Electronic – connect TTL gates directly to the address bus of a Z80 CPU

currentmaximum-ratingsttlz80

I need to know when the 14 MSB bits from the address bus of a z80 CPU are high. To do so my first idea is to use 4 74LS08 ICs (TTL AND gates) reading those lines but I don't know the maximum amount of current that can be drained from the CPU.

The 74LS08 has a maximum input current is of 0.1mA hence 1.4mA from the z80 but I cannot find the maximum z80 ratings on the zilog datasheet. The z80 datasheet mentions a TTL chip select decoder connected to the address bus but no mention of the IC ratings are given.

so, can I connect 14 TTL inputs to the z80 address bus without overloading the z80 outputs?

z80 datasheet
74LS08 datasheet

Clarification
One TTL input per address line, not 14 TTL inputs per address line.

Best Answer

The Thomas Scherrer Z80-Family Official Support Page has a file called Z80 CPU DATASHEET (ZIP'ed PDF file) that contains some additional information on the DC characteristics of the Z80A. While it doesn't specifically mention a maximum that I can see at 250uA the minimum output high voltage is listed as 2.4V.

That should be a a good consertaive value as a few 74LS devices I checked listed 2V as the minimum input high voltage. Certainly if connecting 14 device inputs to a single output I think you'd need a buffer, however in this case when you have 14 different outputs going to 14 different inputs that won't be necessary.

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Electronic – Confused about IN/OUT architecture of z80 chip

The address bus of the Z80 is 16-bits wide (bits A0 through A15), meaning it can address 65536 locations. The low 8 bits of the bus are bits A0-A7, which can address 256 locations. Each of the I/O ports connected to the Z80 has a corresponding address of 0-255 (256 total addresses). Only one port responds to a particular address.

If the contents of register C are 01H, and the contents of register D are 5AH, at
execution of OUT (C),D byte 5AH is written to the peripheral device mapped to I/O port
address 01H.

The address of the peripheral (01) will be placed on the address bus A0-A7. The contents of the D register will be placed on the 8-bit data bus, and written to the peripheral whose port number is 01.

Electronic – 8bit: Gate array or dedicated chip

Yes, it can be done. The Ferguson BigBoard (basically the same design as the Xerox 820) is one example — it had a full 64 kB of RAM for applications, plus dedicated video RAM that could be bank-switched into a 16-kB region of the address space. All done with regular SSI/MSI TTL logic.

Best Answer

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