chancecasey

I thought it might be a good idea to try to organize future posts on this very popular thread - so I'm starting one for making the interface itself.

I started building my interface before the pre-printed boards were available. I'm using the Gregg K interface schematic, and so far it isn't working. The software says "connecting" and I can get an echo back from hyperterminal if I jumper the tx/rx lines on the DB9 connector, but scantool never gets past "connecting" when those lines are not jumpered.

Two components I did not connect when building the interface are NAND gates who's inputs are ground and output is connected to an LED and then the other end of the LED looks attached to nothing. Does it attach somewhere? I didn't see how connecting two inputs of a NAND to ground would do anything.

I did connect the one other LED which I assume indicates 12v power and it does come one when I connect the interface to the car.

mojorizing

You need to connect the un-used inputs to ground for CMOS devices, they shouldn't be left floating. I'm not sure how non-connected inputs for a quad gate effects the connected inputs, but that's a general rule. I'll try to get a schematic of my circuit up soon, it differs in components, but does essentially the same.

chancecasey

Thanks! I am using the quad NAND with a dedicated GND and Vcc, but I'll give it a try and report the results...

chancecasey

Darn! No luck after connecting the 2 pairs of unused NAND inputs to ground. One other thing I did, sort of "connecting the dots" perhaps incorectly, is that when I saw the AGND and DGND connected together at the bottom right-hand side of the drawing, I assumed that meant they were common, and therefore I have only one ground line which I connect everything to, whether it calls for AGND or DGND. Was that a mistake?

mojorizing

I think that's Analog ground and Digital ground and in this slow speed application it is one and the same.
I would check the orientation of the 2N7000's, make sure the source(ground) and drain (V+) are correct.
Then check the polarities of all the polarized capacitors. Then check that you have the correct connections on the Sipex (T1-IN, etc) You're using a Sipex 3222EC?

chancecasey

Yeah, that's what I understood the different GNDs to represent but since they were connected in the schematic, that made it easier to build. I will go back and double-check all my polarites tonight. Just so I'm clear, the + on the schematic is the same as the + printed on the capacitor, right? Also, I'm using a mix of ceramic, metal film (or oxide?), and polarized caps - had to as Radio Shack did not have a great selection.

mojorizing

Here's a link to the schematic of the interface I built, basically parts that I have on hand, but also available from glitchbuster.com


interface