8.3.3 FOUNDATION Fieldbus or PROFIBUS PA Manchester Encoder

FOUNDATION Fieldbus or PROFIBUS PA data is loaded into a transmit FIFO via UART or SPI interfaces which is translated into the Manchester encoded binary analog signals used in both FOUNDATION Fieldbus and PROFIBUS PA bus protocols through an internal Manchester encoder.

The Manchester encoder interacts with the DAC to transmit positive and negative amplitude signals, with respect to a positive common mode voltage, to create the Manchester encoded analog outputs at 31.25 kHz baud. A binary 0 is represented by a low-to-high transition and a binary 1 is represented by a high-to-low transition.

In both UART and SPI interfaced device, the encoder is activated any time there is data available in the transmit FIFO and the decoder is not receiving data. In order to prevent FIFO buffer overflow, for UART mode the CD pin acts as an interrupt to indicate when the FIFO level has exceed a programmed threshold in the packet initiation code. In SPI mode the transmit FIFO threshold programmed in the FIFO LEVEL SET register can trigger an interrupt on the IRQ pin. Once the IRQ interrupts is triggered, the MODEM STATUS register can then be read to determine the source of the interrupt, which includes a bit for the FIFO level in DB4. More details are explained in the respective Device Functional Modes sections.