SIGNAL CONDITIONING, DATA ACQUISITION, AND SPECTRAL ANALYSIS - Design and Development of Medical Electronic Instrumentation - page 215

TABLE 5.2 Some Output Ranges of Sensors Used for Environmental
Conditions in a Small Hyperbaric Research Chamber
ADC Channel

Sensor Output Range (V)

1 Inside oxygen partial pressure 0-5

2 Inside CO 2 partial pressure 0-5

3 Atmospheric pressure 0-5

4 Hyperbaric chamber pressure 0-5

5 Inside relative humidity 0-10

6 Inside ambient temperature 0-1.2

7 Outside ambient temperature 0-1.2

8 Specimen temperature 0-1.2

5-V linear regulator (IC3). Whenever either
the D/A converters or the current sources are needed, an external

The D/A converters are powered from a

12-V supply
must be provided. In some cases, powering the A/D section through an external power sup-
ply may also be desirable. To do so, do not install R1, R2, C3, and L1. Instead, install R3
and place a jumper on JP1. These changes supply the A/D section of the Universal Sensor
Interface from the regulated

9- to

5 V supplied by IC3.
The D/A converters are controlled through a serial protocol that requires only three dig-
ital lines. Bits 0 to 2 of the LPT control port (hex address 37A for LPT1:) are used for this
purpose. As shown in Figure 5.4 b , data-conversion initiation and data-write operations are
controlled by the *DACS (D/A chip select) and DACLK (D/A serial clock) lines. Data are
supplied to the D/A converters serially through the DADAT line. The data on the DADAT
input are loaded into a shift register (internal to the D/A converter control circuitry) on the
rising edge of the DACLK clock. Data are loaded as one 24-bit word where the first 12 bits

are for DAC 1 and the second 12 are for DAC 2. For each 12-bit segment the MSB is
loaded

fi

first. Data from the shift register are loaded into the D/A register when *DACS is
pulled high. The clock is disabled internally when *DACS is high. DACLK must be low
before *DACS is pulled low to avoid an extra internal clock pulse.
The following VisualBasic code module (DTOA.BAS) shows how the D/A serial control
protocol is implemented.

fi

Function dtoa(dtoa1 As Integer, dtoa2 As Integer) As Integer
'
' Function DtoA (dtoa1, dtoa2) executes the serial protocol to update D/A
' converters with the values defined by dtoa1 and dtoa2, where
' dtoa1 contains the code for the desired D/A channel 1 output
' dtoa2 contains the code for the desired D/A channel 2 output
'
'
'
' Define variables
' ---------------
Dim bit3 As Integer

' value of Control *D3

Dim clocknum As Integer ' clock pulse counter

Dim delay As Integer ' delay loop dummy variabe

' D/A loop
' --------
bit3 Inp(princont) And 8

' evaluate current status of Control *D3

Out princont, notdaclk bit3 ' set D/A clock line low without

' upsetting bit 3 of the Control port