MBI block diagram, advantages and DMA

Block diagram of an MBI system

Sensor -> Signal conditioner -> S/H -> ADC --+
  ^                                          v
  |                                +------------------+
PLANT                              | Microprocessor   |<-> ROM/RAM
  |                                | (CPU, I/O ports) |<-> Keypad,
  v                                +------------------+    Display,
Actuator <- Driver <- DAC <----------------+  |           Printer,
                                              +-> Serial link

Sensor converts the measured quantity to an electrical signal; the signal conditioner amplifies, filters and scales it; the S/H and ADC digitise it; the microprocessor with ROM (program) and RAM (data) processes the data; I/O ports connect keypad, display and printer; the DAC, driver and actuator apply the control action to the plant; a serial link connects to a supervisory computer.

Advantages of MBI systems

1. Programmable, so functions are changed without hardware change.
2. Higher accuracy, linearisation and automatic calibration.
3. Automatic decision making, alarms and safety interlocks.
4. Many channels handled by one processor.
5. Data logging, display and remote communication.
6. Less wiring, lower cost, size and power, higher reliability.
7. Easy expansion and maintenance (self-test and diagnostics).

DMA (Direct Memory Access)

DMA transfers a block of data directly between an I/O device and memory without the CPU executing instructions for every byte. The DMA controller (e.g. 8237) requests the bus with HOLD, the CPU finishes its cycle, floats its buses and replies HLDA. The controller then generates the memory address and the read/write signals and transfers bytes at memory speed. After the count is finished, it releases HOLD and the CPU resumes.

 Device --DREQ--> DMAC --HOLD--> CPU
 Device <--DACK-- DMAC <--HLDA-- CPU
          Device <==== data ====> Memory

DMA is used for fast block transfers such as ADC data acquisition and disk transfer.