+Vcc to 54LS series mil spec XOR chip - Page 1
Forum section: electronics / beginners

author: Joule Thief
author_rank: Regular Contributor
timestamp: November 20, 2014, 04:31:10 am
content: At first, I was surprised to see a MIL spec IC chip in the status indicator pcb of a 1980's garden tractor. But then I thought, at times the tractor will be used to plow snow, so there is the -20C temp rating and certainly the PCB could see +40C temps being located near the engine area. And then there is the high vibration and weather exposure ... so OK .... MIL spec chip.The chip is a 54LS86 XOR used to monitor the status of 4 mechanical components on the tractor. If the operator is in the seat, if the clutch is depressed, if the power takeoff engaged and if the brake is on. The pcb only INDICATED and DOES NOT control any of these functions.The biggest surprise I came across when repairing the PCB was the Vcc connection to the 54LS86 chip. The pcb is expected to operate in a 10 - 15 Vdc range and the 54LS85 is rated at 5.5 volts maximum. So where is the 5 volt regulator on this pcb? None to be found.The chip is powered off the 12 volt input to the pcb. A 470R resistor is wired in series with the #14 pin on the IC. No other power regulation is present on the pcb.My questionIs it possible the 54LS86 BCAJC (circa 1985) has an internal voltage (shunt?) regulator?On the defective pcb, I measure the supply voltage 12.6 volts at pin #14On a properly functioning pcb, I measure 5.5 volts at pin #14 with 14.5 dc supplied to the board. As the supply voltage to the board is lowered to 12.6 volts, the Vcc voltage drops to around 5.3 Vdc.

author: SeanB
author_rank: Super Contributor
timestamp: November 20, 2014, 05:02:15 am
content: No, it does not have an internal regulator, it is meant to be operated from 4.75V to 5.25V over the full temperature range, or 4.5 to 5.5V over 0-70c range, like all TTL. It should have a 5V1 zener diode across the supply pins in this application. That might be open on the faulty board.However TTL is quite robust, it often will survive operation at 10V for a short period, but some chips might fail, as they are only warrantied to operate up to 7V, where some will fail.

author: Joule Thief
author_rank: Regular Contributor
timestamp: November 20, 2014, 05:08:29 am
content: Thanks SeanB, you would think that - but no Zeners - no voltage regulators of any kind are present. Battery voltage is supplied thru a 3 diode OR gate so the pcb gets powered when the ignition switch is in the RUN position OR the START position. That's it. From the 1N4007 diodes (in parallel) to a 470R resistor to Vcc on the chip.I will take a few pics later today. I have never seen anything like it.

author: Joule Thief
author_rank: Regular Contributor
timestamp: November 20, 2014, 05:43:53 am
content: best attempt at an Xray viewthe diode seen in the extreme lower right corner is also confirmed to be 1N4007 on both the good and bad pcb's.

author: coppice
author_rank: Super Contributor
timestamp: November 20, 2014, 05:47:12 am
content: Quote from: Joule Thief on November 20, 2014, 04:31:10 am54LS is not MIL spec. It is merely wide temperature range. The MIL spec parts have completely different numbers. There were small quantities of 64 parts for a while, with a temperature range between the 74 and 54 parts. Other semiconductor product lines used similar numbering schemes to differentiate temperature ranges. E.g. Nationals 1xx, 2xx and 3xx linear devices. National also dropped most of the 2xx parts quite early on.

author: Joule Thief
author_rank: Regular Contributor
timestamp: November 20, 2014, 05:57:41 am
content: Thanks Coppice,on the functional pcb, the same chip is marked DM54?LS86J-MIL.Any idea what the MIL suffix refers to?

author: coppice
author_rank: Super Contributor
timestamp: November 20, 2014, 06:05:56 am
content: Quote from: Joule Thief on November 20, 2014, 05:57:41 amDM means it was made by National. I can't remember what MIL might mean, but if that really was how National labelled a military qualified and fully traceable device, someone used a $10 part where a 10 cent one would have done. Maybe the part has been replaced at some point with a military surplus one.The biggest cost difference between 74LS and 54LS was that 74LS was available in a plastic package, while 54LS was usually only available in ceramic. A 54LS was still a fairly inexpensive part.

