Mallinckrodt NELLCOR NPB-4000, Service Manual

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Section 13: Microprocessor Computer and Control –Theory of Operation
13-7
13.5 ISOLATED FRONT END INTERFACE
Figure 13-4: Isolated Front End Block Diagram
See Figure 13-4. The interface to the Front End electronics is shown above and
consists of using the microprocessor’s synchronous serial unit to interface to the
A/D converter. The synchronous serial unit interfaces with the DMA unit,
which passes the data to be transmitted to the A/D. The data received from the
A/D goes to the other DMA channel and is transferred to a data buffer.
13.5.1 A/D Converter Transmit Control
The A/D converter initiates a conversion when it receives a serial stream from
the microprocessor. The processor software sets up the DMA, synchronous
serial unit, and timer channel 0 to work together to transfer control words to the
A/D and Front End. The timer is set for a rate of 800 Hz and its output is
programmed to go directly to the DMA unit. The DMA unit passes data from
the DRAM to the SSIO unit, which in turn transmits the 16-bit word to the A/D
and the Front End. The first eight bits transmitted are stored by the A/D and
interpreted. A conversion starts. At the same time, the previous conversion is
transmitted back to the SSIO unit (this is discussed in the next section). The last
eight bits are read by the Front End and are used to select the Front End mux
channel (three bits), initiate baseline reset (one bit), lead select (two bits), and
select a 0.5 Hz filter (one bit). The eighth bit is a parity bit, which is checked by
the Front End circuit. The software puts together a buffer with this data and
programs the DMA channel to transmit the data words each time the timer
generates a DMA request.