Building the Tauntek Logic IC Tester

Completed Tauntek Logic IC Tester Version 1.1 with cover

I just fin­ished a pop­u­lar kit project used for test­ing 7400 series, and some 4000 series log­ic IC’s. It was recent­ly fea­tured in CuriousMarc’s YouTube chan­nel titled “Viewer-designed, Tauntek TTL IC tester!”.

I have been plan­ning on build­ing my own log­ic IC tester for sev­er­al years, but have been putting it off for many rea­sons. I decid­ed to pur­chase this kit for two rea­sons. Either it would com­plete­ly fit my needs, so that I would not need to design and build my own. Or it would give me addi­tion­al ideas or inspi­ra­tion to build my own. (or both)
The kit cost me $43.40 USD includ­ing ship­ping, and includ­ed an unpop­u­lat­ed PC board and two pro­grammed PIC micro­con­troller chips.

Tauntek Logic IC Tester Version 1.1 unpop­u­lat­ed board top
Tauntek Logic IC Tester Version 1.1 unpop­u­lat­ed board bottom

Parts to pop­u­late the board from DigiKey cost $65.00 USD not includ­ing tax­es or ship­ping. It took me a full after­noon and evening to assem­ble and test the kit, which also includ­ed test­ing all tran­sis­tors, diodes, resis­tors, and capac­i­tors before sol­der­ing then on the board. There were a total of 162 indi­vid­ual com­po­nents to sol­der to the board, and I used some Kester 44 rosin-core 0.020″ sol­der which worked well with a Hakko T15-BLL fine point con­i­cal tip sol­der­ing iron.
I did find one design issue while build­ing the board with the oper­a­tional ampli­fi­er U7B. The inputs and out­put of the 2nd unused op-amp were left float­ing with no con­nec­tions. This is some­thing that many peo­ple over­look, and is not rec­om­mend­ed prac­tice. a great expla­na­tion why not to do this can be found in a Texas Instruments TechNotes pub­li­ca­tion sboa204a.

Bodge to fix unused float­ing oper­a­tional ampli­fi­er issue

The fix was fair­ly easy using a pair of 12K ohm 1/10th watt resis­tors and a jumper. This forms a volt­age divider between ground and +5 volts result­ing of a volt­age of approx­i­mate­ly 2.5 volts tied to pin 5 of U7 the + input. The out­put pin 7 is tied to the — input pin 6 with a jumper.

Solid alu­minum poly­mer capac­i­tors for +5 volt reg­u­la­tor circuit


I also made some oth­er small changes to the design by chang­ing the val­ues of the 5 volt reg­u­la­tor capac­i­tors. The board lay­out had an odd lead spac­ing of 3.00 mm, while most radi­al capac­i­tors have a stan­dard spac­ing of either 2.5 or 3.5 mm. I end­ed up using some Conductive Polymer Aluminum Solid Capacitors that I had in stock. I low­ered the input capac­i­tor C10 to 33 uF at 20volts and raised the val­ue of the out­put capac­i­tor C8 to 150 uF at 10 volts.
I also changed capac­i­tors C4 thru C7 on the RS-232 lev­el shifter U3 from 0.1 uF to a ceram­ic 1 uF at 50V X7R capac­i­tor. This is the rec­om­mend­ed val­ue in the recent Texas Instruments spec­i­fi­ca­tion sheet for the MAX232

Diodes and resis­tors pop­u­lat­ed on the board
Completed board with the excep­tion of the ser­i­al data connectors
Tauntek Logic IC Tester V1.1
Completed Tauntek Logic IC Tester V1.1

One last mod­i­fi­ca­tion before I can use the board. The P2 con­nec­tor is set­up for con­nect­ing a RS-232 ser­i­al con­nec­tion using a 2X5 10 pin IDC to a DB9 IDC con­nec­tor. This was prob­a­bly use­ful 10 years ago when this was orig­i­nal­ly designed, but not as use­ful today with the lack of 9 pin ser­i­al ports on most com­put­ers. The design­er did include a 6 pin head­er P1 for con­nect­ing a USB to Serial adap­tor. The design­er states that this “Maybe the cor­rect pinout” but is untested.

