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@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
simultaneously.
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
arrangement:
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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