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MTBbus Architecture

Purpose of MTBbus is to connect general IO modules to the computer. Each module monitors its inputs and sends computer information when inputs are changed. Computer can instruct modules to set outputs.

Basic Bus Parameters

  • Hardware layer: RS485, 2-wire communication (+ common ground).
  • Communication speed: variable. 38400 Bd, 57600 Bd, 115200 Bd.
  • Data bits: 9.
  • Stop bits: 1.
  • Parity: None.
  • Max bus length: 100 m.
  • Max modules: 255.

Hardware Structure

  • Single master, multiple slaves.
  • Bus should meet all RS485 requirements, straight bus topology is advised.
  • Bus should be terminated on all ends via terminator.
    • 560 Ohm resistors are advised for pull-down & pull-up.
    • 200 Ohm resistor is advised between bus wires.

Packets

Data are sent in packets. MTBbus is single-master bus bus. This implies any slave device is allowed to send data only when it is provided transmission window via Module Inquiry Command by master module. Master module periodically scans all slave addresses and ask them whether the have any data to send. Slave module must always respond to this request, even if it has no data to send. This mechanism ensures master module can detect faulty or disconnected modules, which is critical for ensuring safety.

Master module holds list of addresses of active modules and requests these modules periodically in short interval. It also requests all other addresses to detect new modules in longer intervals. This mechanism allows present modules to communicate fast and to detect new modules too.

Timing

Each slave module must start answering to any request addresses to the module after ≤ 200 microseconds. After 250 microseconds master module is allowed to consider addressed module as non-communicating.

Packet structure

Packet consists of variable numbers of bytes.

  1. Address byte contains slave 8-bit address. Only this byte has ninth bit = 1. This byte is present only in direction master → slave. Each slave module should receive data addressed only for the module and broadcast data. Broadcast commands have address = 0. Thus module address 0 is forbidden.
  2. Command length byte contains number of data bytes following. This number excludes checksum bytes. Max value = 121.
  3. Command code byte.
  4. Data bytes. Up to 120 data bytes (128 bytes buffer).
  5. Checksum CRC-16 (2 bytes).

Note that when slave module sends data to master, packet starts with Header Byte. All bytes in packet from slave modules have 9. bit = 0.

CRC-16

  • Polynomial: x¹⁶ + x¹⁵ + x² + 1 (CRC-16-ANSI also known as CRC-16-IBM, normal hexadecimal algebraic polynomial being 0x8005 and reversed 0xA001).
  • Initial value: 65536, 0xFFFF.
  • CRC-bytes: least significant byte is transmitted first.
  • Checksum includes whole packet: starting from address byte to last byte of data bytes.
  • Example of packet: 0x101 0x001 0x001 0x091 0x090.

Modules addressing

Each MTBbus slave module has its address in range 1-255. Address could be determined by hardware switches/jumpers on module or just programmed in volatile memory. MTBbus supports mechanism for readdressing modules from master module/PC.

Errors

If slave module receives bad checksum, it must not respond.

Diagnostics