Add UDP packet format docs

doc/specific_iocs/datastreaming/Datastreaming_udp_packet_formats.md

@@ -0,0 +1,117 @@
+# Datastreaming: UDP packet formats
+
+This page describes the UDP packet format for instruments which stream UDP from detectors (e.g. HRPD-X).
+
+Translation between this UDP format and the {doc}`flatbuffers format <ADRs/000_kafka>` is done by [`event_udp_to_kafka`](https://github.com/ISISComputingGroup/event_udp_to_kafka), which initially get written to the `_rawEvents` Kafka topic.
+
+The data comes in over UDP packets, each of which is made up of 32-bit words. All data is transmitted in big-endian
+format.
+
+## Header format
+
+The header is 16 words (64 bytes). 
+
+### Word 0: marker byte
+
+Always `0xFFFFFFFF`.
+
+### Word 1: header information
+
+- **Bit 0**: End of run header marker (active low)
+- **Bit 1**: Veto frame packet header marker (active low)
+- **Bit 2**: Sample environment packet header marker (active low)
+- **Bit 3**: Not used
+- **Bits 4..=31**: Always `0xFFFFFFF`
+
+### Word 2: Information
+
+- **Bits 0..=3**: Reserved for header type
+- **Bits 4..=8**: Length of header
+- **Bits 9..=10**: Not used
+- **Bit 11**: Bad frame (not currently used by streaming control board)
+- **Bit 12**: Run will continue (not currently used by streaming control board)
+- **Bit 13**: No frame sync
+- **Bit 14**: Frame Memory Full veto
+- **Bit 15**: Frame data overrun
+- **Bits 16..=31**: Not used
+
+### Word 3: Frame number
+
+- **Bits 0..=31**: frame number as u32
+
+### Word 4: GPS timestamp
+
+- **Bits 0..=3**: seconds (most significant bits; combine with least significant bits from word 5)
+- **Bits 4..=9**: minutes
+- **Bits 10..=14**: hours
+- **Bits 15..=23**: days
+- **Bits 24..=31**: years (as offset from year 2000)
+
+### Word 5: GPS timestamp
+
+- **Bits 0..=9**: nanoseconds
+- **Bits 10..=19**: microseconds
+- **Bits 20..=29**: milliseconds
+- **Bits 30..=31**: seconds (least significant bits; combine with most significant bits from word 4)
+
+### Word 6: period number
+
+- **Bits 0..=15**: Period number
+- **Bits 16..=31**: Not used
+
+### Word 7: events in frame
+
+- **Bits 0..=31**: number of neutron events in this frame.
+
+:::{note}
+This is not necessarily the same as the number of events in this UDP message, as the events may be split between
+multiple UDP messages. This is the *total* number of events in the ISIS frame.
+
+See header word 13 if looking for length of *this* message.
+:::
+
+### Word 8: protons-per-pulse
+
+- **Bits 0..=7**: protons-per-pulse in this ISIS frame.
+- **Bits 8..=31**: unused
+
+To convert to {math}`\mu Ah` delivered during this ISIS frame, multiply by {math}`1.738{\times}10^{-6}`.
+
+### Word 9: vetoes
+
+- **Bit 0**: FIFO veto
+- **Bit 1**: SMP veto
+- **Bit 2**: TS2 pulse veto
+- **Bit 3**: Wrong pulse veto
+- **Bit 4**: Unused
+- **Bit 5**: ISIS slow
+- **Bits 6..=9**: External vetoes
+- **Bits 10..=13**: Fast chopper vetoes
+- **Bits 14..=15**: Reserved vetoes
+- **Bits 16..=23**: Unused
+- **Bits 24..=31**: Frame repeat number
+
+### Word 10: next frame address
+
+- **Bits 0..=31**: Address of the next frame, in bytes.
+
+### Word 11: next frame address
+
+- **Bits 0..=31**: Address of the next frame, in 64-bit words.
+
+### Word 12: streamed frame number
+
+- **Bits 0..=31**: streamed frame number.
+
+### Word 13: packet length
+
+- **Bits 0..=11**: number of 32-bit words from the beginning of this header to the start of the next header.
+- **Bits 12..=31**: Unused
+
+### Word 14: unused
+
+- **Bits 0..=31**: Unused
+
+### Word 15: checksum
+
+- **Bits 0..=31**: Pre-DDR checksum

doc/spelling_wordlist.txt

@@ -192,6 +192,7 @@ dbs
 dcalc
 dcom
 dcomcnfg
+DDR
 deadband
 deadbands
 deadman