Hard: Can anyone give me a sample packet by running function:"Reading Current Position" for Dynamixel Pro Motors?

[maynight]

Hi everyone,
I am trying to write a labview driver for dynamixel pro motors and I am in trouble. I think a sample packet will help a lot. I also try to get a sample packet by running Dynamixel SDK C++ code under my Ubuntu system but I am in trouble again %>_<%, so can anyone help me by getting a sample packet by hooking up one dynamixel pro motor and running your C++ code to read the current position of the motor under linux system?

There is a "readvalue" example under /example folder if you download Dynamixel Pro SDK for linux.
According to my understanding, it runs
DXL.WriteDWord(1, GOAL_POSITION, GoalPos, &error);
to send a signal, then it runs:
ReadDWord(1, Present_Position, &val3, &error)
which fits into
ReadDWord(id, address, *value, *error)
by id=1, address=Present_Position=611

Then it runs
Read(id, address, 4, data, error)
which fits into
Read(int id, int address, int length, unsigned char* data, int *error)

Then it runs
TxRxPacket(txpacket, rxpacket, error);

So the packet gets read will look something like this:
-----header-----
0xFF
0xFF
0xFD
0x00
--------------------
0x01 =ID

0x04
0x00 //Read data length

0x55 =INST_STATUS

?????
Then I really need the rest ????? part. Can anyone help me by print that part out to see how long it is and what it looks like? I think there're 1 error big, 2 data bits as well as 2 checksum bits.

My motor is Dynamixle pro H54-200-s500-r but I think any dynamixel pro motors will work. I would imagine this to be not easy so if somebody happens to have a motor on hand and is able to help me I will greatly appreciated.
Thank you!

[tician]

Don't have any Dynamixel Pro, but the entire communication protocol is documented on the Robotis support site. The Instruction/status packet link is especially useful as it gives complete structure of all packet types.

So, if that site is correct, the ???? of the command packet is simply the CRC checksum. The status return packet will be three byte header (0xff 0xff 0xfd, one byte reserved (0x00). one byte ID, two bytes of packet length, one byte instruction (0x55 for status), one byte error (hopefully none), then N requested bytes of data from the servo (4 bytes in your case), then the two byte CRC.

[maynight]

Actually, my colleague and I think the driver follows exactly with that e-manual, but the result just doesn't seems right. The packet gets sent to motor is:
(under decimals)
----headers------
255 255 253 0
----ID----
1
----Length----
7 0
----INST=READ----
2
----Address----
2 99 //611=0x02 0x63 = 02(d) 99(d)
----Data Length---
0 4 //length=4 because of readDword
----Checksum-----
19 115

Then I read this status packet:
----headers------
255 255 253 0
----ID----
1
----Length----
4 0 //Consistent with data_length
----INST=STATUS----
85 //=0x55
----supposed to be Error+Data----
3 171 12

the last 3 digits confuse me because I will expect they to be longer. This is the reason we are asking for help. If someone can give us absolute correct packets which is sent as well as read, I think we may be able to fix some mistakes we made.
Thank you!

[maynight]

Also, if anyone can help please give me the current position you see under Roboplus also.

[tician]

From the DXL2.0 instruction/status packet page on error numbers:

0x03

CRC Error

Failed Packet’s CRC value match

Also, everything is sent least significant byte first, so 611 decimal = 0x0263 should be sent as 0x63 then 0x02, should it not?
0x63 0x02 = 0x0263 => 611
0x02 0x63 = 0x6302 => 25346

Same goes for data length of 0x0004 should be 0x04 then 0x00.


And no, packet length of 0x0004 is not consistent with a successful return. 4 = 1 instruction byte + 1 error byte + 2 CRC bytes; which is the minimum valid length of a status return packet.

[maynight]

Yes, I also realized that the order is wrong and I fix it just now. But I am still getting CRC error. Does return data length =4 correspond to
Instruction Packet Parameter (Write data length: N)

Parameter 0	Error Byte (please refer to the basic structure)
Parameter 1	Read data start address 1st byte
Parameter 2	Read data start address 2nd byte

So am I supposed to read 5 digits with 2 data digits and 2 CRC?

[tician]

The packet:
----headers------
0xff 0xff 0xfd 0x00
----ID----
0x01
----Length----
0x07 0x00
----INST=READ----
0x02
----Address----
0x63 0x02
----Data Length---
0x04 0x00
----CRC_L -> CRC_H-----
0x1B 0xF9

CRC_calc
((crc_accm>>8) ^ data[j]) & 0xFF = i => (crc_accm<<8) ^ crc_table[i] => crc_accm
((0x0000>>8) ^ 0xFF) & 0xFF = 0xFF => (0x0000<<8) ^ 0x0202 => 0x0202
((0x0202>>8) ^ 0xFF) & 0xFF = 0xFD => (0x0202<<8) ^ 0x820D => 0x800D
((0x800D>>8) ^ 0xFD) & 0xFF = 0x7D => (0x800D<<8) ^ 0x010E => 0x0C0E
((0x0C0E>>8) ^ 0x00) & 0xFF = 0x0C => (0x0C0E<<8) ^ 0x0028 => 0x0E28
((0x0E28>>8) ^ 0x01) & 0xFF = 0x0F => (0x0E28<<8) ^ 0x0022 => 0x2822
((0x2822>>8) ^ 0x07) & 0xFF = 0x2F => (0x2822<<8) ^ 0x80E1 => 0xA2E1
((0xA2E1>>8) ^ 0x00) & 0xFF = 0xA2 => (0xA2E1<<8) ^ 0x83CF => 0x62CF
((0x62CF>>8) ^ 0x02) & 0xFF = 0x60 => (0x62CF<<8) ^ 0x0140 => 0xCE40
((0xCE40>>8) ^ 0x63) & 0xFF = 0xAD => (0xCE40<<8) ^ 0x83ED => 0xC3ED
((0xC3ED>>8) ^ 0x02) & 0xFF = 0xC1 => (0xC3ED<<8) ^ 0x8285 => 0x6F85
((0x6F85>>8) ^ 0x04) & 0xFF = 0x6B => (0x6F85<<8) ^ 0x8179 => 0x0479
((0x0479>>8) ^ 0x00) & 0xFF = 0x04 => (0x0479<<8) ^ 0x801B => 0xF91B

...should produce a status return packet of length 0x0008: 1 instruction byte + 1 error byte + 4 returned parameter bytes + 2 CRC bytes.

[maynight]

Thank you so much!!!!!! According to this, I find that actually the CRC function is wrong. I am able to get the correct status packet now.
Thanks! It's always great to come back to this forum.

[tician]

Since the first four bytes of all packets will be identical, you can seed the CRC function with '0x0E28' and start processing the packet data at the ID byte if you want to save a few cycles.