1975/76 distributor databook page settles it: “RL20 and RL21 Red L.E.D.s”, under the Litronix masthead, gallium arsenide phosphide in a red diffusive moulded lens, 0.7 millicandela typical at 20 milliamps, and a 180° viewing angle that made it “extremely suitable for panel use”. The brighter RL20 sibling ran 1.2; MITS bought the dim one, which fits sixty cents. The designation is MITS-wide, not a one-off line item: the 8800b's own assembly instructions call for “36 LEDs, RL-21” and draw the package with its anode and cathode called out (pp.5-33 to 5-34). |
|---|---|
| 17 × switch ST-1-1-C | at $2.35. The ones that stay where you put them: sixteen address switches and the power switch. |
| 8 × switch ST-1-3-C | at $2.45. A different part, and there are exactly eight control switches, which are the ones that spring back to the middle. This page already drew them that way from photographs. The assembly manual then says it outright: “Eight of these are momentary contact SPDT switches and seventeen are latching type SPDT switches.” |
| 1 × 8080, Intel | where every other line has a price, this one says Factory Quote. |
The boards, photographed
The cage on the main page draws its cards, because drawings can be consistent where surviving photographs cannot. But cleanly licensed photographs of the real boards exist for some of them, and they belong here, where the evidence lives. All five below are by Eric Smith, from his own machines, and carry a licence that permits reuse; each is linked to its source. They are resized and otherwise untouched.
Public-domain advertisement scans exist for several more boards — the 88-2SIO with the 88-PIO in Byte's January 1976 issue, the 4K static board that April, the 88-80LP's control card in December 1975, and a labeled 16MCS in Kilobaud, January 1977 — and the ledger of what is licensed, what is not, and what was searched without result lives in the project's research notes. Nothing cleanly licensed has been found for the 88-ACR, the 88-VI, a labeled 88-PMC, the small memory boards or the 88-EC, which is why the cage stays drawn.
The switches were not silver
They are here, or they were until August 2026: every one of the twenty-five drawn with the same chrome gradient. The reference photograph shows a four-colour panel, and the colours are not decoration. Switches 15 down to 8 are magenta and 7 down to 0 are cream, and that is exactly the sense-switch boundary this page spends a chapter explaining. MITS printed the distinction on the hardware in colour, at the factory, and this page had been teaching it in prose while drawing it away.
The six real control switches are teal. The two AUX switches, which were never wired to anything, are the only black ones, so the panel tells you they are different before you read a word. The power switch matches the data half.
And the silver is real too, which is why the memory is a fair one. The coloured caps are moulded plastic, but the bushing and collar under each of them are bare nickel-plated metal, and there is a bright ring of it at the base of every switch on the panel. A photograph of the back of a real machine shows nothing but metal switch bodies. Both answers are right about different parts of the same switch.
Why the lamps blur, from the schematic
This is the part of the page most likely to be compared against a memory or a video, so it is worth saying exactly what the circuit does. On drawing 880-105, Computer Front Panel Display, each of the sixteen address lamps runs from its bus line through a single 220 ohm resistor to the lamp and on to ground. No latch, no buffer, no driver chip. Nothing is multiplexed: every lamp has its own resistor, thirty-six of them, R24 to R59, one per lamp. The data lamps are the one exception and only electrically: they hang from +5V down into a 74LS04 inverter that sinks the current, so the polarity is flipped on the way but a lit lamp still means a one.
Wired to the bus with nothing in between, a lamp shows whatever the bus is doing, two million times a second. It cannot do anything else. MITS said so themselves, in the operator's manual, immediately after explaining that a lit lamp means a one: “While running a program, however, LEDs may appear to give erroneous indications.” The blur is not an effect this page adds. It is the honest behaviour of a lamp soldered to a bus, and the manual warns you about it.
The eight status lamps are the ones that are different, and the difference is not on this board. Their signals come through an 8212 latch on the processor board that catches the status byte at the start of every machine cycle and holds it for that cycle. So they step once per cycle where the address lamps run free. This page samples the bus once per access, which is once per machine cycle, so it already reproduces both.
Brightness has a number too. 220 ohms against a TTL high, with a period red LED, puts about 10 milliamps through each lamp, roughly half what the catalogue used for its ratings. Not blazing, not dim: readable at arm's length in a lit room, which is what an indicator had to be.
Even the moment of switch-on is specified. MITS's troubleshooting table for the successor machine — bus-level behaviour, so it carries — says a memory board present at address zero “will show the random pattern for that board” on the data lamps, because static RAM does not wake up blank; and with no board fitted at all, every data lamp lights, because nothing pulls the open bus low. This page does both: flick the power switch and the RAM wakes scrambled, and a machine with its memory pulled wakes with the data row solid.
