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Signametrics 42

2040 DMMs

40 DMM must be connected to the A-Bus or the scanner, or to both, the A-

hese include the scanner’s relay actuation times, the DMM input settling and

aries with the type of measurement. For instance, when making high value

ent source level could contribute significant delay due to the capacitance charge

riggered measurements. The triggers are generated by the

ngs, and the DMM must continue to send all readings during the

an, it is important to have a tight loop that reads the measurements fast enough so that no overrun error occurs.

Refer to Figure 4-16 for proper trigger connec

SCANTriggerOu , PosE

nReadings = 100

DMMSetTrigRead(nDmm, 4, nReadings, Neg

SCANAutoScan(nScan, nSteps)

For I = 0 to nReadings -1

while(DMMReadMeasurement(nD ait for each reading and store it

SCANOpenAllChannels(nScan)

4.15.2 Multiplexing with the SM

For two wire measurements, the SM20

Bus and C-Bus for 4-Wire measurements (assuming an SM4040 or SM4042 scanner). It is important to consider

system-settling time when making measurements. Time delays exist in any measurement system. These delays are

contributed by various sources. T

wiring capacitance. The latter will v

Ohms measurements the DMM curr

time. For example, with 1,000pf cable capacitance, the source current of the SM2044 DMM using the 33MΩ range

is 0.1µA which translates to 33ms (dt = C*dV/I). It is also recommended to set the appropriate number of settlin

measurements for the DMM (a minimum of 4 is recommended regardless of measurement rate).

4.15.3 Interface Commands and Timing

he sequence requires the SM2040 DMM to make tT

SM4040. Start by setting the SM4040 to the desired configuration, with Trigger Output enabled and positive

polarity. Each channel selection will generate a positive pulse with duration equal to the actuation time. This could

be generated by one of the scanning. The SM2040 must be set up for triggered readings by using the

DMMSetTrigRead() command. In the following VisualBasic® example, the SM2040 sends readings during the

can. Since it’s on board FIFO is limited to 5 readis

tion.

tState(nScan, Enabled

dge) // Set trigger output to Positive edge.

// Total number of measurements to take

Edge) // Total of 100 readinigs and 4 settling readings

// Start auto scan

// read values as they come

mm, reading) = NO // w

// Good idea to open all channels when done

Figure 4-17. Triggered reading process and timing of SM4042 Scanner and an SM2044 DMMs.

Unlike the previo MSetBuffT

measurements to it’s on-board buffer, which i

is limited to a maximum of 64 readings per sc

SCANTriggerOu abled, PosE

nReadings = 50

DMMSetBuffTrigRead(nDmm, 4, nReadings, NegEdge) ‘ Use 4 settling readings each

SCANAutoScan(nScan, nSteps)

While DMMReady(nDmm) = NO

Wend

For I = 0 to nReadings -1 in the buffer

while(DMMReadBuffer(nDmm, reading(I)) ‘ Store each reading

SCANOpenAllChannels(nScan) ‘ Good idea to open all channels when done

While SCANReady(nScan) = NO n is a polled operation,

DoEvents ' Make sure Scanner is ready

Wend

us example, DM rigRead() is not time critical since the DMM saves all

s read after the scan is complete. However, this function

an.

tState(nScan, En dge) ‘ Set trigger output to Positive edge.

‘ Total number of measurements to take

‘ Set off AutoScan

‘ wait for the DMM to indicate completion

‘ read values stored

‘ Since AutoSca