Mallinckrodt NELLCOR NPB-4000, Service Manual

Page: 120 / 219

Section 13: Microprocessor Computer and Control –Theory of Operation
13-6
Figure 13-3: Recorder System Block Diagram
13.4.1 Recorder/Microcontroller Interface
The recorder is interfaced to the microcontroller via a DUART that has two
UARTs programmed by the software. The DUART has a UART0 and a
UART1, each with a full UART signal complement. The circuit uses UART1 of
the DUART for the recorder interface. This UART has 16-byte fifos on each of
the receive and transmit channels. The software programs the UART by setting
the chip select unit, CS4 to I/O addresses for the recorder and then programming
the internal registers for baud rate, parity, and bit count. Up to 16 bytes can be
transferred to the UART, which will transmit serially to the recorder . The
UART generates RS-232 signals in TTL/CMOS compatible signal levels. The
recorder takes 0-5 volt signals, so each of the signals to/from the recorder must
be translated from/to 3.3 volts/5 volts. This is done with the HCT244 buffers.
The UART signals and recorder signals are listed below.
Host transmit -> printer receive
printer reset bit -> printer reset
Host receive <- printer transmit
Host CTS <- printer clear to send
The host signals are generated by the programmable UART.
13.4.2 Recorder Power
The recorder requires 5 volts for logic operation and 10-18 volts for print head
operation. The power for the recorder is generated on the power supply board
and goes directly to the recorder from the power supply board. It is merged with
the control signals to make one 50-pin cable going to the recorder interface
board.