And lamp depth, the one dimension the successor machine specified at 13/16 of an inch, was on the original machine not specified at all: the assembly manual has the builder align each LED by eye against its dress-panel hole, then concedes that “due to supply variations, the LED's in your kit may or may not fit all the way through the holes” (p.21). Every original panel's lamps sit at whatever depth that day's parts allowed, which is worth knowing before treating any photograph's lamp depth as a dimension.
The loader on this page, against the ones MITS printed
The walkthrough has you toggle in a sixteen-byte bootstrap loader written for this machine. MITS published three of their own, in Appendix A of the Altair BASIC Reference Manual, 1975. Theirs are twenty and twenty-one bytes, and they are the same shape as ours: ask the card whether a byte has arrived, take it, store it, move along. There is no checksum in any of them. This page said otherwise until August 2026 and was wrong.
They will not load this tape, though, and the reasons are about the machine rather than the loader. They talk to an 88-SIO at ports 000 and 001, where this page has an 88-2SIO at 020 and 021. And they fill memory downward from the top of an 8K Altair BASIC, stopping on a byte that matches a register, where the tape here is 1,920 bytes of Tiny BASIC that has to land at address zero. Ours is four bytes shorter because it needs no stopping condition: you stop it.
The nicest thing in that appendix is the stack pointer. MITS set it to 022, which is inside the loader itself, so every RC and RZ and RNZ "returns" to address 0003 and drops back into the polling loop. A return instruction used as a jump, to save a byte, in a program twenty bytes long.
Their procedure is also the one this page walks you through, down to the order: put the switches down, EXAMINE, set 041, DEPOSIT, then DEPOSIT NEXT for every byte after it. Their checking step is here too. Six of MITS's twenty-one steps have you examine the whole loader back, one address at a time, against what you meant, and the walkthrough keeps that step: it does the finding, then puts the offending address on the switches and leaves the fixing to you on the panel, which is where their step 11 leaves you anyway.
And there was a way out of the morning ritual. MITS's own software FAQ, asked when the bootstrap would be available on PROM, answers: “The PROM boards are available and PROMs with the bootstrap programmed on them cost $40.” Fit the 88-PMC in the cage on the main page and the machine becomes that one: the sixteen bytes are simply there, burned in at 377 000, still there after the power goes off, and DEPOSIT cannot dent them.
How the PROM card finds itself on the bus
Three stages of decode. The top five address bits go to a DM8131 comparator against five switches, each settable true or complement, which is how one card straps to any 2K boundary in the 64K space. The next three bits drive a 74154 wired as a three-to-eight decoder that picks one of the eight 256-byte PROM sockets. The bottom eight go to every PROM in parallel, and the chip itself decodes the byte.
The read gate is the careful part. Board-select alone is not enough, because an I/O device can share the same bit pattern on the address lines, so the card enables its bus drivers only on board-select AND SMEMR — the status signal that says this cycle is a memory reference and not an I/O one. And that is the entire bus interface: eight tri-state drivers pointing at the bus. There is no write line anywhere on the schematic, which is why DEPOSIT into the window does nothing here. Nothing was left out; there was nothing to leave out.
Two details this page does not model, named so they are not mistaken for modelled: the card could insert zero to three wait states depending on PROM speed (a 1 µs 1702A wanted one), and it powered its PROMs up in pairs, switching VGG in under 30 nanoseconds, so only two chips drew current at a time. Both are electrically real and lamp-invisible.
MITS 88-PMC documentation, 1976 (reprinted April 1977): Theory of Operation pp.1–5, Memory Address Selection pp.25–27, schematic 8800-30 (drafted 16 Oct 75).
What a BASIC tape actually sounded like
Paper tape was not the only way in. Owners who bought the 88-ACR loaded BASIC off an ordinary cassette recorder, and the sound of it was two tones: 2400 Hz for a one, 1850 Hz for a zero, one tone per bit at 300 baud — a start bit, eight data bits and a stop bit, ten bit times a character — which works out to thirty characters a second and about a minute of warble for a Tiny-BASIC-sized tape. Between bytes and before the data starts, the line idles at a solid 2400. The manual even hands you the arithmetic: a good tape reads back near 2125 Hz on a frequency counter, which is the two tones' average.
This is not the Kansas City standard, and the proof is numeric rather than a MITS denial that does not exist: Kansas City encodes a zero as four cycles of 1200 Hz, where MITS uses 1850, and the ACR shipped before the November 1975 symposium that produced the standard. The mark tone coincides at 2400 Hz; the space tone gives it away.
Electrically the ACR is an 88-SIO B serial board with a modem board bolted to its back — the manual reprints the whole SIO manual inside itself — wired by the book to addresses 006 and 007, status and data. Jumpers could put it anywhere even-numbered; MITS's software expected 6 and 7, so that is where a machine that ran MITS's own software had it.
MITS 88-ACR documentation, 1975 (second printing Feb 1977): the assembly sections, the Modem Board Theory of Operation, and the alignment procedure.
