SMRD communication

Normally, the SMR C library copes with communication between a user's program and the SMR sensor daemon SMRD.

This page documents this communication.

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Connect

SMRD listens for TCP/IP connections on port 24901. Multiple clients (up to ten) can be connected at once.

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Data Packets

The communication protocol is byte oriented and binary coded.

Data both from client to server, and from server to client, is sent in packets. Each packet starts with a length byte, specifying the number of bytes to follow. For example:

0x04 0x10 0x20 0x30 0x40

Only the lower 5 bits of the first byte give the length. The upper 3 bits describe the source/destination of the packet:

  0b000?????:   RS485 master/slave serial bus
  0b001?????:   RS232 serial port
  0b011?????:   AU simulator packet
  0b111?????:   SMRD control packet

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Poll

Clients start the communication by sending a client ready packet:

0xE1 0x02

The server responds with a number of packets. First, a start of period. This is followed by a series of data packets. Finally, an end of period packet is sent.

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Start of period

0xE9 0x00 <sample no.> 
  <sample no.>:   4 byte (integer) sample number
  :   4 byte (integer) cpu clock time stamp

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Motor data

The two motors on an SMR are numbered 1 and 2. In the following, replace i with the motor number.

Some motor modules return more data than others. The length byte distinguishes between module types.

0x03 0xAi <encoder position>
0x05 0xAi <encoder position> <status> <pwm>
  <encoder position>:   2 byte (integer). 2000 positions per wheel turn.
  <status>:   1 byte status
  <pwm>:   1 byte motor drive voltage

MSB is to the left, for example: encoder position = 256*byte2 + byte3.

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IR proximity data

0x09 0x88 <proximity data>
  <proximity data>:   8 bytes (array of bytes) proximity data. The first 6 bytes contain measurement data.

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Serial line sensor

The serial port line sensor sends 16 bytes of sensor data; typically split into 2 or more packets. So, the length fields can vary from packet, and from sample to sample. A typical period might look like this:

0x27 0x73 <6 bytes serial data>
0x2B 0x73 <10 bytes serial data>
  <serial data>:   Up to 16 bytes (array of bytes) serial data. The first 8 bytes contain measurement data.

Other packets may be received between serial port packets.

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RS485 line sensor

0x09 0x17 <line sensor data>
  <line sensor data>:   8 bytes (array of bytes) line sensor data.

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Power module

0x08 0x19 0b<fedcbaAA> 0b<BBCCDDEE> 0b<AAAAAAAA> 0b<BBBBBBBB> 0b<CCCCCCCC> 0b<DDDDDDDD> 0b<EEEEEEEE>
  fedcba:   Digital input channels 5 to 0.
  AAAAAAAAAA...EEEEEEEEEE:   Analog input (10 bits) channels 0 to 4. The two MSBs come from one of the pairs in byte 2 or 3. LSB is to the right.

The module inputs are connected as follows:

  Digital input 0:   Power on.
  Digital input 1...5:   Unused.
  Analog input 0:   Battery voltage.
  Analog input 1:   External supply voltage.
  Analog input 2,3,4:   Unused.

To convert analog input readings to volts:

Vin = 4.1 * 3.5 / 1.1 / 1024 * ADin

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New Power Module v. 3.3b (firmware 3.31+)

Data is sent in same format as the old module

The module inputs are connected as follows:

  Digital input 0 (a):   Power on.
  Digital input 1 (b):   External Power Connected.
  Digital input 2 (c):   Robot is charging.
  Digital input 3 (d):   Robot is calibrating battery
  Digital input 4..5 (e..f):   Unused.
  Analog input 0 (A):   Battery voltage.
  Analog input 1 (B):   Robot Power Supply voltage.
  Analog input 2 (C):   Battery charge current.
  Analog input 3 (D):   Battery temperature sensor 1.
  Analog input 4 (E):   Battery state-of-charge.

To convert analog input readings into usable values:

Battery voltage:     Vbatt = 4.1 * 3.5 / 1.1 / 1024 * ADin. [V]
Robot PSU voltage:   Vpsu = 4.1 * 3.5 / 1.1 / 1024 * ADin.  [V]
Battery Chg Current: Ichg = ADin * 3 [mA].
Battery Temp 1:      Tbatt = ADin [0.1 °C].
Battery SOC:         Bsoc = ADin [% remaining capacity]

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End of period

SMRD signals that the period is finished by sending end of period.

0xE1 0x01

No more packets will be sent to the client until a new poll is started.

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Simulation time

When SMRdemo is connected to the simulator, it will receive the simulation time just before the end of period packet.

0x69 0x01 <simulation time>
  <simulation time>: 8 byte (double) simulation time.