Data acquisition modes

The AC3 has six modes (0 to 5) for acquiring measurement data. These are described below:

Mode 0 - default mode

  • This is the default mode of operation after power has been applied to the AC3 or a system reset has occurred. AC3 does not respond to the PICS READ command, or reversed direction interrupt, nor generate PICS SYNC or any interrupt. When the user writes a ‘1’ to bit ‘11’ of the command register (‘base +13’) the timer value is latched, the BUSY bit is set and the conversation of the three measurement channels begins. When the data from the measurement channels is available to be read over the ISA bus, the BUSY flag is lowered and the AC3 is ready for another conversation.

Mode 1 - ISA bus acquire with PICS SYNC

  • This mode is identical to mode 0 with additional activity on the PICS interface. During the period that the BUSY bit is set, PICS SYNC is set to its active state.

Mode 2 - PICS READ without interrupt

  • In this mode the AC3 responds as in mode 0 when bit ‘11’ of the command register is written to with a ‘1’. In addition, on the falling edge of the PICS READ signal, the timer value is latched, the BUSY bit is set and the conversation of the three measurement channels begins. The BUSY bit is lowered in the same way as for Mode 0.

Mode 3 - PICS READ with interrupt

  • This is identical to mode 2, except that when the BUSY bit is lowered at the end of the data acquisition, the interrupt selected by bits ‘7’, ‘6’, ‘5’ and ‘4’ of the acquisition mode selection register becomes active. The interrupt is cleared by reading any AC3 register. This feature has been included so that customers who do not wish to reset the interrupt bit can just read the measurement register.
  • It is essential that any user who wants to read the interrupt bit at address ‘base +10’, reads this address first after the interrupt has occurred. If the user does not, the interrupt will be cleared and the bit in the status register will be reset to ‘0’ before the software has read it.

Mode 4 - Reversed direction interrupt without PICS SYNC (used by some CMM controllers)

  • In this mode the AC3 responds as in mode 0 when bit ‘11’ of the command register is written to with a ‘1’. In addition, when the AC3 sees a rising edge on the interrupt selected by bits ‘7’, ‘6’ and ‘5’ of the acquisition mode selection register, the timer value is latched, the BUSY bit is set and the conversion of the three measurement channels begins. The BUSY bit is lowered in the same way as for mode 0. A second conversation will not occur until after the selected interrupt has gone to logic ‘0’ again. Bit ‘4’ of the acquisition mode register is ignored.

Mode 5 - Reversed direction interrupt with PICS SYNC

  • This is identical to mode 4 with additional activity on the PICS interface. During the period that the BUSY bit is set, PICS SYNC is set to its active state for the duration of the BUSY period.