Hi Matt,
This forum post seems to be talking about the same thing: ProXR & AD8 Relay Activator / Event Generator.
The breakdown as stated there is:
170 NCD API Frame: Header Byte
6 NCD API Frame: Number of Payload Bytes
171 Payload Byte 1: Spontaneous Event Generated by Controller
0 Payload Byte 2: E3C Device Number
1 Payload Byte 3: Data Source (1 = AD8 Inputs)
8 Payload Byte 4: Indicates the Status of the 8 Inputs
24 Payload Byte 5: Indicates Which Bits have Changed
0 Payload Byte 6: Status of Relay Bank 1
124 NCD API Frame: 8-Bit Checksum (Sum of all Payload Bytes Truncated to 8 Bits)