The Extensible switch is an updated version of the switch portion of the MOAS. It uses RJ45 connectors and could be easily built on a single PC board.
This was an interim design. It has been superseded by the MOAS II which has more features and more flexibility.
This page and the schematic exist for reference only. If you want to build something please look at the MOAS II instead.
This switch is capable of controlling 64 relays, 32 with high side switching and 32 with low side switching. If a station requires more than 32 of one type of switching a second board can be used to allow up to 64 high or low.
The switch connects to the amplifier keying lines. It can handle amplifiers which have high positive keying voltages such as the old SB-220 or L4B.
The switch prevents hot switching by not switching a radio’s antennas while it is transmitting. The switch is fast enough that it will protect antenna relays as long as the radio has sufficient delay between keying the amplifier and producing RF. Radios normally do this because it is how they protect the T/R relay in the amplifier.
The switch allows selection of different antennas for transmit and receive. This can be useful, for example, if it is raining and precipitation static makes it difficult to hear on the top antenna in a stack but that top antenna is still desired for transmitting.
The switch detects conflicts and will defer setting them. When it starts up the switch requests a table which tells it which antennas conflict which others. If a radio tries to use an antenna which is in use by another radio the switch will hold the request until the other radio switches to a different antenna. For example, suppose radio 1 is using antenna 1 and radio 2 is using antenna 2. If radio 1 is switched to antenna 2 it will actually remain on antenna 1 until radio 2 is switched to a different antenna. If radio 2 is then switched to antenna 1 both radios will be changed at the same time (and when neither is transmitting).
This is the schematic. J9 is a 12xRJ45 connector, Digikey part SS-73100-046.