General purpose switching refers to a class of switching architectures constructed from individual relays. Users can interconnect these relays to form standard structures such as MUX's or matrices to their own architecture or simply be used to switch individual signal lines. They are relatively expensive and sometimes low performance when used to create larger standard switching architectures - principally because the density is low due to the high connector pin usage and the need to wire the relays together externally.
However, that also allows them to be very flexible, especially when used with external interconnection systems like mass interconnect solutions that allow the system to be reconfigured for different applications. For this reason they can be popular on test systems designed to support a diverse array of products where it is important to minimize the number of test systems required.
Single Pole Single Throw (SPST) switches have a single input and single output connection. The relay is either open or closed. The most common (and considered the most reliable on reed relays) configuration is where the relay is open when the switching system is not powered or is in its default state.
Some applications prefer to use a default state of closed, for example applications involving Fault Insertion, since through connections is maintained even if the switch is reset or becomes disconnected form power.
Also referred to as a changeover switch Single Pole Double Throw switches connect a single input connection to one of two output connections. The input connection is usually referred to as the common and the outputs are the normally closed (NC) and normally open (NO) connections. On the drawing the common is C1, the NC contact is B1 and NO contact A1.
Double Pole Single Throw (DPST) switches have a pair of input and output connections. The two relay paths are either open or closed, if one rely is open then so is the second so the drawing shows a dotted line to indicate the two sets of paths state are linked. As with the SPST the most common default (unpowered) state is to be open but normally closed versions are available.
Double Pole Double Throw (DPDT) switches have two changeover switches (SPDT) which are mechanically operated by the same actuator.
Although operated from a common actuator in the event that one contact welds it does not prevent the other contact from moving (though the mechanical arrangement result in degraded operation on the second pole in the event of a weld on the first).