Signametrics Corporation SM4020, Operator'S Manual

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1.3 Description
The SM4020 and SM4022 are 20 channel models arranged in two groups of 10:1 differential channels. The
SM4040 and SM4042 are 40 channel models arranged in four groups of 10:1 differential channels. The “2” suffix
indicates an instrumentation quality scanning structure, while the “0” indicates standard general purpose scanning
structure. The Instrumentation models are necessary in applications requiring precision and low leakage. The most
outstanding feature of the Instrumentation models is a very low Thermal EMF, resulting is highly accurate Ohms,
low Voltage, temperature and other sensitive stimulus and measurements. The standard scanners in this family, the
SM4020 and SM4040, exhibit much lower Thermal EMF than most other plug-in relay scanners.
When using an SM2020 or an SM2040 series 5-1/2 and 6-1/2 DMMs, or similar DMM, it is necessary to use the
Instrumentation quality scanners in order to maintain full measurement accuracy.
These scanners are software configurable on the fly, and can handle several pre-defined switching configurations,
such as
TwoWire , FourWire , SixWire , Universal , TwoGroups, FourGroups, and Disabled . In the TwoWire
configuration, the scanner acts like a 20 or 40 channel multiplexer. In the FourWire configuration, it automatically
selects two simultaneous channels, allowing a DMM or other device to connect to four individual lines. In the
SixWire configuration six wires, or three differential channels are selected automatically, providing such
applications as six wire guarded in circuit measurements. The Universal configuration turns the scanner into a
pseudo matrix structure, allowing any relay, including channel relays, tree relays and configuration relays, to be
closed or open, including multiple relays. The
TwoGroups and FourGroups configurations split the Scanner into
several independent Two-Wire multiplexers.
Figure 1-1. Simplified diagram of the SM4000 relay scanner.
In the TwoWire , FourWire , SixWire , TwoGroups, and FourGroups configuration, the switching sequence includes
the opening of the previously closed channels and then closing of the selected channels. In the case where multiple
channels are supposed to be closed, such as in
FourWire configuration, these channels are closed simultaneously so
that no additional time beyond the actuation time is consumed. Break-before-make operation is maintained due to a
high speed turns off circuit, which shuts off the active coil current very quickly as compared to the turn on time.
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