author: Joule Thief
author_rank: Regular Contributor
timestamp: November 20, 2014, 06:56:15 am
content: Another question would be what expectations I should have if I replace the 54LS with a 74LS series.Given the primary reason of this post was to inquire about the Vcc into the chip, I would consider mounting a 100 ma 5V regulator on the board to power the (more readily available and much less expensive) 74LS86.Will the plastic 74LS survive as well as the 54LS in the outdoors environment? The circuit board will be covered with a conformal coating.Given the 0 deg C spec on the 74LS, and given the circuit may be stored and operated in -20 C environments, what, if any, modes of failure could be expected? What parameters could be compromised if a 74LS is operated at -20C ? Remember, the inputs are micro switches attached to the mechanical components of the tractor and the outputs from the chip are driving 2N3904's into grain of wheat indicator bulbs.

author: Paul Moir
author_rank: Frequent Contributor
timestamp: November 20, 2014, 08:03:04 am
content: I was curios, so I was googling around and found this:http://www.wheelhorseforum.com/topic/32049-indicator-lite-circuit-board-repair/It suggests some of the suffix letters indicates it's got some capability for dealing with high supply voltages. The author apparently went the zener route.I think you might be able to sub in a 74HC(T)86 or AC(T)86, which are readily available in -40-85 parts.

author: Kjelt
author_rank: Super Contributor
timestamp: November 20, 2014, 08:56:51 am
content: Perhaps exchange it with a chip from the CMOS4000 family, at least they can handle 15V, not sure if there is a pin equivalent part though.

author: Joule Thief
author_rank: Regular Contributor
timestamp: November 20, 2014, 09:29:48 am
content: Quote from: Paul Moir on November 20, 2014, 08:03:04 amThanks Kjelt,Thanks Paul.I am a member of that particular forum and know the gentlemen very well. Andy had similar comments about how "unusual" the circuit design was and attempted to "force fit" the output states to his liking. We both scratched our heads over the Vcc connection thru a 470R resistor. The board was never made fully functional and interest waned for a few years.Unfortunately, many pcb's are now failing due to corrosion under the conformal coating and failure of the LS86 chip. Thousands of these tractors still function with the 12 volt thru 470R to Vcc connection intact. The original question remains. Documentation on this chip does little to explain why the manufacturer would consider Vcc wired in this fashion.I have 74LS86 chips coming tomorrow, and an addition of a 5v reg should get me up and running. We will see if I can perform some "poor man's" environmental testing this winter.There seems to be a line forming at my door with much interest in having defective indicator pcbs repaired. The manufacturer wants nearly $200 USD for a replacement and stock is NOS. Maybe an interesting winter design project is calling.

author: Kjelt
author_rank: Super Contributor
timestamp: November 20, 2014, 11:32:55 am
content: Best to draw the entire schematic, perhaps that clears things up.For instance if they have one or two outputs on the ic that will continuously draw a total of around 18mA so the total comsumption of the ic will be around 19mA the Vcc will drop to the expected 5V - 5,5V because the other 9V will drop over the resistor of 470. If that outputs resistors goes high ohms (fails) than the Vcc will go up and the chip will fail.

author: SeanB
author_rank: Super Contributor
timestamp: November 20, 2014, 05:03:20 pm
content: OK, the chip is a genuine mil spec part, with a batch code and testing lot number. It can be replaced with the 74LS version with no problems, and simply add a 5V1 400mW zener to Vcc ( between pin 14 and 7 on the solder side is easiest) to give a regulated supply to it. That it ran for decades with the bad power is simply because the chips are old, and the processing gave a part that would not blow up at 12V, while those that would blow up at 12V were weeded out very early as field failures. The 7V rating was because some parts would blow up at 8V, not that they all would fail, just every so often a batch would not survive it.I had a computer where the power supply did a smoke test and applied the unregulated 16V supply rail to the whole unit, and the only failure after it ran for a few hours was a PM5403 hex inverter with 30V open collector output, which showed how unhappy it was by delidding itself then blowing the lead wires off. All the other TTL survived, even the unobtanium Intersil IIL DAC, which was on the board which made up half of the ADC system, and even the Intersil MK4007 serial ram chips survived unharmed.

