[PD] comport and servos

[Hans-Christoph Steiner]

Sounds like a crazy hack, but fun stuff for sure.   Maybe this will  
give some insight to serial port jitter in general when using  
[comport].  I measured 0.5ms to 22 ms for roundtrip latency on the  
serial port, with an average of around 12ms.  So I guess that  
translates to 0.25ms to 11 ms with an average of 6ms for just sending  
or receiving.

But that is some serious jitter.  I am wondering how much is caused  
by the hardware and/or OS buffering and how much is caused by Pd and  
the [comport] object.  It would be nice to reduce the jitter caused  
by [comport] as much as possible.


.hc




On Jun 19, 2006, at 10:04 PM, Martin Peach wrote:

> Continuing the cheapest servo controller in the world method, (not  
> counting the computer)...
> I used a digital oscilloscope running at 50ks/s, so the best time  
> resolution is 20us.
> Using a metro set to 20ms to send the number 0 through [comport],  
> the jitter can be brought down to about 300us on my 200MHz linux  
> pentium MMX linux machine. It is necessary to set the audio delay  
> in Media->Audio Settings... to 20ms, that is, the same value as the  
> metro. It is also necessary to enable and then disable  "compute  
> audio" on the main pd window. Leaving the audio enabled adds about  
> 300us to the metro period, causing it to jitter at the 2ms level.  
> Also, don't move the  mouse or try to do anything else with the  
> machine :(
>
> It should be possible to control a single servo with this, but the  
> output needs to be clipped so that it runs between 0 and 5 Volts  
> instead of +/- 12 Volts. The easiest way to do this would be to put  
> a 5Volt zener diode in series with a 1k resistor between the  
> transmit data pin and ground:
>
> TxD-----+
>        \
>        /
>        \  1000 Ohm 1/4 Watt resistor
>        /
>        \
>        +-------------->to servo control
>        |
>       ___
>        ^  5V zener diode
>       / \
>        |
>        |
> Gnd-----+-------------->to servo ground
>
> A better way would use at least a transistor. For the same price as  
> a single transistor an IC comparator like the LM311 running on the  
> same power supply as the servo would be the best interface:
>
>          +5Volts-+-------to servo 5V
>       |\ |       /
>       | \|       \ 10k
> Gnd----|+ \       /
>       |   \______\________to servo control
>       |   /
> TxD----|- /
>       | /|
>       |/ |
>          + Gnd-----------to servo Gnd
>
> To control a servo, using 1200 baud, the numbers 0, 128, 192 and  
> 224, 240 should position the servo roughly at 1/5 range steps. 240,  
> 248, 252, 254 and 255 may be too short. The binary representation  
> of these numbers is all 1s ending in all 0s. RS232 communications  
> sends a 1 level (-12V) by default, and starts every character with  
> a 0 bit (+12V). The least significant bit of each character is  
> transmitted next, so to make variable pulse widths, 255 (all 1s)  
> gives the narrowest pulse (just the start bit). Servo pulses are  
> supposed to be at least 1ms and no more than 2ms, so 248 is  
> probably as narrow as you can go at 2400 baud.
>
> I fixed some more things with comport, but can't get them into cvs  
> today (authorization failed, worked OK yesterday...). I'll try  
> again tomorrow. Also I plan to make the DTR line settable so  
> possibly that could be used as a cheap digital output as well.
> Martin
>
>
>
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