Session Lifecycle

This document describes the complete communication sequence between the host and the FNIRSI DPS-150 power supply, from connection establishment through operation to disconnection.

Source

All sequences confirmed from captures dps150_connect_set_10v_set_1A_disconnect.txt and dps150_connect_enable_out_set_v_set_i_disable_disconnect.txt (2026-03-29).

Connection Sequence

sequenceDiagram
    participant H as Host
    participant D as DPS-150

    Note over H: Open serial port<br>9600 baud, 8N1<br>DTR=off, RTS=on

    H->>D: CONNECT (c1 00 01 01 02)
    Note over H: Wait ~200 ms

    H->>D: GET_READY (a1 e1 01 00 e2)
    D-->>H: READY (a1 e1 01 01 e3)

    H->>D: START_SESSION magic (b0 00 01 01 01)
    Note over H,D: Session established — device begins periodic push

    rect rgb(230, 245, 255)
        Note over H,D: Periodic push stream (~600 ms interval)
        D-->>H: PUSH_VIN_A (a1 c0 04 ...) — Vin [V]
        D-->>H: PUSH_OUTPUT (a1 c3 0c ...) — Vout, Iout, Pout
        D-->>H: PUSH_VIN_B (a1 e2 04 ...) — Vin alt [V]
        D-->>H: PUSH_MAX_I_REF (a1 e3 04 ...) — 5.1 A constant
        D-->>H: PUSH_VIN_C (a1 c4 04 ...) — boost rail [V]
    end

    rect rgb(255, 245, 230)
        Note over H,D: Host commands (interleaved with push stream)
        H->>D: SET_VOLTAGE (b1 c1 04 ...)
        H->>D: SET_CURRENT (b1 c2 04 ...)
        H->>D: SET_OUTPUT enable (b1 db 01 01 dd)
        D-->>H: SET_OUTPUT echo (a1 db 01 01 dd)
        H->>D: GET_FULL_STATUS (a1 ff 01 00 00)
        D-->>H: STATUS blob (a1 ff 8b ...)
    end

    H->>D: DISCONNECT (c1 00 01 00 01)
    Note over H: Close serial port

Phase 1: Connection Handshake

The handshake consists of three mandatory steps:

Step 1 — CONNECT

The host sends a CONNECT control frame to wake the device:

TX: f1  c1 00 01 01 02
        │  │  │  │  └── CHKSUM = (00+01+01) mod 256 = 0x02
        │  │  │  └───── DATA = 0x01 (connect)
        │  │  └──────── LEN = 1
        │  └─────────── CMD = 0x00 (CONNECT_CTRL)
        └────────────── START = 0xc1 (control)

Step 2 — Poll GET_READY

The host polls the ready status. The device may take a few hundred milliseconds to become ready:

TX: f1  a1 e1 01 00 e2       (query GET_READY, DATA=0x00)
RX: f0  a1 e1 01 01 e3       (response: ready=1)

If ready ≠ 1, the host retries up to 10 times with 100 ms delays.

Step 3 — Start-Session Magic

An opaque 5-byte sequence sent by the manufacturer tool in every session:

TX: f1  b0 00 01 01 01

Non-standard checksum

This frame uses START byte 0xb0 and its checksum does not follow the standard (CMD + LEN + Σ DATA) mod 256 algorithm. Expected 0x02, actual 0x01. Treated as an opaque constant.

Phase 2: Active Session

Once the session is established, two independent data flows run concurrently:

Periodic Push Stream (Device → Host)

The device pushes measurement data approximately every 600 ms, unsolicited:

CMD	Name	Payload	Description
`0xc0`	`PUSH_VIN_A`	1 × float32	Input voltage channel A (~20.1 V)
`0xc3`	`PUSH_OUTPUT`	3 × float32	Vout [V], Iout [A], Pout [W]
`0xe2`	`PUSH_VIN_B`	1 × float32	Alternate Vin measurement (~19.9 V)
`0xe3`	`PUSH_MAX_I_REF`	1 × float32	Max current constant (5.1 A)
`0xc4`	`PUSH_VIN_C`	1 × float32	Boost rail voltage (~23.6 V)

All push frames use START byte 0xa1.

Host Commands (Host → Device)

Commands can be sent at any time during an active session:

Action	START	CMD	Payload	Response
Set voltage	`0xb1`	`0xc1`	float32 [V]	None (fire-and-forget)
Set current	`0xb1`	`0xc2`	float32 [A]	None (fire-and-forget)
Enable output	`0xb1`	`0xdb`	`0x01`	Echo with START=`0xa1`
Disable output	`0xb1`	`0xdb`	`0x00`	Echo with START=`0xa1`
Query status	`0xa1`	`0xff`	`0x00`	139-byte status blob

Interleaving

The host must be prepared to receive periodic push frames between sending a command and receiving its response. The read_frame() method reads one complete length-delimited frame at a time to handle this.

Phase 3: Disconnection

The host sends a DISCONNECT control frame:

TX: f1  c1 00 01 00 01
        │  │  │  │  └── CHKSUM = (00+01+00) mod 256 = 0x01
        │  │  │  └───── DATA = 0x00 (disconnect)
        │  │  └──────── LEN = 1
        │  └─────────── CMD = 0x00 (CONNECT_CTRL)
        └────────────── START = 0xc1 (control)

After disconnect, the serial port is closed. No response is expected.

Timing Diagram

gantt
    title Session Timeline
    dateFormat X
    axisFormat %s

    section Handshake
    CONNECT frame           :a1, 0, 1
    GET_READY poll          :a2, 2, 3
    START_SESSION magic     :a3, 3, 4

    section Push Stream
    PUSH_VIN_A              :b1, 5, 6
    PUSH_OUTPUT             :b2, 5, 6
    PUSH_VIN_B              :b3, 5, 6
    PUSH_VIN_A              :b4, 11, 12
    PUSH_OUTPUT             :b5, 11, 12

    section Host Commands
    SET_VOLTAGE 12.0V       :c1, 7, 8
    SET_CURRENT 1.0A        :c2, 8, 9
    SET_OUTPUT enable       :c3, 9, 10

    section Teardown
    DISCONNECT              :d1, 15, 16

Python Implementation

The DPS150 class implements this lifecycle as a context manager:

from fnirsi_ps_control.device import DPS150

# __enter__ performs Phase 1 (CONNECT + GET_READY + START_SESSION)
with DPS150("/dev/ttyACM0") as ps:
    # Phase 2: active session
    ps.set_voltage(12.0)
    ps.set_current_limit(1.0)
    ps.enable_output()
    status = ps.get_status()
# __exit__ performs Phase 3 (DISCONNECT + close port)