Deramp Altair 8800c User manual
Page 1
Altair 8800c Front Panel Board Set
Ver 1.2, Feb 2019
Introduction
The Altair 8800c front panel
board set is a drop-in re-
placement for the original Al-
tair 8800 front panel board.
The ne board set as creat-
ed for the follo ing reasons:
1) Availability
Finding an original front panel board to restore
or build an Altair is nearly impossible. The ne
board set provides a drop-in equivalent that is
readily available.
2) Elimination of the Hand Wired Harness
The original front panel board picked up bus sig-
nals and connected to the CPU board through a
large hand- ired harness. This harness as te-
dious to build and as often a reliability issue –
especially if you had to remove and ork on the
front panel board. The 8800c board set replaces
the hand- ired harness ith a 50 pin ribbon ca-
ble that connects to a bus interface board (sho n here) to pick up and drive the required bus signals.
3) Mechanical Compatibility with the Altair 8800 Clone Cabinet
The original Altair 8800 cabinet made by Optima is no longer manufactured, and like the original
front panel board, finding an original cabinet by itself is nearly impossible. As an alternative, the cab-
inet for the Altair 8800 Clone is still available, ho ever, the original front panel board covers mount-
ing points in the Clone cabinet’s front panel bracket. This ne front panel board is designed to fit
properly in the Clone cabinet’s mounting bracket. The ne front panel board also fits in an original
Altair cabinet.
4) Compatible with the 8800b CPU Board
The original front panel board does not ork ith an Altair 8800b CPU board and vice-versa. This is
due to a change in ho the 8800b front panel injects data into the CPU and ho it stops the CPU. For
increased flexibility, the ne front panel set orks ith both the original and the 8800b CPU boards.
5) Logic Error Fixes
The original front panel board had a fe logic problems that are fixed in the ne front panel board
set. These same problems ere also fixed by MITS hen they designed the 8800b computer.
CPU “Stop” Timing Error
The Stop s itch is designed to put the 8080 into a ait state on an instruction fetch bounda-
ry (the M1 cycle). Ho ever, due to a timing issue in the run/stop circuit, the front panel
doesn’t al ays stop on an M1 cycle.
Page 2
Indeterminate Power On State
On po er-on, the run/stop flip-flop on the front panel board is not initialized and the 8080
processor is not given a reset signal. These omissions tend to leave the 8080 in an invalid
state on po er-on. This is hy the “STOP/RESET” s itch sequence is required to put the
computer into a valid state. This fix can be optionally enabled or left like the original.
Reset not Synchronized with phi2
An undocumented 8080 reset timing requirement is that reset be released a minimum of
about 200ns prior to the rising edge of phi2. This is implemented in the Intel 8224 clock gen-
erator that as used on the 8800b CPU board, but this chip is not used on the original Altair
CPU board. The ne front panel board enforces this timing for resets generated by the front
panel.
Front Panel PRDY Output is not Open Collector
Clients on the bus should drive the PRDY signal ith an open collector driver. The signal is
terminated on the CPU board ith a 1K pull-up resistor. Ho ever, the original front panel
board actively drives this signal high. Therefore, hen any bus client drives PRDY lo to in-
sert ait states, its output driver is sinking the active high drive of the front panel’s PRDY
driver.
6) Enhancements
In addition to duplicating the functionality of the original front panel board, the 8800c front panel
board set offers several enhancements that can be optionally enabled:
• An output latch at I/O address 0FFh that can display on the data LEDs (similar to IMSAI 8080)
• Auto repeat single-step at selectable rate from 1hz-8hz, similar to the 8800b
• Code snippet injection: Common Altair bootstrap loaders, port echo routines, Kill-the-Bit, etc.
can be injected into RAM by selecting the desired code snippet on the lo er eight address
s itches, then depressing the AUX-Left s itch.
• Auto jump on po er-on (can be independent of reset operation)
• Auto jump on reset (can be independent of po er-on operation)
Page 3
Front Panel Board Assembly
Assembly of the front panel board is not a beginner’s project and these instructions on’t teach you
ho to solder components on a PCB, solder connectors, or sho you the difference bet een a resistor
and a diode. If you have any concerns about building this board, please contact me first.
Many assembly details are illustrated in photographs that follo . Rather than print this manual, you
may ant to vie this document on your tablet hile you ork. A tablet makes it especially easy to
zoom into the photos to see the details you may need.
