While the biggest advantage the MIL-STD-1553 standard provides for avionics engineers is compatibility, you might not realize that there are actually two specifications that standard covers. Those who have worked in defense for a long time probably remember when every aircraft manufacturer had a contract with their own hardware vendor. Each vendor had their own way of doing things. That made it hard to interface computer components with avionics equipment. Every device that's compliant with the MIL-STD-1553 standard features the same interface, but devices that meet the 1553B specifications actually have some additional features. Whether you actually have a need for them is a different story. Optional Broadcast Transfers When engineers first announced MIL-STD-1553B equipment, they were quite excited about a technology that permits avionics equipment to send data to all RTs without any responses. All RTs that support the standard can receive the data, but they don't actually reply because this could cause a conflict on the bus. This is perfect if you have data you need to send in only a single direction. These transfers can be sent one of four different ways. Controller to RT(s) In this mode the bus controller sends a single receive command word. It features a Terminal address registered to 31, which signifies a single broadcast type command. This is followed by a series of up to 32 data words. RT to RT The bus controller can actually send information from one remote terminal to another. It can even transmit data to a number of different terminals connected to a network. Mode Command Without Data Words A properly configured bus controller could transmit a single mode code command to all devices that are connected to it. No extra information is transmitted at all, which makes it faster than the other modes. Mode Command With Data Word This function behaves almost identically to the previous one. It sends a single mode code command, but it additionally transmits a single data word along with it. While errors are highly unlikely when working with this kind of technology, this will help to reduce the possibility of them even further. Using these Advanced Functions There are a few reasons that you might want to deploy these in an avionics environment. If a number of technicians are examining a single flight system, then you could share information with all of those terminals. This is especially useful for certain types of crew installations where people use their remote terminals merely to access information about certain flight devices. They're also useful when collecting large amounts of data for processing and testing reasons. Keep in mind that the bus controller can download data straight to a PC running standard Microsoft Windows system software, which is ideal in this sort of an environment. Some users will want this additional connectivity while others won't. If you do need it, then any 1553B-compliant device can work with it. You won't have to worry about anything if you're not making optional broadcast transfers, however. You can safely ignore the issue because all newer devices are backwards compatible.