Signametrics Corporation SM4020 Operator'S Manual Download Page 25

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the Scanner is ready to accept a new command, and in the case of channel selection, the selected channel is closed
and settled. If

SCANReady()

returns

TRUE

, it should not be used again until the next polled command is issued.

The following is a list of all

polled

functions:

SCANSelectChannelCmd()

SCANAutoScan()

SCANTrigAutoScan()

SCANTrigScan()

SCANStepCmd()

and

SCANCleanRelays()

4.4 Interfacing to the SM2040 series 6-1/2 Digit DMM

The SM4000 series Scanners are designed to interface to the SM2040, SM2042, and SM2044 Digital
Multimeters. The following section describes both, the hardware interface and the software functions used
to implement a synchronized operation.

4.4.1 Triggering the SM2040 DMMs

The SM2040, SM2042, and SM2044 can be triggered to measure selected Scanner channels. The
interface requires a single jumper between the SM4000

Trig_com

and

Common

lines, and two interface

wires connected to the SM2040 series Trigger input. Once connected, the Scanner can be setup to
produce a trigger signal for each relay selection operation. The various auto scanning operations can run
independently from the computer, whereby the Scanner selects a list of channels stored in its Scan List
table, and the DMM is triggered to take measurements following each channel selection.

Figure 4-12. Trigger interfacing connection to an SM2040 class of DMMs.

4.4.2 Multiplexing with the SM2040 DMMs

For two wire measurements, the SM204X DMMs must be connected to the A-Bus or the scanner, or to both, the A-
Bus and C-Bus for 4-Wire measurements (SM4040, SM4042). 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. These include the relay Actuation times, the DMM input settling and wiring burden. The latter is primarily
due to capacitance, and will vary with the type of measurement. For instance, when making high value Ohms
measurements the DMM current source level could contribute significant delay due to the capacitance charge time.
For example, with 1,000pf cable capacitance, the source current of the SM2044 DMM using the 33M

Ω

range, is

0.1

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

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

4.4.3 Interface Commands and Timing

The sequence required for the SM2040 DMM to make triggered measurements that are generated by the SM4040,
starts with the preparation of the SM4040. Set the SM4040 desired configuration, with Trigger Output enabled and
positive polarity. Each channel selection will generate a positive pulse with a duration equal to the Actuation time.
This could be generated by one of the scanning operations, or simply by sending channel selection commands to the
SM4040. The SM2040 must also be set up for triggered readings by using the

DMMSetTrigRead()

command. In

the following example, the SM2040 must send readings during the scan. Since it’s on board Fifo is limited to 5
readings, and the DMM must continue to send all readings during the scan, it is important to have a tight loop that
reads the measurements fast enough. Refer to Figure 4-12 for proper trigger connection.

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