Packet Size Reference

Radio Payload Limits

Radio	FIFO Size	Adafruit Header	`\n` Delimiter	Usable Payload
RFM69	66 bytes	4 bytes	1 byte	61 bytes
RFM95	252 bytes	4 bytes	1 byte	247 bytes

The configured chunk sizes in config.yml provide additional margin:

Radio	Config `chunk_size`	Effective Payload
RFM69	50 bytes	50 - 1 (`\n`) = 49 bytes data
RFM95	240 bytes	240 - 1 (`\n`) = 239 bytes data

Telemetry Packet Sizes (including 4-byte ID)

All sizes include the 4-byte identifier but exclude the \n appended by radio.py.

Packet	Total Size	Fits RFM69?	Fits RFM95?
GYRO	16	Yes	Yes
ACCL	16	Yes	Yes
MAGN	16	Yes	Yes
GRAV	16	Yes	Yes
EULR	16	Yes	Yes
BME2	16	Yes	Yes
TEMP	8	Yes	Yes
QUAT	20	Yes	Yes
ADCS	32	Yes	Yes
SOLR	36	Yes	Yes
EPSS	28	Yes	Yes
OBCC	8	Yes	Yes
OBCR	8	Yes	Yes
OBCD	8	Yes	Yes
HOST	15	Yes	Yes
BECN	24	Yes	Yes
XFRC	8	Yes	Yes

Chunked Data Sizes

For multi-packet responses (OBCP, OBCL), each chunk is:

[4-byte ID] + [payload] + [\n from radio.py]
max_chunk_payload = chunk_size - 5

Radio	chunk_size	ID	`\n`	Max Payload per Chunk
RFM69	50	4	1	45 bytes
RFM95	240	4	1	235 bytes

Image Packet Sizes

Image packets use a different calculation because they're text-based:

SEND[base64_data][total:04d][current:04d]
  4  +  b64_len  +    4    +     4      = total

Radio	Raw Chunk	Base64	+ ID + Meta	Total	Within Limit?
RFM69	33 bytes	44 chars	+ 4 + 8 = 56	56 bytes	Yes (< 61)
RFM95	168 bytes	224 chars	+ 4 + 8 = 236	236 bytes	Yes (< 247)

Size Calculation Examples

RFM69 Image Packet

Raw data chunk:   33 bytes
Base64 encoded:   ceil(33/3) * 4 = 44 characters
SEND identifier:  4 bytes
Metadata:         8 bytes (4 total + 4 current)
                  ─────────
Packet total:     56 bytes
+ \n from radio:  1 byte
                  ─────────
On-air total:     57 bytes  (< 66 byte FIFO limit)

RFM69 OBCP Chunk

OBCP identifier:  4 bytes
Text payload:     45 bytes max
                  ─────────
Packet total:     49 bytes
+ \n from radio:  1 byte
                  ─────────
On-air total:     50 bytes  (= chunk_size config)

Exceeding Limits

If a packet exceeds the radio FIFO size, the Adafruit library raises an AssertionError. This most commonly occurs with:

Image packets when chunk sizes aren't properly tuned
OBC process/file listing chunks when using the wrong default size
Any packet that doesn't account for the 4-byte Adafruit header + 1-byte \n

The chunk_data() function in main.py automatically calculates the correct max payload size based on the active radio's chunk size configuration.