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 groundA 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 |\ | / | \| \ 10kGnd----|+ \ / | ______________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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