The datasheet is a warning about brightness. The RL-21 was rated 0.7 millicandela at 20 milliamps, the panel gives it about ten, and light output tracks current, so each lit lamp made something near a third of a millicandela, where a garden-variety modern indicator is twenty to a hundred times brighter. And the lens is fully diffused, a 180° viewing angle with no beam and no hot spot. That is why the panel glows in period photographs instead of glaring, and it is the reason a lit lamp here is drawn as a soft bloom rather than a bright dot with a shine in the middle.
The RL-21 datasheet, line by line
Gallium arsenide phosphide in a red diffusive moulded lens. The plain RL-21 was rated 0.7 millicandela typical at 20 milliamps; its sibling the RL20 ran 1.2, and both could take 100 milliamps continuous. MITS bought the dimmer one, which fits a sixty-cent unit price. The page quotes no forward voltage because the datasheet gives none — its “3.0 V” column is peak inverse voltage, a reverse rating, and borrowing a VF from general knowledge would be dressing a guess as a citation.
The package: a 5.08 mm dome — exactly 0.200 in, the 5 mm envelope, though the page does not call it “T-1¾” because the sheet never does — on an 8.64 mm body, rectangular leads on 2.54 mm centres, short lead the cathode. The line that matters most for rendering is the sales pitch: a 180° viewing angle that made it “extremely suitable for panel use”. No beam, no hot spot, the same apparent brightness from any seat in the room.
Restorers today fit an Everlight HLMP-D150A behind a 2 K resistor — about 1.6 milliamps, a sixth of the original's current, because a modern part is that much more efficient. Its 65° cone is nothing like the RL-21's 180°, which is why a replica panel does not read the same from off-axis as an original.
Litronix data page “RL20 and RL21 Red L.E.D.s”, 1975/76 databook scan; Altair 8800c Front Panel Manual (deramp.com), parts list, Sept 2024.
The Theory of Operation is where the rest of the panel's behaviour comes from, including the sentence that this page got wrong until August 2026 and now obeys: “The machine must be stopped for any of the front panel switches except RESET to be active.” A HLT stops the processor but does not hand the machine back, so the panel stays dead until you press STOP yourself. The successor machine's manual states the same rule more bluntly — “The RUN position… disables all functions on the front panel except reset” (Altair 8800b documentation, p.2-2) — so it is a design rule of the family, not a quirk of one machine.
No MITS document names the typeface. Two people who went looking independently, one identifying type from a Computer History Museum specimen and one making reproduction panel artwork, both land on Helvetica for the switch and lamp labels, with the ALTAIR 8800 nameplate in Tuxedo. The machine photographed for the January 1975 Popular Electronics cover wore Microgramma instead — and that unit was the non-functional mock-up built after the prototype vanished in air freight, so it is evidence about a photographic shell, not a shipped machine. Take the identifications as good ones rather than documented facts. The two-typeface split has a mechanical explanation the assembly manual supplies: the nameplate is not part of the dress panel's silkscreen at all but a separate adhesive plate, specified silver, stuck on “on the very bottom, beneath the switches” after the case is assembled (p.77). Two typefaces because two parts, made in two different steps. The panel-maker adds one detail that is checkable and checked: MITS drew the numeral one as a bare vertical stroke, with no flag and no foot, which is what this page draws.
Where the panel's actual size comes from
There is no dimensioned drawing of the dress panel, and the assembly manual explains why: the panels arrived from MITS already drilled and already printed. The builder never laid anything out, so there was never a template to publish. That is a better answer than not finding one.
But the lamps are not positioned by the dress panel. They are positioned by the Display/Control board, because the LEDs mount to it and stick through. And the board's circuit artwork survives, which is a manufacturing document: exact by construction, with no lens and no perspective in it. Every layout of that era sits on a tenth-of-an-inch grid, so the chip footprints on the board are a ruler with known markings. Measuring two of them gives the scan's scale, and measuring the lamp footprints against that gives a lamp pitch of 0.6 inches, six grid units, to within eight tenths of one per cent.
That is one real length, and this page already had every ratio off the photograph. Together they give the panel's size without anybody's tape measure: 0.6 divided by the measured lamp pitch of 0.0365 makes the face 16.44 inches wide, and the measured aspect makes it 6.48 tall. An owner who put a tape across their own machine got 16.5 by 6.5, give or take an eighth. Both numbers land inside that.
Three sources, three different kinds of instrument, one answer. A photograph can be foreshortened, a tape can be off by an eighth, and a circuit board can be neither.
Still not found, after the MITS manuals, the schematics, bitsavers and the restorer forums: any authority on switch handle colour, which varies across surviving units. That one needs a photograph survey rather than a document search.
Sources for every claim above are on the main page's source list, and the machine itself is one click up. If you find a document this notebook needs, the address on the FAQ reaches a person.