Parallax USB Servo avoiding the Javelin Stamp Code using C++

Tyrone · 2009-12-25 09:43

Hi

I am interfacing C++ code into·a Parallax USB Servo # 28823 avoiding the Javelin Stamp Code. I can control the servo

· but not efficiently from position 250-1250.

I understand that I need to send the following data:

!SC [noparse][[/noparse] SERVO ] [noparse][[/noparse] RAMP ] [noparse][[/noparse] LOW POSITION BYTE ] [noparse][[/noparse] HIGH POSITION BYTE ] [noparse][[/noparse] 0x0D ]

"!SC" I understand I need to send as ASCII in the data type of CHAR. The rest I understand that I have to send as an INT which will be passed on as decimals to the COM Port.

I am using the following code to write the data using Writefile().

(Code to send "!SC" is ASCII)

char szMessage[noparse]/noparse = "!SC";

int nBytesSent = serial.SendData(szMessage, strlen(szMessage));

(Code to send "SERVO")

int szMessageNumber = 00;

nBytesSent = serial.WriteCommByteNumber(szMessageNumber);

(Code the send "RAMP")

szMessageNumber = 00;

nBytesSent = serial.WriteCommByteNumber(szMessageNumber);

(Code to send "LOW POSITION BYTE")

szMessageNumber = 100;

nBytesSent = serial.WriteCommByteNumber(szMessageNumber);

(Code the send "HIGH POSITION BYTE)

szMessageNumber = 1;

nBytesSent = serial.WriteCommByteNumber(szMessageNumber);

(Code to send CARRIAGE RETURN)

szMessageNumber = 13;

nBytesSent = serial.WriteCommByteNumber(szMessageNumber);

I have not been able to move it from position 250-1250 a notch at a time. The only possible way I can make a 180 degree turn is if I change the HIGH POSITION BYTE from 1 - 4. That is if I change the HIGHBYTE from 1 to 2, the servo moves 45 degrees, and then another 45 degrees if the HIGHBYTE is changed to 3 and also 4 eventually making a full 180 degree turn. 5 strains the servo.

What am I doing wrong? Can you clarrify how one would use and/or manipulate the HIGHBYTE·and LOWBYTE.·I eventually burnt an expensive servo.... I'm a junior. How does Javelin pass these values onto the controller.

Thanks

Tyrone

Peter Verkaik · 2009-12-25 10:38

Attached is the class for the Parallax Servo Controller (serial version).

You should·be able to figure out the send and receive byte sequences from that code.

I am assumimg the psc usb version uses the same byte sequences.

regards peter

Peter Verkaik · 2009-12-25 10:45

The high byte and low byte are extracted from position value like this:

int position = 878;
int lowbyte = position & 255;
int highbyte = (position >> 8) & 255;

position range is 250 to 1250, centerposition is 750

regards peter

Peter Verkaik · 2009-12-25 11:00

Open source for psc control:
http://www.crustcrawler.com/software/psc/index.php

regards peter

Tyrone · 2009-12-25 13:00

Thank you Peter for a fast response. Great resource material.

In your suggestion:

int position = 878;
int lowbyte = position & 255;
int highbyte = (position >> 8) & 255;

What is the & 255 and the >>8 for?

I haven't got past the 4th chapter of C++.... haha Maybe that is why I burnt my servo!

Tyrone · 2009-12-25 14:46

Okay I see they are BitWise Operators and reading more up on them and playing. Maybe you want to explain the logic behind it so that I can understand what I am doing by performing bitwise operations on the decimal values.

Tyrone · 2009-12-25 15:35

int position = 878;
int lowbyte = position & 255;
int highbyte = (position >> 8) & 255;

1.) 878 (decimal) = 1101101110(binary)
2.) Performing a BitWise Shift Right 8 places will leave a byte looking like 0000000011 which is 3 in decimal. So this explains why I could only use values 1-4 on the HIGHBYTE without BitWise operators.
3.) The & "255" takes the binary representation (1101101110) of the decimal value 878 and does a calculation against the binary value of 255 (11111111)
For example: 878 = 1101101110
255 = 11111111
________
0001101110 = 110

OR

878 = 1101101110
255 = 11111111
________
1101101100 = 876

(THIS IS WHERE I GET LOST BECAUSE I AM NOT SURE HOW THE CALCULATION IS DONE WITH A 8-BIT AND 10-BIT BIT-STREAM.)

Peter Verkaik · 2009-12-25 16:35

The &255 means bitwise AND with 0b11111111
in other words: keep lower 8 bits only
878 = 1101101110
255 =·0011111111
________
······· 0001101110 = 0x6E = 110 decimal

regards peter

Tyrone · 2009-12-25 17:29

Peter thank you. You have been an awesome help.

Off to buy another servo and will post the results here.

Tyrone · 2010-01-22 17:31

I have been meaning to post the results here!

BitWise Operators work like a charm!

I can control the servo any which way I want to.

Peter thanks....

Rob311 · 2010-05-27 00:23

I've been trying to talk to my psc for a little while now, and all i get is an echo. Im using vb express 2010, this is my code

serialPort.WriteLine("!SCVER?")

and it responds !SCVER?

i dont suppose anyone knows what im doing wrong?

Tyrone · 2010-05-27 05:49

It seems the code is not opening the port properly because you should get a response showing the version 1.6 I think?

Find sample code to open the COM port but it should not be that different from previous versions of Visual C++.

I dont have any of my code anymore, so I can't help with sample code right now. I'm a beginner in C++.

Rob311 · 2010-05-27 10:32

the com is opening and sending. the psc echos if it doesnt understand the command. maybe the format of the command?

Peter Verkaik · 2010-05-27 11:22

Any command should end with a CR character (ascii code 13 decimal).

regards peter

Parallax USB Servo avoiding the Javelin Stamp Code using C++

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