Parts List
Designator Description Part Number Distributor
C1-C5 0.1uf, 50v 2146302 Jameco
C6 100uf, 16v electrolytic 647-TVX1C101MAD Mouser
D1-D36 Red LED Everlite HLMPD150A Digi-Key 1080-1064-ND
J1 50 pin, dual ro IDC straight header 517-D3433-6302-AR Mouser
J2 5-pin, 0.156 locking header B5P-VH-B-LFSN Digi-Key 455-1642-ND
Female Housing VHR-5N Digi-Key 455-1186-ND
Crimp Pins SVH-21T-P1.1 Digi-Key 455-1133-1-ND
IC-A,B,C,D,E 74LS165 Parallel In Shift Register 595-SN74LS165AN Mouser
R1-R36 2K, 1/4 , 5% 291-2K-RC Mouser
R37-R52 4.3K,1/4 , 5% 291-4.3K-RC Mouser
S1 Toggle S itch On-None-On (Silver) APEM 5236AB Search for best deal
S10-S25 Toggle S itch On-None-On (Gold) APEM 5236CDB Search for best deal
S2-S9 Toggle S itch (On)-Off-(On) (Gold) APEM 5237CDB Search for best deal
Switches
If at all possible, do not use C&K switches for the front panel. The C&K s itches have an erratic and stiff
feel – especially the momentary s itches – compared to the American S itch brand used in the original
Altair. In addition, I’ve had several C&K s itches break hile gently tightening the bushing nut as ell as
during normal use. The APEM s itch part numbers listed above are much closer to the original American
S itch brand. In fact, APEM purchased American S itch a number of years ago.
The po er s itch must have silver contacts as noted for S1. Ideally, use gold contacts for all remaining
s itches. Do not mix s itch brands as most have slightly different body heights and this causes prob-
lems hen mounting the front panel to the mounting plate/bracket.
The APEM s itches can be expensive. I may be able to help ith a bulk buy. Contact me if interested.
Additional Part Notes
Use only LS-TTL parts. Some IC inputs are tied directly to Vcc. Multi-emitter TTL inputs do not tolerate
this ell (e.g., 74xxx and 74Sxxx).
The specified LEDs provide an appropriate brightness ith the 2K resistors specified for R1-R36. You may
need to choose a different resistor value if a different LED is used.
Page 4
Front Panel Assembly Steps
First, insert and solder lo -profile parts
1) Insert and solder R1-R52.
2) Insert and solder IC-A through IC-E (or sockets).
3) Insert and solder C1-C6. Note polarity on C6.
The follo ing instructions for installation of the s itches and LEDs assume you have an Altair Clone cab-
inet. If you’re installing in an original Altair cabinet, follo the instructions in the original manual for me-
chanically aligning the s itches and LEDs hile soldering into the PCB.
Use the front panel mounting bracket from the Altair Clone cabinet as a jig to help install the front panel
s itches, S1 through S25. Do not attempt to solder the switches without usin a ji .
Note: The follo ing pictures are from the Altair Clone assembly manual, so the PCB looks a bit different
than the 8800c front panel board. I nore the LED orientation/polarity shown in the pictures.
1) Prop the front panel bracket an inch or t o above your ork surface and orient the bracket as
sho n in the picture belo . Note, the APEM switches are blue, not red as show in the picture.
2) Place the po er s itch, S1 (toggle s itch ith silver terminals) in the bracket (far left corner as
pictured). Notice that each s itch has a slot in the threaded bushing. Orient the slot of all s itches
a ay from your vie as you place them in the bracket.
3) Place S2-S9 (momentary s itches ith gold terminals) in the eight holes across the bottom of the
bracket (ro furthest a ay from your vie ). Again, orient the slot of all s itches a ay from your
vie as you place them in the bracket.
4) Place S10-S25 (toggle s itches ith gold terminals) in the 16 holes above the eight momentary
s itches. Again, orient the slot of all s itches a ay from your vie .
Page 5
5) Check that all s itches are as squarely aligned as possible, then carefully lo er the circuit board,
component side do n, onto the s itches. Don’t let o of the board after you have it settled on
the s itches as the board ill tend to rock to ards you and fall off the s itches.
6) To solder the s itches, keep pressure on the board ith your “spare” fingers placed in bet een
the t o ro s of s itches and near here you are soldering. This keeps the board level and square
on the s itches.
The next step is to install the front panel LEDs. In order to hold the LEDs at the proper and consistent
height above the board, use the front
dress panel as a jig. Do not cut or prep LED
leads until after soldering is complete!
1) Insert D1-D36 fully into the circuit
board. The anode of the LED is indi-
cated by the longer lead and the
cathode by the flat spot on flange
(for the specific LED in the parts list).
