To fully understand the FF-821 and FF-822 specifications, you need the associated IEC 61158 standards. Pay close attention to the exact standard needed as later standards shifted the section numbers, and the reference will not line up.


That said, the DL address takes the form of LLLLNNSS (32 bits). However, for most H1 segments, the local link (0x000) is assuming, so the addressing may only be represented as NNSS. The exception is when a true FF Linking Device is used.


Generally, NN is the node address and SS is a selector. However, the valid range of NN node addresses is 0x10-0xFF. NN below 0x10 are special cases, and you don’t necessarily view it as NNSS. For example, 0140 is not Node Address 0x01 and Selector 0x40, it’s actually viewed as a flat address at 0x0140. Flat addresses are used by publishers to send data to subscribes that are node independent. The primary use case is redundant interfaces, where either could publish from 0x0140.


Section A.2.2 provides some additional reserved ranges and how to interpret them.


The selectors also have a defined range to their use case. The table included by reference to the FF specifications. Here is an excerpt from the IEC standard for reference.



So, for example, first configurable DL-CEP (connection end point) starts at selector 0x20. DLCEPs are used for connection oriented services like publisher-subscriber and client-server connections. Each service would have a unique DLCEP. In this case, when you configure the VCR for the endpoint, you specify the FasDLLConfiguredRemoteAdr. It follows the LLLLNNSS format, where LLLL is typically 0000. So, if the publish is using Node Address 0x18 and selector 0x20 (you would see this address on the bus monitor either in the DT packet or the prior CD packet), then the subscriber would configure the FasDLLConfiguredRemoteAdr to 0x1820.


0xF8 is a reserved DLCEP for the Management VCR (defined in FF-801)


Report Distribution (Source/Sink) are connection-less services, and so they use DL-SAPs (service access points). These can be individual or group (e.g. send alerts to multiple devices).