Serial port head­ers and RS-232 lev­el shifter U3 with jumpers


It is not the cor­rect pinout for a TTL ser­i­al to USB adap­tor. So a small adjust­ment is need­ed to make it com­pat­i­ble. As designed the cir­cuit routes RS-232 out­put lev­els, which are +/- 10 volts to pin 5 of head­er P1, and looks for the same input lev­els from pin 4. I used a Sparkfun FTDI Basic TTL ser­i­al to USB adap­tor which requires 0 — 5 volt TTL lev­els in its inputs and out­puts. The sim­ple fix for this is to remove the RS-232 lev­el shifter IC U3, and add two jumpers. This routes the RX and TX TTL sig­nals from micro­proces­sor U1 to pins 4 and 5 of head­er P1. Pin 1 of head­er P1 is ground.
I used sev­er­al trimmed leads from installed capac­i­tors to make the jumpers. Jumper 1 con­nects pin 11 to 14 on sock­et U3, and jumper 2 con­nects pin 12 to 13 on the same emp­ty socket.

I tried sev­er­al ser­i­al ter­mi­nal pro­grams at baud rates up to 19.2K which is the list­ed max­i­mum speed with good results. Best results are obtained using a VT100 mode emu­la­tion as this is the for­mat that the board outputs.

Testing a 74LS158 IC using a non-VT100 terminal
Testing a 7400 IC using a non-VT100 terminal
Testing a 74HC138 IC using a Putty VT100 ter­mi­nal emu­la­tion also V & D tests
Simulation of a failed test (chip removed) with high­light­ed bad values

Testing a 74AHC125 CMOS 3‑state out­put quad bus buffer log­ic IC.
When dis­abled the out­puts are turned off pre­sent­ing a high-imped­ance to the bus as can be seen in row 3 of the test as float­ing out­puts (F).

Testing a 74AHC125 quad bus buffer


Overall it is a good log­ic IC tester kit, but needs a few tweaks to make it excel­lent. The firmware works well with no issues that I noticed. I was fair­ly unim­pressed with the trace sizes of the 5 volt and ground busses on the board. There was plen­ty of room on the board to increase the trace sizes, and add sep­a­rate cur­rent paths for the device under test (DUT), micro­proces­sors, pull-ups, and RS-232 lev­el shifter. There were ade­quate bypass capac­i­tors installed on most active devices though, which is always good to see.
Only the first 19 pins on the DUT are tied to A to D con­vert­ers for ana­logue volt­age mon­i­tor­ing on indi­vid­ual pins. For most com­mon log­ic IC’s this is good enough, as there are very few log­ic IC’s with over 20 pins.
It is not open source soft­ware or hard­ware which is fine, but may have ben­e­fit­ed from the com­mu­ni­ty by going that route. The devel­op­er is active­ly updat­ing the library of log­ic IC’s that it can test. To update the library the pri­ma­ry micro­proces­sor must be repro­grammed with an updat­ed hex file.

I have also been look­ing at the Retro Chip Tester Professional by 8Bit-Museum.de , and may build my own ver­sion of a log­ic IC tester lat­er this year if my bud­get permits.


Spoiler below,

Prototype 28 pin Logic IC tester Rev 0.6

2 Replies to “Building the Tauntek Logic IC Tester”

  1. Thanks for the re-engi­neer­ing hints, espe­cial­ly the ser­i­al inter­face and the float­ing op-amp pins.

    When do you expect to release your own tester?

    1. Hi Jack,
      A work­ing revi­sion of the hard­ware is com­plete (0.14), and a low-lev­el test­ing pro­gram has been installed for the ADC’s on the SPI bus, and the I/O Expander on the I2C bus.
      Reading all 28 A to D con­vert­er input chan­nels in less than 1 mil­lisec­ond with 16 sam­ple aver­ag­ing per channel.
      Screen cap­ture of ADC read­ings with Pullup and Pulldown resis­tors enabled on all chan­nels. Pin 14 is select­ed as ground, and pin 21 set as 5 volt VCC.
      Prototype tester Rev.  0.14
      ADC readouts from Logic IC tester
      I cur­rent­ly have anoth­er board revi­sion ready with mos­fets and oth­er changes, but am low on funds with the hol­i­days so it may be a while before it is ordered.
      The soft­ware will like­ly take the most time, as I am much bet­ter with the hard­ware side of projects. So may release it next Fall.

      Thanks, Happy Holidays
      Greg (Barbouri)

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