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The 315-5308 is found on earlier revisions of the MegaDrive I.

It is the bus arbiter that controls all bus accesses.



OpenOffice Draw file : File:315-5308.odg


Address Buses
A1-A23 M68K Address Bus
ZA0-ZA15 Z80 Address bus
Data Buses
D0-D15 M68K Data bus
ZD0-ZD7 Z80 Data bus
IO port lines
EM1X Port 1 lines
EM2X Port 2 lines
EM3X Port 3 lines
Control lines
M68K related control lines
/BG Bus Grant
/BGACK Bus Grand Acknowledge
/BR Bus Request
FC0 Function Code 0, 1 = Accessing user data
FC1 Function Code 1, 1 = Accessing user program
/HALT Halt 68K execution
/RESET Reset 68K
/VPA Valid Peripheral Access
/AS Address strobe
/DTACK Data Acknowledge
/UDS Accessing even bytes
/LDS Accessing odd bytes
R/W Read/Write pin, 0 = write 1 = read
Z80 related control lines
/ZM1 Opcode fetch strobe
/ZRD Read strobe
/ZWR Write strobe
/ZMREQ Memory request
/ZIORQ IO request
/ZNMI Non-maskable IRQ
/ZBR Bus request
/ZBAK Bus Acknowledge
/ZRES Reset Z80
Address decoding related control lines
/ASEL Read or Write on $000000-$7FFFFF region
/CAS0 Read or Write on $000000-$DFFFFF region
/RAS0 Read or Write on $E00000-$FFFFFF region, act as /CE signal for the RAM chips
/CE0 Chip Enable for the cartridge, low when accessing $000000-$3FFFFF region when expension unit isn't inserted or $400000-$7FFFFF when it is.
/TIME Read or Write on $A13000-$A130FF region
/LWR Lower byte WRite
/UWR Upper byte WRite
/NOE /OE signal for the lower part of the RAM
/EOE /OE signal for the upper part of the RAM
/ZREF Unknown, tied to unused pin #1 of Z80 RAM
/ZRAM /CE for the Z80 RAM
Configuration related control lines
/DISK 0V Expansion Unit connected
FREQ 5V = 60Hz, 0V = 50Hz
LANG 5V = English, 0V = Japanese
/M3 Mark III compatibility mode
VRAM related control lines
/CAS1 Column address strobe
/RAS1 Row address strobe
/SE0 Serial data bus /OE
SC Serial clock
/WE0 Write enable
/OE1 Output enable
Clock lines
MCLK Master Clock
VCLK 68K clock
ZCLK Z80 clock
EDCLK Unknown clock