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Openbench logic sniffer pull-up/down wing

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By: madworm

Floating inputs are nasty, as you'll get erratic data. Something you don't want to see on your screen, it only adds confusion and simply doesn't look nice. And if you monitor many channels, your eye might slip and end up on one that shows 'crap'. Of course you will only find out about that mishap much much later.

This add-on board adds selectable pull-up or pull-down resistors to the Openbench logic sniffer (open source logic analyzer, by dangerousprototypes).

The pull-up voltage is selectable from 2.5V, 3.3V and 5.0V in 2 groups, so you can use it with different voltage level signals at the same time.

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Bill of Materials

Qty Part # Description Schematic ID Source
4 4609X-101-103 Resistor Network,10kOhm,2%,Bussed,+/-100 Source
4 3860fd3cc1a6c9a97621ba5ec1e7f7772e47fb1a BD08 DIP SWITCH, POSITION 8 Source
1 E62687d59b5a5eed84b35bac83a29f1238d88e6e 929500-01-36 Pin Strip Header Source
1 F4dc5ec1b074c28680817a97645b4ba6adb9f6e4 PPTC181LGBN-RC CONN FEMALE 18POS .100" R/A TIN Source
2 Ac1f5e43fda4efc820860987ed282d3c9f810b92 2-881545-2 AMP SHUNT LOW PROFI Source
1 11c90230db66971f11e0216efcded943c2d7cf68 PEC36SAAN CONN HEADER .100 SINGL STR 36POS Source
3 CR0603-FX-1001ELF RESISTOR, 0603, 1K, 1%, 0.1W Source
loading total from bomfire.com...

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1. Have the PCB made

Grab the gerber files and send them off to a PCB manufacturer of you liking. The usual suspects located in Shenzhen have pretty good deals for small quantity orders. This time I only posted the gerber files and the schematic. You can still do what you want with it, but this board is so simple that everybody can re-create it on their own if you want to change things. If you're absolutely desperate, try searching for it on github.


2. Get the parts and fire up the soldering iron

Assembly is rather simple. Everything except 3x 1k 0603 resistors is just large through-hole parts. And if you don't want to deal with the SMD parts, you can replace them with a solder bridge (now you shouldn't misplace the voltage select jumpers, or you'll probably fry your OLS). The only parts that need a bit of special care are the resistor networks. Make sure the common pin (#1) is placed into the square pad on the PCB.

You'll have to cut the female headers to size. A file will help to clean up the edges. You'll also need a 4-wire double female header connector cable to 'steal' the pull-up voltages from the OLS board itself.

The individual 10k (brown-black-black-red) resistors you can see in the image were used for testing purposes only. The empty footprint below them will be filled with a 8-component resistor network.


3. Happy logic analyzing !

First make up your mind if you want to use pull-ups or pull-downs. In case of pull-down resistors, you can just flip the dip-switches of unused channels to ON. In case of pull-up resistors, you have to make sure the pull-up voltage is compatible with the signals you use. The board has two banks, so you can choose e.g. 2.5V for channels 0-7 and 5.0V for channels 8-15. Make sure to enable either pull-up or pull-down resistors on _all_ unused channels.

Now you shouldn't see any signals on unconnected logic probes appearing out of nowhere anymore.

Sidenote: YES, you can just use a piece of wire, connect it to GND and clip all unused logic probe hooks to it. It can be done, but isn't funny at all. A) these probe clips have a life of their own and like to go somewhere else. You'll hear a tiny 'click' when it happens, B) you'll end up with a big mess of wires. And you don't want that either ;-)

6279734492_a536738f0e_b 5916331217_d3ec581960 5916329679_957d159c95_o 5909586763_1b8f88828a_o 5916329817_a38dc7536a_o

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2 - Added more logic analyzer traces.
1 - Initial project release
Add revision

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