Yamaha YIS-503IIR
This page was last modified 23:31, 16 December 2021 by Mars2000you. Based on work by Gdx and Rderooy and others.




The Yamaha YIS-503IIR is the adaptation of the YIS-503II computer for Soviet Union. It was not sold in shops and could only be used in schools. The "КУВТ" logo is the abbreviation of "Комплект учебной вычислительной техники" and means "Class of Teaching Computing Equipment".

This computer has a Russian keyboard and the specific music connectors have been replaced by 2 serial port connectors and a switch allowing the machine to work in teacher mode or in student mode. The communication between MSX computers is actually controlled by a serial network interface called Serial I/O Unit.

Note: This communication system is completely incompatible with the system used on the MSX2 YIS-503IIIR and YIS-805/128R2 computers.

Brand Yamaha
Model YIS-503IIR
Year 1985
Region USSR
Launch price
RAM 64kB in slot 3-2
Media MSX cartridges, (only teacher machine) DD floppy disks
Video Yamaha V9938 (MSX2 VDP!)
Audio PSG (YM2149 integrated in MSX-Engine S3527)
Chipset Yamaha S3527
Keyboard layout JCUKEN/Cyrillic
Extras 60 pin module port, mini cartridge port, serial network interface + switch + RS-232C BASIC , internal RAM / VRAM expandable, (only teacher machine) 1 external 720kB 3,5" floppy disk drive
Emulation ROMs dumped and emulated with exception of the network module, side module slot and mini slot


By default, the network firmware is enabled and the content of the function keys is modified for F1 and F6 to F10:

Key Display String Key Display String
F1 color "color" F6 run"COM:" "run"+CHR$(34)+"COM:"+CHR$(34)
F2 auto "auto" F7 save"COM:" "save"+CHR$(34)+"COM:"+CHR$(34)
F3 goto "goto" F8 load"COM:" "load"+CHR$(34)+"COM:"+CHR$(34)
F4 list "list" F9 merge"COM:" "merge"+CHR$(34)+"COM:"+CHR$(34)
F5 run "run"+CHR$(13) F10 _comterm "_comterm"+CHR$(13)

In this system, several student machines are connected to a teacher machine with an external floppy disk drive for centralised file storage. Note that the corresponding machine for the teacher in the blueMSX emulator includes an internal floppy drive.

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 MSX with the 'normal' content of the function keys.


Yamaha YIS-503IIR
Yamaha YIS-503IIR left side
Yamaha YIS-503IIR back
Yamaha YIS-503IIR RAM board
Yamaha YIS-503IIR VRAM board
Yamaha YIS-503IIR underside
Yamaha YIS-503IIR label
Yamaha YIS-503IIR main PCB


The YIS-503IIR is a MSX1 with a Z80A from Sharp (LH0080A), a V9938 and an MSX-Engine S3527 from Yamaha.

This computer can be easily upgraded internally because RAM and VRAM are each on an daughterboard. Simply just to change the corresponding board.

The mini slot allows to add software as on the Yamaha CX5MII.

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
Page 0000h~3FFFh
  • By default the Module Slot contains a serial network interface module (8kB ROM - 4000h-5FFFh)