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https://github.com/bkw777/molex_7880x_bus_tap

Bus tap to connect a logic analyzer between a Molex 7880x socket & chip carrier. (TANDY 100/200, Epson PX-8 Option ROM sockets)
https://github.com/bkw777/molex_7880x_bus_tap

Last synced: 25 days ago
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Bus tap to connect a logic analyzer between a Molex 7880x socket & chip carrier. (TANDY 100/200, Epson PX-8 Option ROM sockets)

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README 

        
# Molex 7880x BUS TAP

Bus tap to fit between a Molex 78805 socket and a chip in a Molex 78802 carrier



![](Molex_7880x_28_BUS_TAP.jpg)



More pics: https://photos.app.goo.gl/PHQ1rTQFpPRF6NSh8



Provides a connection point for a logic analyzer or oscilloscope to tap the bus between a Molex 78805 socket and a mating Molex 78802 carrier module that would normally be in the socket.



## Parts



Carrier for the small PCB: https://github.com/bkw777/Molex78802_Module/blob/master/Carrier/Molex78802_PCB_28.stl  

  Some places to buy from. Select Material: Nylon, Process: SLS or MJF  

  https://www.sculpteo.com/en/print/molex78802_pcb_28-18/nC7YgK6X  

  https://www.pcbway.com/project/shareproject/Molex_7880x_28_PCB_Carrier_9419cd9c.html



Housing for the large PCB: [CASE/Molex_7880x_bus_tap_cover.stl](CASE/Molex_7880x_bus_tap_cover.stl)  

  This one can be FDM printed.



FFC connector (x2): https://www.digikey.com/en/products/detail/amphenol-cs-fci/F52R-1A7H1-11028/11564624



FFC cable - 4" gold - Molex 0150390449 : https://www.digikey.com/en/products/detail/molex/0150390449/11581605  

FFC Cable - 6" gold - Molex 0150390453 : https://www.digikey.com/en/products/detail/molex/0150390453/11581883  

Other FFC Cables in other lengths, copper weight, plating options: https://www.digikey.com/short/pqq5pd04  



Molex 78805 socket: These are no longer made. Find me on the M100 mail list. I have some and will sell them one at a time until gone.  



Other possible solutions for the socket:  

* Once in a great while they do turn up on ebay or other places on-line:  

  https://www.dhgate.com/product/for-molex-dip28p-ic-chip-socket-silver-plated/835617844.html  

  https://www.ebay.com/itm/153988317907  

  Search "molex ic socket" and then ignore all the normal dip sockets.  

  Other search terms: 8878 28S, A78805-0518, A78805-0918, 15-29-9282, 15-41-2283  

  Or any other part number from here: https://github.com/bkw777/Molex78802_Module/blob/master/references/015299282_sd_corrected.pdf  

  Particularly detail-1, detail-10, detail-11 as those ones all have key slots at positions 1,5,9. The peg positions on the bottom don't matter because they can just be cut off.  

  (Or maybe I'll update the pcb to include drill holes in all 8 peg positions. Really any version of the socket would be better than nothing if you found any at all, just cut the missing key slots if needed.)  

* DIP-28 (or more) test clip and a standard dip-28 socket or male pin headers to connect the wires.  

  When searching for a test clip, make sure it opens wide enough for 600 mill / aka 0.6" wide dip-28, not just 300 mil.  

  https://duckduckgo.com/?t=h_&q=test+clip+dip+28+32+40+600+0.6

* Solder long wire-wrap pins in place of the socket and wedge the module between the pins.  

  https://www.digikey.com/en/products/detail/samtec-inc/ZW-16-15-F-S-225-175/6614101  

  The width of the module is a little wider than the width of the pins, so the pins would need to be bent, or change the PCB to provide a footprint with just the right spacing.  

* Change the PCB to use 2 of these: https://www.digikey.com/en/products/detail/samtec-inc/SIR1-15-L-S/10218718  

* Change the PCB to use these: https://lectronz.com/products/flexypin-pack-of-100



TODO: Design a socket that can be printed and just needs plain wire for the contacts.



Machines with this socket:  

* TRS-80/TANDY Model 100, 102, 200, 600

* Kyotronic KC-85

* Epson PX-4, PX-8

* Intermec 9440, Telexon 710, and similar hand-held teriminals

* Allen-Bradley SLC 500 programmable controllers



Cable notes:  

4" is long enough to reach out from under the nearest side to the socket on all machines.  

6" is long enough to reach the left instead of the front on a Model 200.  

8" is long enough to reach the back on a Model 100, and does still work, but might be getting marginal at this length.  

10" untested  

12" does not work



# PINOUTS



 TANDY 100/102/200  TANDY 600 / Epson PX-4/8
(STANDARD MASK ROM) 

 VCC128A14   NC128VCC 

 A10/CS   A12A14 

 A7A8   A7A13 

 A6A9   A6A8 

 A5A12   A5A9 

 A4ALE   A4A11 

 A3A13   A3/OE 

 A2/OE   A2A10 

 A1A11   A1/CS 

 A0AD7   A0D7 

 AD0AD6   D0D6 

 AD1AD5   D1D5 

 AD2AD4   D2D4 



 GND1415AD3   GND1415D3 



# GND pins work-around

This thing could really use a bunch more GND pins, both for logic analyzer probes and for a place to clip oscilloscope probe alligator clips.

Currently there is only the single GND pin which is pin 14.



For now a simple work-around that provides both extra pins for logic analyzer probe grounds and a perfect place for a scope probe alligator clip, is just a piece of single row male pin header with a wire soldered to bridge all of the posts, and a single short female-female dupont jumper to connect to the single gnd pin.



![](gnd_pins_workaround.jpg)

![](scope_probe_gnd_alligator.jpg)



# Related

http://tandy.wiki/Molex78802_Module  

https://github.com/bkw777/M4ROM  

http://tandy.wiki/Teeprom  

http://tandy.wiki/REX