author: rr100
author_rank: Frequent Contributor
timestamp: November 20, 2014, 05:12:19 pm
content: Quote from: Kjelt on November 20, 2014, 11:32:55 amThis was PRECISELY what I had in mind. However, it would have been really too "living-on-the-edge" scenario.

author: Kjelt
author_rank: Super Contributor
timestamp: November 20, 2014, 07:35:20 pm
content: Quote from: SeanB on November 20, 2014, 05:03:20 pmI can't believe that. The yield would be catastrophic and the design engineer a moron. If you reread the TS post he says that a good functioning pcb measures 5,5V at the Vcc, just in spec. and the broken one 14,5V !!!The resistor should take care of the 9V drop if the rest of the circuit is working ok.54LS series dont survive three times the voltage that is why the CMOS series were developed.

author: Rufus
author_rank: Super Contributor
timestamp: November 20, 2014, 07:44:05 pm
content: Quote from: Joule Thief on November 20, 2014, 05:08:29 amBut there are two holes where a zener diode would fit and it looks like something used to be in those holes.

author: Kjelt
author_rank: Super Contributor
timestamp: November 20, 2014, 09:42:47 pm
content: Quote from: Rufus on November 20, 2014, 07:44:05 pmThere was a large green 100nF cap see other picture.

author: Joule Thief
author_rank: Regular Contributor
timestamp: November 21, 2014, 03:35:02 am
content: Correct, The two open holes are for a 100nf ceramic cap. The capacitor was found to have two cold solder joints and left the circuit board as the conformal coating was removed.My SN74LS86 have arrived and hopefully some time will open up this weekend for experimentation.Another defective indicator board has arrived. As luck would have it, my next door neighbor has a similar garden tractor - same manufacturer, different model number. The indicator pcb is also defective but with a small twist. This circuit board shows 1.99 volts DC at the Vcc pin of the 54LS86 chip.Stay tuned.

author: Paul Price
author_rank: Super Contributor
timestamp: November 21, 2014, 04:17:17 am
content: This whole outdated tractor circuit could be easily replaced with just a simple but rugged PIC microprocessor circuit that would use much less parts and use diodes/resistors only for worst-case input/output/transient/fault voltage protection.For instance, a single HV 1N4006 diode in series with a 78L05 regulator to supply 5V for a PIC MCU. Two small non-electrolytic caps for bypass regulator input and output to MCU.A single isolating resistor to bias and isolate each micro-switch input. A single 5V zener at A2D inputs to protect the MCU from overvoltage/voltage reversal due to load dumps, surges, tampering etc. Ditto for the oil-pressure sensor switch.One resistor for each LED indicator.Certainly easy enough to add LCD readout, but LED indicators as in the original would be practical, simple and rugged.

author: SeanB
author_rank: Super Contributor
timestamp: November 21, 2014, 04:23:47 am
content: Quote from: Kjelt on November 20, 2014, 07:35:20 pmcrDSC01484bySeanB_ZA, on FlickrYes they do, though they are not happy with it. That the tractor board drew just about enough current to make the supply 5V or so helped, but a simple mod would be to use a zener diode to clamp the supply voltage. No need for a more complex regulator with a single low speed TTL IC.

author: Paul Price
author_rank: Super Contributor
timestamp: November 21, 2014, 04:26:13 am
content: The real challenge is to somehow enclose the circuit/connectors in a rugged metal/plastic box to keep water, hay and other farm dander from causing damage from corrosion/contamination/open and short circuits.

author: David Hess
author_rank: Super Contributor
timestamp: November 21, 2014, 04:59:32 am
content: Quote from: Kjelt on November 20, 2014, 08:56:51 am74C86

author: Joule Thief
author_rank: Regular Contributor
timestamp: November 21, 2014, 05:49:41 am
content: The real challenge is to somehow enclose the circuit/connectors in a rugged metal/plastic box to keep water, hay and other farm dander from causing damage from corrosion/contamination/open and short circuits.This is very true. Molex and manure is not a winning combo!

author: Joule Thief
author_rank: Regular Contributor
timestamp: November 21, 2014, 06:25:11 am
content: and then there is a more recent version (circa 1990) with LED's supported by diode based logic!The 8 pin DIP is a 555 functional equivalent (MC 1455P) used to flash the "Low Oil" warning LED.