[Ovmsdev] OVMS v3 First Board Layout
mark at webb-johnson.net
Thu Mar 30 09:07:46 HKT 2017
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?
The idea is to retain the existing DB9 connector, with the same basic pin arrangement:
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.
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