BUSDIR signal

By Kwik

Expert (127)

Kwik's picture

28-10-2006, 16:30

Hi,

I'm unsure about the BUSDIR signal in the cartridge port of the msx. What does it exactly do? Is it true that when a cartridge contains memory only (so no I/O), it does not need to manage the BUSDIR signal?

I just looked at the schematics of an 8280 and saw that SLTSL1, SLTSL2 and RD are used to determine the direction of the databus buffer. If it is true that BUSDIR is not needed for memory access, than this would mean that the SLTSL-signals contain MERQ. Still true?

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By HansO

Paladin (672)

HansO's picture

28-10-2006, 18:44

Look at the MSX Technical Handbook and you will see that for memory access the busbuffers for a slot, including cartridge, are switched by the applicable slot select signal. Indeed SLSLT contains MERQ, since the slot select signals are all about memory access in a slot.
So indeed if a cartridge contains only memory it does not have to manage busdir.

For I/O on a cartridge generation of BUSDIR is required. And that will have to be derived from the I/O port selects, IORQ and RD.

By Kwik

Expert (127)

Kwik's picture

28-10-2006, 21:34

Thanks very much for your help, Hans. I have two other questions:

1. It is clear now that SLTSL contains MERQ. So when RAM is connected to the cartridge using SLTSL, MERQ does not need to be used. Am i correct?

2. When taking a look at the schematics of the 8280 you can see that the basic rom and extrom use ROMCS and SLT3/30 respectivily. However the CE of these IC's is connected to MERQ. This suggests that ROMCS and SLT3/30 do not contain MERQ. Is this the true?

By HansO

Paladin (672)

HansO's picture

29-10-2006, 09:21

Thanks very much for your help, Hans. I have two other questions:

1. It is clear now that SLTSL contains MERQ. So when RAM is connected to the cartridge using SLTSL, MERQ does not need to be used. Am i correct?

No. But the CS1 CS2 and CS12 are having MERQ included.
See for example the Elektuur SRAM design or the memory (mapper) expansion articles mappers on the DIY page.


2. When taking a look at the schematics of the 8280 you can see that the basic rom and extrom use ROMCS and SLT3/30 respectivily. However the CE of these IC's is connected to MERQ. This suggests that ROMCS and SLT3/30 do not contain MERQ. Is this the true?

Yes.

By Kwik

Expert (127)

Kwik's picture

29-10-2006, 15:50

Regarding your answer on question 1:

Why is it necessary to use MERQ, when it is already included in SLTSL? That does not make sense.

When, for example, connecting 64 kB RAM (not ROM) to the cartridge i thought it would suffice to use SLTSL (connected to CE of the RAM), RD and WR. But you say that i need MERQ as well?

By HansO

Paladin (672)

HansO's picture

29-10-2006, 17:19

I should have said SLTSL + MERQ instead of SLTSL contains MERQ

By Kwik

Expert (127)

Kwik's picture

29-10-2006, 17:26

OOOOHHH NOOO, now i understand... STLSL not only contains MERQ, but it also inlcudes REFRESH.... that why you need MERQ as well. You were right, Hans. Thanks!