The "PCM" term refers to the transmitter-receiver link. It is proprietary information to the various manufacturers. The signal is still FM. By transferring digital information the receiver can send to servo pulse to the servo at the rated speed. Without this strategy the servo cannot achieve its maximum (speed , torquee) ratings. John Ferrell W8CCW "Life is easier if you learn to plow around the stumps" http://DixieNC.US ----- Original Message ----- From: "William Chops Westfield" To: "Microcontroller discussion list - Public." Sent: Monday, May 07, 2007 1:24 AM Subject: Re: [PIC] r/c receiver interfacing > > On May 6, 2007, at 8:25 PM, Robert Rolf wrote: > >> Actually, in most early (non digital) R/C setups the pulse >> DOES drive the motors directly. The leading edge triggers >> a monostable whose period is set by the current servo position. >> an exclusive-or of the incoming and position pulses go to an >> H bridge... > > That's what you call "directly" ?! I think the poster > meant that PWM of the motor power (traditional for some speed > control sorts of things) is not driven off of the "servo output" > PWM channels... > >> >> The receiver demultiplexes the PWM (not PPM) train using a simple >> counter and monostable to detect end of frame > > I see AM, FM, and "PCM" radios advertised, so it's not entirely > obvious that they all encode the bitstream the same way... > The output of the RADIO is some sort of bitstream that includes > the position of each of (12? many?) possible servos and some > synchronization scheme that allows you to tell which bits go > with which servo. This goes into a decoder of some kind (these > days frequently a microcontroller, though SSI logic of the sort > described here used to be popular) that demultiplexes the bit > stream and provides a signal to each of the servos supported > > Radio output: SSSS___XX___XXX__X____XXXX______ > Servo 1 out: _______XX_______________________ > Servo 2 out: ____________XXX_________________ > Servo 3 out: _________________X______________ > Servo 4 out: ______________________XXXX______ > > HOWEVER, My impression was that the original poster was more > interested in the problem: "given a servo pulse of 1 to 2ms > as input, is there a standard way to drive a stepper that > maintains "current position" and needs as output "direction" > and "number of steps."" My guess is "no", cause it's not > too complicated except for taking into account the variables > that exist in range of servo travel and degrees per step for > the stepper motors. (It seems an unfortunate side effect of > electronics economics that relatively beefy stepper motors > are cheaper, more standardized, and easier to find for hobbyists > than geared motors. I'm not sure you're accurately estimating > the relative torque of big steppers and smaller servos, though.) > > BillW > > -- > http://www.piclist.com PIC/SX FAQ & list archive > View/change your membership options at > http://mailman.mit.edu/mailman/listinfo/piclist > -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist