MSX Interface

What might the casette interface be about?
I imagine a casette bit gets fed to chips of serial port gear,
and then casette loading no more is one big disabled interrupt zone but can go multitasking.
like having an interactive GUI while a download from net to casette happens.

Or was that too much phantasy and it is about some MSXes that had no casette port?

I think possibly the MSX-INTERFACE ASIC was also intended to service a market outside MSX systems.

Maybe it was designed to compete with the Yamaha YM3802 MCS. This IC was used in some of the Russian teaching network side slot modules, and also has a UART, clock generator, FIFO and... FSK modulator.

I've found what MSX-INTERFACE J3125 is.
It is likely identical with YM3802(MCS).
http://doc.chipfind.ru/yamaha/ym3802.htm

In spite of its name, the integrated UART is used for general purpose, not only MIDI.
The specification seems to be very similar to the one of YM2148(MKS) - a.k.a. interface companion of SFG-01/05.
So Yamaha or Ascii might have intended to develop MSX sound unit integrates Y8950 and J3125 as like as SFG.

https://www34.atwiki.jp/matomerutokoro/pages/44.html

Japanese comments:

「ソフトウェアを知りつくしたアスキーの、LSI。」
CQ出版 トランジスタ技術 1986年12月
ASCII 半導体広告

左の公告の白黒画像版公告。
CQ出版 トランジスタ技術 1987年04月
スキャン画像が荒いのでわかりにくいが、J3125の表面には3802-Yの表示があるため、YM3802と同じか上位互換が考えられる。アスキー外販版がJ3125、ヤマハ販売分がYM3802なのかもしれない。なお、turboRのテクハンによると、MSX-INTERFACEのI/OアドレスはturboRでは廃止されているとのこと(MSX Datapack ではMSX-INTERFACEの割り当てがC8H～CFH、turboRテクハンでのI/Oアドレス割り当て廃止の記載はC8H～CCH)
またデバイスイネーブルのポート(F5H)のビット4にもMSX-INTERFACEの割り当てがある。

Excite translation:

"LSI of Ascii which knew everything of software."
CQ publication Transistor technology December, 1986.
ASCII Semiconductor advertisement.

Monochrome picture edition proclamation of the left proclamation.
CQ publication Transistor technology April, 1987.
A scan picture is rough, so it's incomprehensible, but for an outside of J3125 to have indication of YM3802, a dominance interchange can think whether it's same as YM3802. ASCII outside sales edition may be J3125 and the one by which the Yamaha sales part is YM3802. Further it's said that the I/O address of MSX - INTERFACE is abolished in turboR according to technology Hann of turboR (for mentioning by which allocation of MSX - INTERFACE is I/O address allocation abolition by C8H-CFH and turboR technology Hann in MSX Datapack, C8H-CCH).
There is also allocation of MSX - INTERFACE in bit 4 in a port in DEBAISUINEBURU (F5H).

Little correction to the translation:

A scan picture is rough, so it's incomprehensible, but for an outside of J3125 to have indication of 3802-Y, a dominance interchange can think whether it's same as YM3802. ASCII outside sales edition may be J3125 and the one by which the Yamaha sales part is YM3802.