1997 F-Body Tools

[Tom H]

I hadn't even looked at the schematic other than to identify you were using a transistor pair to drive the output, etc. It's hard to read, is that rail voltage 6.2 or 8.2 volts? I should have a 7806 and maybe even a 7808 3 pin regulator in my parts bin.

Regardless, if this bus is electrically similar to the older 8192 baud ALDL, I have to wonder (and will experiment with) a simple resistor and diode arrangement as I used on my data logger.

The objective is to send 6.25 - 8V when the output is active and high impedance when it is passive. The rest of the symbol info is in the timing of transitions. Any 8V regulator will allow for the drop in your output stage and should work a treat and you won't need a zener diode to regulate. You could avoid transistors and so on by using a 1488 rs2323 driver and a resistor. While you might not be in spec with the J1850 (rise time/fall time issues) it will work fine. When I did the circuit, I just went to my parts drawer and used what I have. Looking at it with the scope I see no problems.


-Tom

[spfautsch]

The objective is to send 6.25 - 8V when the output is active and high impedance...

Thanks for the clarification.

Out of curiosity, did you test with TTL levels? I know it wouldn't be within spec but the 328P will happily sink 20ma per pin at 5V with no issues driving the outputs active high. When not driving the pin it could be toggled to an input / high impedance state.

[Tom H]

I did try 5V, didn't work for me. Interested if you test.

Didn't understand the 2nd bit... No need to sink any current. Passive state is high Z. Active state sources current. Issue I had is that 5V doesn't seem to be enough to toggle the input stage. If you can get it to work, all the resistors & transistors involved with the TX can be removed with just a small bit of software change. I power the NANO through USB. There are some voltage losses through the chip and also a diode to permit other power schemes. Powering with the NANO regulator might be enough to make it work. So there might be a way, but I didn't look too hard. I also needed to buy the zener. Can't just buy one on the 'zon so i have a bag full.

-T

[spfautsch]

No need to sink any current.

Just so we're speaking to the same semantics, when I say sinking current that to me implies an output pin on the AVR pulled positive sinking current supplied by an external ground. Sourcing current would be the same output switched to ground / negative. This was the terminology I was schooled with, with the underlying theory that current flows towards a positive charge. Not that important, just semantics.

I'm still playing catch-up so haven't looked at any of the source code. But if you're using any of the Arduino PWM libraries, changing a pin's state during operation may not be feasible.

[Tom H]

Just so we're speaking to the same semantics, when I say sinking current that to me implies an output pin on the AVR pulled positive sinking current supplied by an external ground. Sourcing current would be the same output switched to ground / negative. This was the terminology I was schooled with, with the underlying theory that current flows towards a positive charge. Not that important, just semantics.

While you are absolutely correct in what you write, however conventional current flow is the standard in use almost everywhere today. When looking at data sheets for ICs and so on, the reference is to ground. Specifications for sourcing of current are above that potential. This is somewhat of an abstraction from what we want to get to here. I will try not to use source/sink to describe current flow. I believe you should have no trouble adapting the circuit given to the parts you have on hand.

To get rid of the need for an 8V supply, I thought for a moment about using an output of the micro to toggle and building a switch capacitor boost circuit. This took too many parts, i discarded the notion.

I will be interested to see what you come up with. There is no use of PWM libraries in my design. You can do practically whatever you need WRT control of the outputs.

-Tom