Avionics reverse-engineering: navigation display boards - Page 1
Forum section: electronics / projects

author: D Straney
author_rank: Frequent Contributor
timestamp: April 11, 2025, 03:42:53 am
content: Here's a couple boards I got fromIndustrial Alchemy:First, let's talk about where they came from, then about how they work.BackgroundFrom looking up the part numbers, I found out these came from an aircraft navigation display: part of the AN/ASN-124 "Tactical Navigation System", made by Lear-Siegler in the 70's. This was used in theP-3 electronic surveillance aircraft.I think US military documents, including specifications and manuals, may be considered public-domain when no longer classified, and so you can find a surprising amount of detailed information online.You can read the full specifications here, in fact, for the entire display system. The detailed description of the "Digital Display Indicator" which contained these boards:QuoteHere's a drawing of what the display was supposed to look like:Top row is all 16-segment digits (6x), for messages: can easily do the whole alphabet, numbers, and some symbols with all the extra diagonal segments in theseMiddle and bottom rows start off with an alphanumeric 16-segment digit, but the rest are 7-segment numeric-only digits (8x per row): probably for displaying coordinatesThere's also a "Keyer Control" for user inputs, which has a few alphanumeric 16-segment displays of its own for feedback:The system was supposed to interface to an external "tactical display" with a CRT, as well, to show a primitive graphical map with waypoints, markers, etc. I couldn't find an example of the screen contents in the spec document.Numeric Display Card WorkingsThis "numeric display" card receives, decodes, and drives 16x 7-segment digits: this covers both numeric rows on the display, all in one place (which explains the name).The construction is interesting: two very thin fiberglass PCBs are glued to either side of an aluminum plate. I assume this is done both for conduction cooling (TTL logic's static power consumption starts creating significant heat once you pack a lot of chips into a small space!) and for physical compactness.There's also a slightly strange repeating pattern of rectangular pads which are present even where completely unconnected to anything else - I assume this is an artifact of early PCB CAD software, similar toIBM'sidiosyncraticlayout styleon old systems.Each of the two PCBs is almost identical, with 4 columns. Each column handles 2x 7-segment digits.Each column contains a DM54L164 8-bit shift register (black), and 2x HL54397s, plus a blue power-filtering capacitor. The HL54397 isn't a standard TTL part number, butluckily there's data available- these are listed as BCD-to-7-segment decoders and drivers; likely similar to thestandard 74x46/74x47/74x48/74x49 parts. Each 7-segment digit needs 4 bits to represent the numbers from 0 to 9, so one shift register receives 8 bits serially, and produces the parallel outputs to drive both the adjacent decoders/drivers.The shift registers on each PCB are all chained in series, so that one big 32-bit word to drive all 8 digits is clocked in through a serial input. That approach is extended to the rest of the display too, though.The specificationsdescribe the single external synchronous serial input to the "digital display indicator" and the bit position of each digit within the serial data words - from the direct bit-to-digit mapping, it looks like internally the system uses the (appropriate) simplest-possible approach of just making a giant chain of shift registers for every single digit on the front panel.The empty columns with traces connecting to unused pads suggests to me that either...This PCB design was also used in a similar system with more digits,the designers wanted to leave their options open in case requirements changed, orthe same PCB was used for the alphanumeric 16-segment digits too, with different decoder/driver chips replacing the HL54397s.The lack of traces connecting to some of the SMT pads confused me at first, until I looked very closely and realized that many of the pads have vias inside them, made invisible by the covering of solder in most cases. This isn't considered good practice with modern parts and tighter sizing, but I guess you can get away with it here, with the generous pad sizes and amounts of solder(due to the extra-long leads that take up thermal-expansion mismatch, and make it ok to completely cover the pad with a giant solder blob, unlike on modern SOIC packages where that would coat the short leads in solder too and make the whole thing too rigid).Dimmer Card WorkingsThe specification above mentions a dimming control for the displays, and that's what this card handles:LEDs weren't that great in the 70's, and so instead of the 7-segment LEDs we're used to now, multi-segment displays using individual incandescent filaments were more common (like the one in the other navigation display I took apart). The spec specifically mentionsfilamentsin the display digits being dimmed by the "DIM" knob; it seems like this card drives a common high-side connection for all the display filaments, and the per-segment drivers (such as on the "numeric display" card) all have open-collector outputs, driving individual low-side filament connections.The most noticeable feature on this board is the big, heatsinked metal-can power transistor, which provides the current regulation for the display dimming. It wasn't hard to follow the traces on this board, though, so let's look at the actual schematic piece-by-piece:Current regulatorThis is the core of the dimming circuit, implemented all with discrete transistors. The big power transistor (Q8) is a PNP using its current gain to provide a current source to power-output pins (presumably for the high-side connections of the displays). The outputvoltageis summed with a setpoint signal to produce one input to a differential error-amplifier, which subtracts from a fixed threshold and closes the feedback loop to drive Q8 again through a series of amplification stages (Q4, Q6, Qh).This regulates the outputvoltage, which is necessary here: because a variable number of display segments will be on at any one time, the only (simple) way to make them all equally bright is to put the same voltage across each one. However, there's also a feedback path for the outputcurrent. Output current is sensed by some small-value power resistors in series with the output (Rh3-5), added to a fixed offset created by a current sink through a series resistance. This offset current sense signal is compared against the very-rough threshold generated by a couple P-N junctions (Q5's Vbe & CR2's Vf). If the offset current sense is larger than this threshold, then CR1 conducts and the current-sense circuitry suddenly enters the feedback loop, and regulates the output current at this maximum level.I'm guessing this over-current protection is meant to protect filaments in the display at power-up, besides the obvious limiting of damage from faults. Because of the temperature-dependent resistance of most metals, filaments of any kind usually have a lower resistance when cold, and so with a constant voltage they draw much higher current until they warm up to their normal operating temperature. This means that the voltage-control scheme of this dimmer would have to deal with higher current & higher dissipation at power-on, than in steady-state: the over-current protection removes the need to over-spec the heatsinking & thermal handling on Q8 just for the worst-case situation where someone holds the "lamp test" button down after a cold power-on.There's a couple other little features in here:A soft-start with a large capacitor on the error-amplifier output, andControl loop compensation using a series resistor & cap to produce a zero and pole; without this, the chained hfe(/beta) values of all the transistors in the control loop would combine to produce a pretty huge open-loop gain, which would only start rolling off once individual transistor poles due to their internal frequency limitations start showing up and killing the phase margin. Much safer to create a nice, simple single-pole response on your own terms and roll off the gain down to 0 dB long before all the extra poles do bad things to your stability.Lamp driversThere's 5 identical driver circuits, which accept a logic (TTL) input and drive a hefty open-collector output. These match up nicely with the 5 lamps on the front panel diagram shown above: "UPD", "SYNC", "MODE", "CMPTR", and "MALF". The 5416 here, the single IC on the bottom side of the board, is the military version of the TTL 7416: an inverter with a "high-voltage" open-collector output.There's room on the PCB for a 6th lamp driver, but the resistor and transistor aren't populated.PowerNothing too interesting here, except that this is where the 3.6V reference for the current-sense-offset current sink comes from. Not sure why this particular part of the circuit gets a zener diode and some extra precision, when the actual value of the current sink is set more by a random transistor's Vbe anyways.Anyways, hope this was interesting seeing inside some 70's avionics.