Parallax Servo Controller -- posted docs not current; ?? about pull-up resistor
Zoot
Posts: 2,227
The printed documentation that came with my PSC has this slug at the bottom of every page:
"Rev B ver 2.4 (#28023) • 1/2005"
The printed doc contains the full description of the serial protocol used by the PSC (2400/38k4; N 8 2) -- open drain.
However, the POSTED documentation at the Parallax site has this slug on each page:
"Rev B ver 2.2 (#29023) • 4/2004"
And the POSTED docs do not contain any references to the actual serial settings (beyond a general discussion of baud rate and the adding of $8000 to the baud to make it "open").
Now, my PSC works fine, but I want to share that serial I/O pin with some other serial devices in open mode, so I wanted to nail things down and make sure I can do what I want. That's how I stumbled upon the omission from the posted docs.
That said, I realized I have another question:
The Stamp manual discusses the need for a pull-up or pull-down resistor when using SEROUT in open modes -- but neither version of the PSC docs mention the need for the pull-up resistor on the serial line. Is there some magic at work here that pulls the serial line high for my PSC? I looked on the PSC schematic but didn't see a pull-up resistor there.
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When the going gets weird, the weird turn pro. -- HST
"Rev B ver 2.4 (#28023) • 1/2005"
The printed doc contains the full description of the serial protocol used by the PSC (2400/38k4; N 8 2) -- open drain.
However, the POSTED documentation at the Parallax site has this slug on each page:
"Rev B ver 2.2 (#29023) • 4/2004"
And the POSTED docs do not contain any references to the actual serial settings (beyond a general discussion of baud rate and the adding of $8000 to the baud to make it "open").
Now, my PSC works fine, but I want to share that serial I/O pin with some other serial devices in open mode, so I wanted to nail things down and make sure I can do what I want. That's how I stumbled upon the omission from the posted docs.
That said, I realized I have another question:
The Stamp manual discusses the need for a pull-up or pull-down resistor when using SEROUT in open modes -- but neither version of the PSC docs mention the need for the pull-up resistor on the serial line. Is there some magic at work here that pulls the serial line high for my PSC? I looked on the PSC schematic but didn't see a pull-up resistor there.
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When the going gets weird, the weird turn pro. -- HST
Comments
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·· Thanks for pointing that out.· I will send an e-mail to our Web people to have the correct documentation listed.· While a pull-up is needed in many situations using Open Baud Mode, it is not required in this case.··Take care.
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Chris Savage
Parallax Tech Support
Might I ask *why* the pull-up isn't needed? My PSC has been working just fine for months, but I'm curious... and if I required the pull-up for another reason (say I put two Stamps and PSC on the same serial line), would the pull-up cause any problems reading or writing to the PSC?
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When the going gets weird, the weird turn pro. -- HST
·
·· The controller chip used has internal pull-up capability on the I/O lines much like as SX Microcontroller has.· I hope this helps.· Take care.
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Chris Savage
Parallax Tech Support
Would I be able to let the internal pull-up on the PSC take care of the pull-up for the whole serial line, or would I want to add another weak pull-up? (I am guessing the internal pull-up on the PSC chip is weak)
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When the going gets weird, the weird turn pro. -- HST
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·· I don’t know the specifications for the pull-up on that chip.· You could look up the datasheet on it to check.· I know the specs for the SX pull-ups.· In any event you may very well be able to get away with one common pull-up if the distance between the units isn’t very far.· Take care.
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Chris Savage
Parallax Tech Support