The cathode is closer to the top side
of the circuit board and the anode is
closer to the bottom of the circuit
board. You may ant to prop the
PCB off the ork surface as sho n
to accommodate the long LED leads.
Page 6
2) Work the front dress panel over the s itches and then do n the s itch bushings slightly. You
don’t need to push the panel all the ay do n to the s itch body.
3) Turn the PCB/front panel over as sho n belo and let the LEDs fall into the front panel LED holes.
If the front panel sags a bit in the middle, add a third support point near the middle of the panel.
4) Carefully adjust the spacing bet een the PCB and the front panel in order to accommodate the
four 5/8” spacers provided*. The goal is to make the PCB and front panel as parallel as possible.
The spacers are used only for this step – they do not remain in the assembly. (Note that once an
LED is soldered, it can then serve as a “spacer”) See the pictures on the next pa e.
5) Push each LED all the ay do n into its corresponding front panel hole. Re-verify that the front
panel and PCB look perfectly parallel to each other and are resting directly on all of the hite
spacers. See the pictures on the next pa e.
6) Solder the LEDs: Press do n gently on the PCB ith your “spare” fingers as you solder the LEDs to
the PCB. You may ant to slide the hite spacers near the area you are orking as you proceed
through the LEDs. Note that once an LED is soldered, it can then serve as a “spacer.”
* The spacer height determines ho far the LEDs ill protrude through the front panel after final as-
sembly. With APEM s itches in the Altair Clone case, a spacer height of 5/8” to 11/16” gives a good
LED position after final assembly. At 5/8”, the LEDs protrude just slightly through the front panel. At
11/16”, the LEDs protrude a bit further. You can add something thin under each 5/8” spacer to raise it
to ards 11/16” depending on your LED position preference. The spacin requirement is different if
you use a different s itch brand since s itch body height varies slightly bet een brands.
Page 7
Lastly, install J1 and J2 on the back side of the circuit board.
J1 – Note the orientation of the keying slot before soldering the connector onto the board. Again, the
connector goes on the back side of the board.
J2 – Though not a requirement, I’d recommend pulling the center pin out of the J2 connector. If you
leave the pin in place, note that its hole on the PCB is not plated and should not be soldered.
Page 8
Final Assembly (using an Altair Clone cabinet)
Mount the circuit board to the front panel support bracket.
1) Set all toggle s itches in the
do n position. Feed the
s itch batons into the holes
through the rear of the sup-
port bracket. Be sure the
bracket is fully seated
against the body of every
s itch. You may have to
squeeze/push the panel
firmly around a fe of the
s itches to free those spots
here the support bracket
may be caught on the
s itch threads.
2) Install a lock asher and nut (finger tighten only) on the po er s itch, all eight momentary
s itches, and every other address s itch (A14, A12, A10,… do n to A0) as highlighted belo .
3) Incrementally tighten nuts – one on the far left, then one on the far right, orking back to the
middle, then repeat tightening each nut a bit more. A 5/16” deep socket makes this job much
easier. Do not over-tighten the nuts – trust the lock asher.
Page 9
Mount the front dress panel to the front panel sup-
port bracket.
1) Slide a 0.150 hite spacer over each of the six
studs on the back of the front dress panel.
2) Insert the batons from the s itches into the
front panel and ork the studs into the holes on
the front panel support bracket until the front
panel and support bracket have fully sand-
iched the spacers.
3) Work the front panel do n around the s itch
bushings as needed until the front panel is flat
and even over all 25 s itches. When properly
assembled, the bushings generally protrude just
slightly above the surface of the front panel.
4) Install a #6 nut over each of the studs on the back side of the support bracket(do not tighten). A
shallo 5/16” nut driver makes this job easier.
5) Incrementally tighten the nuts – one on the far left, then one on the far right, orking back to the
middle, then repeat tightening each nut a bit more.
Page 10
Install the Front Panel Assembly into the Case.
1) Slide the front panel assembly in from the front of the case. The front panel may be snug as it goes
into the case bezel. If this is the case, ork the panel in bit-by-bit, keeping the amount of insertion
on all four sides approximately equal as you ork the panel back ards into place ( here the
mounting bracket’s threaded holes line up ith the four holes in the bottom of the case).
2) Attach the front panel support bracket using four of the #6 scre s you set aside during disassem-
bly of the case. Lock ashers go under the t o scre s closest to the rear the panel. The
lock ashers ensure an electrical connection bet een the chassis and the front mounting bracket
for better static protection.
Table of contents
