Has anyone replaced a MAX6675 with a MAX31855 on Gen4 electronics?

[Ethan Dicks]

Greetings, fellow makers,

I was noodling around on Maxim's site and confirmed something I think
I heard a while back - the MAX6675 (as found on the Gen4 extruder
controller) has been discontinued. While it will be possible to find
such parts in the retail channel for some time, has anyone done the
legwork to replace it with its appointed successor, the MAX31855?
There are two up-front issues to resolve - the MAX31855 is not
pin-compatible with the MAX6675 and it's not code compatible either.
The Mighty Bboard Rev E can take either chip, but it has 3.3V and SMT
jumper pads to switch around between the differences. The "old" Gen4
Extruder Controller does not have that.

Adafruit sells MAX31855 breakout boards, so wiring one up externally
shouldn't be too hard. Has anyone produced modded firmware that would
work? (or could it be done by fiddling the build to look something
like a Mighty Board build?)

Considering that a number of Gen4 machines have lost their MAX6675
chips, are people just replacing the chips and/or boards with
originals, or has anyone looked into an upgrade hack? The MAX31855 is
1/10th the cost of the older MAX6675, so it seems to me to be a
worthwhile hack, even with having to find/generate 3.3V to power it.

-ethan

[Jetguy]

Just a thought, Makerbot already knows what you know, the MAX6675 is
basically to be considered for production no longer available. The
giant hint here is the the schematics for the Mightyboard in the
Replicator has listed the changes and it's a couple of resistor
values. Meaning likely, new versions of the code auto sense the chip
because they have to support the old installed base and work for the
new boards too and not make the user figure out what chip they have.
http://thingiverse-production.s3.amazonaws.com/assets/6d/94/65/9b/90/MakerBot_MightyBoard_REVE_Schematic.pdf

Easy enough to prove, get the breakout from Adafruit, connect it to an
Arduino Mega 1280 or 2560 and load the latest firmware. They branch
the code between models of board so likely the change was made across
the board. You would know in jsut a few seconds if the code works if a
correct temp is reported to rep-g.

Just my thoughts.

[Thomas Charron]

On Mon, Oct 8, 2012 at 4:45 PM, Ethan Dicks <ethan...@gmail.com> wrote:
> Considering that a number of Gen4 machines have lost their MAX6675
> chips, are people just replacing the chips and/or boards with
> originals, or has anyone looked into an upgrade hack? The MAX31855 is
> 1/10th the cost of the older MAX6675, so it seems to me to be a
> worthwhile hack, even with having to find/generate 3.3V to power it.

There is 3.3V, synthesized by the FTDI chip, at least on typical
Arduino boards. Board already has it out, with the resistor not
populated, connecting it to 3.3

--
-- Thomas

[Jamesarm97]

Basically the 31855 is a 3.3v version so you are changing the CS pullup to use 3.3v and rescaling the input to the 3.3v range by changing the voltage divider on the input.

[Ethan]

On Mon, Oct 8, 2012 at 4:45 PM, Ethan Dicks <ethan...@gmail.com> wrote:
> Considering that a number of Gen4 machines have lost their MAX6675

> chips, are people just replacing the chips and/or boards with
> originals, or has anyone looked into an upgrade hack?  The MAX31855 is
> 1/10th the cost of the older MAX6675, so it seems to me to be a
> worthwhile hack, even with having to find/generate 3.3V to power it.

I had a MAX6675 on hand (from a free sample order last year).  It's installed and works great.  Much less effort than reworking an old Extruder Controller.

-ethan