Yamaha YIS-503IIIR
This page was last modified 23:36, 16 December 2021 by Mars2000you. Based on work by Gdx and Rderooy and others.




The Yamaha YIS-503IIIR is the adaptation of the YIS-604/128 or the CX7M/128 computer for Soviet Union.

It was not sold in shops and could only be used in schools. The "КУВТ2" logo is the abbreviation of "Комплект учебной вычислительной техники 2" and means "Class of Teaching Computing Equipment 2".

This computer has a Russian keyboard, it was generally used by the students while the teacher had a YIS-805/128R2 including two disk drives. The communication between MSX computers is actually controlled by a sophisticasted interface called Serial I/O Unit Mk II.

This interface has 2 specific serial ports and an internal DIP switch with 4 configurable bits that define the number of student machine (1 - 15). Setting DIP to 0 makes the machine a teacher's machine.

In this MSX-Network, up to 8 student machines are connected to a teacher machine using one of the floppy disk drives for centralised file storage.

Note: This communication system is completely incompatible with the system used on the MSX1 YIS-503IIR computer.

Brand Yamaha
Model YIS-503IIIR
Year 1985
Region USSR
Launch price
RAM 128kB in slot 3-2
VRAM 128kB
Media MSX cartridges
Video Yamaha V9938
Audio PSG (YM2149 integrated in MSX-Engine S3527)
Chipset Yamaha S3527
Keyboard layout JCUKEN/Cyrillic
Extras Reset button, 60 pin module port, Network unit + switch + Network BASIC, Net-Shell version of CP/M 2.2
Emulation ROMs dumped and emulated with exception of the networking function


When booting, the network is automatically initialized, you can switch it off by CALL NETEND or _NETE and eventually switch it on again with CALL NETINIT or _NETI (all CALL commands can be shortened to 3-4 letters and a space is not required after the underscore).

The network firmware can be disabled by pressing DEL key while booting. It is recommended to disable it for playing games, this to avoid eventual incompatibility or for using the machine as a standard MSX2. However, some games were adapted to play them in network.

Another firmware is included in this computer, it's a special version of CP/M 2.2 for working in network. From MSX-BASIC, this firmware can be launched with CALL CPM or _CPM.



Yamaha YIS-503IIIR
Yamaha YIS-503IIIR underside
Yamaha YIS-503IIIR box
Yamaha YIS-503IIIR left side
Yamaha YIS-503IIIR back
Yamaha YIS-503IIIR right side
Yamaha YIS-503IIIR inside
Yamaha YIS-503IIIR mainboard
Yamaha Serial I/O Unit Mk II connectors


CP/M 2.2 NET-SHELL Version
CP/M 2.2 NET-SHELL Version


The YIS-503IIIR is a MSX2 with a Z80A from Sharp (LH0080A) and an MSX-Engine S3527 from Yamaha.

This model can be easily upgraded internally because RAM and VRAM are each on separate boards with a connector, you just need to change the corresponding board.

Slot Map

Slot 0 Slot 1 Slot 2 Slot 3-0 Slot 3-1 Slot 3-2 Slot 3-3
Page C000h~FFFFh Cartridge
Slot 1
Slot 2
Page 8000h~BFFFh
Page 4000h~7FFFh Main-ROM CP/M ROM
Page 0000h~3FFFh Sub-ROM
  • By default the Module Slot contains a serial network interface module from 4000-BFFF