You may have read in our last article that we’ve been digging around in the Logic Cube and found that, in a hardware sense, it’s an easy mod to install the other 32 channels. And of course, we pointed out that the RAM installed on our board was a 128K, not a 32k. So it seems that with a minimum of work, we can upgrade our $120 16032 to a 32128 which would have cost us a few hundred bucks more.
As gedesu mentioned in a comment, the famed bushing already discussed this hack a few months ago on a hardware forum page. Sadly, we never saw this page and I suppose we’ve gone and duplicated efforts a bit as we developed our mod independently. But we were actually able to catch bushing and a few other developers for a few minutes today and had a very nice discussion on modding the device. Seems both they and we had discovered a few secrets that the other hadn’t, and a lively exchange took place.
The mod really is as simple as installing the missing channels and trigger buffer circuitry. Now when we say simple, we mean simple for a person that is pretty skilled in soldering. This is not a project for a first time user, as the risk of damaging the device would be high. But if you’re careful and thoroughly check the connections before powering up, you should be fine.
The total cost is less than $5 in parts. We bought everything but the 74LVT16245 from HSC, so the prices listed are what we paid. We bought the 74LVT16245′s from Mouser because they don’t gouge like Digi-Key or Arrow.
Here’s what you will need:
1x 74LVT16245B 3-state Bus Transceiver ($0.56) – You need to get the DGG (“74LVT16245BDGG”, 48-TSSOP) package. We couldn’t find the TI part and got an NXP LVT16245 as well as a 74ABT16245. We ended up using the ABT chip but the LVT is probably the better choice. Hereafter will be called the ’245.
NOTE: the NXP 74LVT16245B is NOT SUITABLE for this mod. It latches up when an external signal is applied. Our suspicion is the 10pF cap in the input circuit drives the input pin a little beyond the rail and NXP did not account for this in their circuit. Amateur mistake! Especially when cloning a chip that has been around for what, 15 years, with no latchup issues. Booo! Credit for this catch goes to Jake, who traced the problem back and raised the issues when we failed to mention the latchup and just switched to the ABT without telling you valuable readers why it was done. Apologies!
- 1x NXP 74ABT16245B or genuine TI 16LVT245B. These are two chips that are verified not to latch up in the application. Both come in the DGG package.
- 48x 1MEG SMT resistors (48x $0.02) – 0603 package is good and not too tiny. You need 3 for each channel you’re adding, so get 55 or so. They always land upside down when you dump them out of the tape & reel and the natural inclination is to press one side to get it to flip neatly over. But sometimes that ends up in your resistor shooting off somewhere never to be found. Getting extras of all the SMT components is probably a good idea.
- 16x 510 Ohm SMT Resistors (16x $0.02) – Again, we used 0603 size. One for each channel you’re adding. Get extras.
- 16 10pF SMT Capacitors (16x $0.05) – One for each channel. 0603 if you can find them, otherwise 0402. Get extras.
- 2x 10k SMT Resistors (2x $0.02)- One for each of the two new trigger buffers (each buffer drives 8 lines. eg: C0-C7 and D0-D7)
- 2x 36k SMT Resistors (2x $0.02)- One for each of the two new buffers.
- 8x 120nF SMT Capacitors (8x $0.05)- Two each for each of the two new buffers, and 4 for the decoupling caps of the ’245. 0.1uF is probably fine as well.
- 2x 1.2uH SMT Inductors (2x $0.07)- One for each of the two new buffers. Debatable. We lost ours and used wire to short these pads, but we’re very willing to debug the PCB if noise issues arise. If you don’t want to mess around, install them.
- 2x 12pF SMT Capacitors (2x $0.05)- One for each of the two new buffers. You can probably use a 10pF if you just want to buy more of those from above.
- Fine Point Tweezers – For holding the SMT components.
- Fine Point Soldering Iron – The tip should be pretty small to solder the leads of the ’245 accurately. Similar to a toothpick tip if you can find it. We’ll give you a technique to make ithe TSSOP soldering as easy as possible, but a fat tipped soldering iron still won’t work. Get a fine point tip before starting.
- Thin Solder – that 10 gauge crap for pipes ain’t going to cut it. That stuff would be acid core anyway and will eventually eat away your PCB so don’t even consider it. Invest the $30 in a spool of nice fine rosin core solder. Thinner than 0.8mm would be great. Our spool has lasted 5 years or more and we solder like mad bastards, so the initial investment can be amortized over many projects.
- Liquid Solder Flux – Try to get a “flux pen”, but the brush-on stuff should also be fine. You’ll paint the leads of the 16245 when soldering to hopefully lessen the chances of solder bridges. You can also paint the pads of the RC networks if you’re having trouble with solder slop. Flux is awesome like that – it makes solder want to stick to metal and not want to stick anywhere else. So your solder will jump right on the pad and won’t slop around. If you don’t use it – get it. Cleans up with Ethyl Acetate (Flux cleaner) or Acetone and a cotton swab.
- Solder Wick/Braid – A loosly braided mass of thin copper wires soaked in flux. It sucks up solder in order to desolder components or clean up bridges and blobs. To tell the truth, desoldering usually requires MORE solder so the thermal intertia keeps it liquid long enough for you to pull off the part. But then you still need the braid to get those little puddles of solder off your PCB and device.
OK, got all your stuff? Are you ready to start the build? Continue on, intrepid reader!
Overview of the LAP-C Modifications
Below is the overview of the areas we’re going to work in. We have 3 main sections where we will be doing mods.
- Input Circuit (BLUE) – One for each channel. Starts with 3 1M resistors in a resistor divider chain. Across the middle resistor is a series connection of a 510 Ohm resistor and 10pF cap. The external port connects to the 510 Ohm side of this network, and the ’245 input connects to the 10pF capacitor side.
- Trigger Voltage Buffers (YELLOW) – An interesting circuit that uses an opamp to generate a dac-controlled voltage for each port (set of 8 channels). This trigger buffer voltage is fed to the top of all 8 resistor divider chains in the input circuit. By varying the buffer voltage, the device adjusts various input levels up or down to match the input threshold of the 3.3v ’245.
- Unused Channel Shorts (RED) – On the back of the board are 16 empty resistor padstacks. In the center of these are a small trace shorting the pads together. This is shorting the 16 unused channels to GND. You’ll need to carefully cut the shorting trace without gouging too deep into the board. We used a tiny flathead screwdriver tip to gouge a channel which worked fine.
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