WHAT IT IS

CIS-40.5 is the monitor name for a Russian military 2-tone FSK telegraph at 40.5 baud (measured ~40 Hz on poor samples). Hardware name on the cards: T-206. T-206 «Весна» (Wesna, “Spring”) is the Soviet online telex encryptor family introduced about 1974 as the solid-state successor to T-204 Wolna and T-205 Wecha. The waveform page is the air interface those boxes keyed; the steel box and the FSK pair are not the same object.

Use: encrypted telegraph channel on HF military circuits. Sigidwiki marks the mode Inactive as a regular catch. Dual-channel sibling is CIS-81-81 (T206-M 3M1; old nickname Moroz / Frost) — two 40.5 Bd streams time-multiplexed so the line rate reads ~81 Bd.

WHAT THE WATERFALL LOOKS LIKE

Two rails. Book ID: ~500 Hz shift, occupied width about 600 Hz. USB. Idle on the 250 Hz-shift cousin is a 12-bit mark/space block (6+6); 81-81 idle is 24 bits (12+12). Traffic looks like any slow FSK tweet, slower and usually wider-shift than navy BEE 50 Bd / 200–250 Hz.

HOW THE AIR INTERFACE WORKS

Synchronous FSK. Single-channel frame in the old 81-81 handbook: 6-bit elements at 36.5 / 40.5 Bd; dual-channel: 12-bit frames at 73 / 81 Bd. Alphabet on those cards: Cyrillic M2. Hoka files the family under 81-81 and 81-29. I-56578 notes on the 250 Hz-shift pair: idle, then a block scrambled with PRBS x^11 + x^9 + 1, then idle inverted. Traffic encrypted. WUN-era shift-to-user notes (historical, not a current allocation table): 125 Hz navy; 500 Hz railway; 200 / 250 / 500 Hz military; 1000 / 1500 Hz also logged.

THE T-206 BOX

Wesna was Warsaw Pact inter-governmental online telex crypto. First T-206 MT used tubes, diodes, and core memory in a T-205-sized case. From 1982: 2MT, 3, 3M, 3M1 — 3M / 3M1 transistor-IC, half size, the variants that went into armored comms vehicles. GDR approved it to GVS (SECRET-equivalent). That is the encryptor. CIS-40.5 is what a monitor sees when that class of modem is on HF.

BANDS AND WHERE IT IS HEARD

HF USB, 3–30 MHz when it was a regular. RadioReference lists FSK 40 / 40.5 as sporadic (noted 2021). Not a diplomatic X06 / Serdolik circuit and not the navy 36/50 BEE clock.

HARDWARE

T-206 / T-206-M 3M1 on the cipher-modem side. Radio is whatever HF set takes the FSK pair. Decoders: Hoka 300-32 (81-81 / 81-29). No public plaintext.

NOT THESE

CIS-36-50 / BEE / T-600 — navy FSK, 36 Bd idle / 50 Bd traffic, different box and different clock.
CIS-81-81 — dual-channel version of this family; own page later.
CROWD-36 — 36-tone MFSK, not 40.5 Bd FSK.
Project 206M boats — different T-206.

CAVEATS

40.5 vs ~40 is measurement slop. Shift is not locked at 500 Hz in every log. Sigidwiki Inactive means the classic 40.5/500 catch is rare, not that T-206 never existed. Wesna model letters (MT, 3M1) are box variants, not extra waveforms.

STATUS

Legacy military telegraph FSK. Catalogue it; do not expect it every night next to CIS-12.

IN ONE SENTENCE

CIS-40.5 is the T-206 Wesna family’s single-channel HF FSK telegraph — about 40.5 Bd, commonly ~500 Hz shift in a ~600 Hz slice, encrypted — the slow half of 81-81, not navy BEE and not CROWD-36.