[Ovmsdev] OVMS v3 First Board Layout
collink at kkmfg.com
Thu Mar 30 22:33:42 HKT 2017
I'm unsure of why the 3.3v transceiver would be a problem. CAN wiring
is supposed to be differential and it isn't supposed to matter if it
is 20V and 18V relative to local ground or 4V and 2V. There is isn't a
requirement to keep a shared ground between two CAN devices so how
would a remote node even know what the "voltage" of CAN-L is? Voltage
is only defined between two points so if you don't share a ground
between two nodes then the only voltage the transceiver knows is CAN-L
to CAN-H which will range by the proper amount whether you use a 5V or
3.3V transceiver. I've used the SN65HVD series transceivers in all
sorts of things and never had a single problem. I've also used the
ISO1050 isolated transceiver and also had no problems with different
devices. But, if the 5V transceiver is cheaper and you can get 5V
easily enough then maybe that's still the way to go. I'm just saying
in my experience the concern isn't terribly warranted. YMMV.
On Wed, Mar 29, 2017 at 9:07 PM, Mark Webb-Johnson
<mark at webb-johnson.net> wrote:
> I’m trying to finalise the OVMS v3 final board layout, with the factory in
> China. We have some questions and seek your opinions:
> CAN transceivers / power
> Overall, the OVMS v3 system runs at 3.3V. We have two power supply sources:
> USB (where we use a 5V -> 3.3V regulator), and +12V vehicle power (where we
> use a +12V -> 3.3V switching power supply, to be as energy efficient as
> possible). Diodes are used for reverse-polarity protection as well as coping
> with the situation where both usb and vehicle power is applied
> Our problem is with the CAN transceivers. I’m used to the MCP2551 (been
> using it for a decade or more), but that is 5V so greatly complicates the
> power supply arrangements at the +12V side. We can switch to something like
> the SN65HVD233 transceiver that works at 3.3V.
> But, I am concerned about comments I am reading about 3.3V CAN transceivers
> and their inability to meet the ISO11898 dominant condition requirement of
> 3.5V. From my understanding, these 3.3V CAN transceivers get around this by
> driving CAN-L to 1V, to still get the differential of about 2V (recessive
> condition?). My concern is compatibility.
> What do people think about this? Any recommendations?
> External Connectors
> The idea is to retain the existing DB9 connector, with the same basic pin
> DB9-M Signal
> 3 Chassis/Power GND
> 2 CAN-L (primary)
> 7 CAN-H (primary)
> 4 CAN-L (alternate CAN)
> 5 CAN-H (alternate CAN)
> 9 +12V Vehicle Power
> That leaves pins #1, #6, and #8 free for expansion uses. It gives us
> compatibility with existing OVMS cables.
> We would then add a second connector. The suggestions here are DB15 normal
> density, DB25 normal density, or DA-26 high density. My preference is the
> DA-26 (as DB25 is the old parallel printer style connector and very bulky).
> As well as power lines, expansion cards could wire to this connector to
> expose external inputs/outputs.
> What do people think about the DA-26 connector? I’m suggesting a female
> version (as power is carried there, and I don’t want the pins to get pushed
> together for a short).
> Note that we’ve also got a micro-usb socket, as well as space for GPS and
> GSM/GNS antennas.
> Other than that, we are good to go. Things have stabilised now with
> Espressif, so we can proceed with building developer boards.
> Regards, Mark.
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