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Serial_Protocol.md

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Ubiquity Robotics Motor Controller Serial Protocol V3

Packet Structure

0x7E AA BB CC DD EE FF GG

0x7E : Delimiter

This is the start character of the protocol, Motor controller looks for this character along with the Protocol Version to establish the validity of the incoming protocol.

AA : Protocol Version and Message Type

The most signifcant 4 bits are the protcol version (currently 3), and the least significant 4 bits are the Message Type (READ/WRITE/RESPONSE/ERROR).

BB : Address

This specifies the register address the command is to be acted on.

CC DD EE FF : Data

Data is a BIG ENDIAN representation of the information being sent. It has 4 bytes of data - 8 bits each, that together represents a 32 bit data string.

Negative numbers are represented using 2s complement: +568 = 0x00000238 -568 = 0xFFFFFDC8

GG : Checksum

This contains the checksum for the packet, see the section on Checksum Algorithim below.

Checksum Algorithim

Checksum can be computed by the following method (computing checksum): Example command: 7E 2A F3 C2 D3 3E 4F XX (Where XX is the checksum to be computed)

  1. Compute the sum of all characters without including the command delimiter and the checksum. Ex: 2A+F3+C2+D3+3E+4F = 33F
  2. Keep the last eight bits and discard the rest. Ex: =3 3F = 3F
  3. Subtract the result of step 2 from FF. Ex: FF - 3F = C0

The result is the checksum, and should be the last byte of the packet.

Psudeo-code for checksum calculation:

sum = 0
for byte in command:
     sum += byte
sum = sum & FF
return FF - sum

Checksum Verification

  1. Compute the sum of all characters without including the command delimiter and the checksum. Ex: 2A+F3+C2+D3+3E+4F = 33F
  2. Keep the last eight bits and discard the rest. Ex: =3 FF = FF
  3. The result of above should be 0xFF

Psudeo-code for checksum verification:

sum = 0
for byte in command:
     sum += byte
sum = sum & 0xFF
return (sum = 0xFF)

Message Type

There are 4 message types: READ (A) WRITE (B) RESPONSE (C) and ERROR (D).

Read: 0x0A

Used to read a register from the motor controller, which returns the data in a response message.

Example: read register 0x21. 0x7E 0x3A 0x21 0x00 0x00 0x00 0x00 0xA4

Write: 0x0B

Used to write a register on the motor controller.

Example: write all 0s to register 0x21. 0x7E 0x3B 0x21 0x00 0x00 0x00 0x00 0xA3

No response is sent after a write, if confirmation is needed, a read message can be sent to confirm the values.

Response: 0x0C

A response to the read message, has same register address.

Respond to read of register 0x21 with 1. 0x7E 0x3C 0x21 0x00 0x00 0x00 0x01 0xA3

Error: 0x0D

Sent by the motor controller upon receiving a message with a bad checksum.

The register value in the message is the same as the bad message, and the data is all 0s.

Register Map

ID (Hex) Name Units Read/Write
0x00 DEPRECATED
0x01 Brake / Stop RW
0x02 DEPRECATED
0x03 Left PWM R
0x04 Right PWM R
0x05 DEPRECATED
0x06 DEPRECATED
0x07 Left Motor Speed Set tics/100ms RW
0x08 Right Motor Speed Set tics/100ms RW
0x09 DEPRECATED
0x0A DEPRECATED
0x0B Left Motor Tics (Clears on Read) tics
0x0C Right Motor Tics (Clears on Read) tics
0x0D Deadman Timer RW
0x0E Left Current Sense mA R
0x0F Right Current Sense mA R
0x10 ERROR COUNT R
0x11 5V MAIN ERROR 0x00 = OK R
0x12 5V AUX ERROR 0x00 = OK R
0x13 12V MAIN ERROR 0x00 = OK R
0x14 12V AUX ERROR 0x00 = OK R
0x15 5V MAIN OL 0x00 = OK R
0x16 5V AUX OL 0x00 = OK R
0x17 12V MAIN OL 0x00 = OK R
0x18 12V AUX OL 0x00 = OK R
0x19 LEFT MOTOR ERROR 0x00 = OK R
0x1A RIGHT MOTOR ERROR 0x00 = OK R
0x1B PID P RW
0x1C PID I RW
0x1D PID D RW
0x1E PID C RW
0x1F Debug LED 1 Bool RW
0x20 Debug LED 2 Bool RW
0x21 Hardware Version RW
0x22 Firmware Version RW
0x23 Battery Voltage mV RW
0x24 5V Main Current Sense mA R
0x25 12V Main Current Sense mA R
0x26 5V Aux Current Sense mA R
0x27 12V Aux Current Sense mA R
0x28 Left Motor Speed Read tics/100ms RW
0x29 Right Motor Speed Read tics/100ms RW
0x2A Both Motor Speed Set tics/100ms RW
0x2B Moving Buffer Size RW
0x2C Integral Limit Reached R
0x2D Both Motor Error 0x00 = OK R
0x30 Both Odom R
0x31 Robot Id R
0x50-5F Reserved for debugging R