Above is the circuit drawing for picoSam2. This version is comparable to picoSam1 with the distinction:
- Slightly different circuitriy around HCT243 buffer
- Opto couplers with LED indicators
- Dual power supply, isolated on computer side
- Voltage reference
If you are new to electronics, print out the circuit drawing and keep it aside for reference while reading
this section.
The picoSam 2 circuit is laid out as follows. We will scan the drawing from topleft to bottom right.
- The DB25 connector mates with the LPT port (or an appropriate extension cable).
-
I designed an auto detection circuit around it. Pins 14/15 are connected, just like pinpairs 8/10 and 9/13.
Now, software will be able to auto detect the presence of the picoSam 2 adapter.
-
The signals are buffered by means of a 74HCT243 quad bus buffer with Schmitt trigger inputs. D6 controls the
tristating of the busbuffers.
-
D0 and D1 each drive one PC 817 opto coupler. A LED is placed in series with the opto coupler and the
current limiter so that the user can SEE that an opto coupler is working.
-
Damn.
At this very moment I see that I made a mistake. I made this circuit by starting out with picoSam 1 and adding
things. I added so much and changed things accordingly, that I lost track of the big picture.
The TLC 0831 ADC only has a DO output. The TLC 0832 ADC is multi purpose. So it needs to be 'programmed' and
hence needs a DI input. At this moment I have several options:
-
Keep the TLC 0832 ADC and change the circuit
If I do this, then I need to
- add another opto coupler with LED indicator
- add pull up resistor and tracks on the hot side
- redo the complete PCB layout
This is a hell of a job.
-
Change over to the TLC 0831 ADC and change the circuit
If I do this, then I need to
-
change the analogue input section to single channel:
- I can skip one opamp section
- I can change the input section to a difference engine
- redo the complete PCB layout
This is a hell of a job.
- Start again from an empty sheet
Option 3 appeals most to me right now. Perhaps it is even time to reconsider the type of the ADC. The TLC 083x
series can do 22 ksps. But a well controlled LPT port can do 50 - 100 ksps.
I think it's time to get the picoSam 2 towel and throw it into the ring.
Page created on 2 Februari 2008 and
Page equipped with FroogleBuster technology
