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speculatrix
 Post subject: Driving the bus
PostPosted: Sun Dec 12, 2021 9:29 am 
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I'm plodding away with my first 6502-based homebrew project (https://bit.ly/zolatron64). It's planned to run at 1MHz and no more.

I'm learning a lot (the entire reason for the project), mostly by making mistakes. Which is fun.

It's a backplane design and, currently, the signals from the 6502 just go straight to the backplane connector.

It has occurred to me that it might be better if these signals were buffered. I don't know why - just a hunch. So I was thinking of introducing something like 74HC245 tranceivers between 6502 and backplane, with the DIR pin connected to R/W. Does this make any sense?

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GARTHWILSON
 Post subject: Re: Driving the bus
PostPosted: Sun Dec 12, 2021 9:40 am 
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Do go through the 6502 primer. Section 5, on 74xx logic families does address this. The W65C02S's pin drivers are a lot stronger than 74HC's are, so you definitely don't need the 74HC245 bus transceiver for strength. There might be some value in the 74HC's slower slew rates, for preventing AC-performance problems, but the most important thing to keep clean there will be the clock signal, not so much the address and data lines. In section 10, on expansion buses and interfaces, you'll see that I'm not fond of the card-cage idea anymore, and the reasons why it's not like things were 30 years ago.

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speculatrix
 Post subject: Re: Driving the bus
PostPosted: Sun Dec 12, 2021 10:04 am 
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Thanks for that. I have read large portions of the primer, but obviously didn't recall that bit!

I understand the arguments against backplanes, but for a first project this approach offers a handy degree of modularity. Currently, the computer consists of three boards - a main board with CPU, RAM, ROM, clock and misc decoding. Then there's a VIA board with a single 6522 and a serial board with a 6551. I've just ordered PCBs for all this, but have already thought about improvements for the VIA board. The VIA and serial boards have their addresses selectable on the boards via jumpers, so I can add more VIA & serial boards if I like. Or add upgraded versions of either without changing other boards. I feel this mix'n'match approach is highly suitable to learning, which is what this is all about.

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BigDumbDinosaur
 Post subject: Re: Driving the bus
PostPosted: Sun Dec 12, 2021 8:28 pm 
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speculatrix wrote:
It has occurred to me that it might be better if these signals were buffered. I don't know why - just a hunch. So I was thinking of introducing something like 74HC245 tranceivers between 6502 and backplane, with the DIR pin connected to R/W. Does this make any sense?

My POC V1.2 unit is clocked at 20 MHz and runs fine without any bus drivers. My POC V1.3 runs at 16 MHz, also without bus drivers (its speed is timing-limited). As Garth notes, the WDC processors have strong output drive, much more so than the older CMOS and NMOS incarnations.

With the 65C816, a data bus driver is appropriate for timing reasons, not due to low drive.

I'd be cautious with using a backplane. They tend to produce transmission line effects that are not counteracted solely by bus drivers. These issues can arise even at relatively low clock speeds.

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I understand the arguments against backplanes, but for a first project this approach offers a handy degree of modularity. Currently, the computer consists of three boards - a main board with CPU, RAM, ROM, clock and misc decoding. Then there's a VIA board with a single 6522 and a serial board with a 6551.

I would consider that hardware de rigueur and would incorporate all of it onto a single mainboard (although I wouldn't be using the 6551). If you think you will be adding to it in the future, set up an expansion port.

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GARTHWILSON
 Post subject: Re: Driving the bus
PostPosted: Sun Dec 12, 2021 8:40 pm 
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BigDumbDinosaur wrote:
I would consider that hardware de rigueur and would incorporate all of it onto a single mainboard (although I wouldn't be using the 6551). If you think you will be adding to it in the future, set up an expansion port.

or just leave places for the extra ICs on the board even if you don't initially put them in; or you can leave a prototyping area on the board to add stuff later. With the today's smaller packages (even SOJs, PLCCs, and SOICs which are still easy to solder by hand), and the higher-scale integration, the reduced pin count and number of connections afforded by things like SPI, and the greater memory densities, you can put more on a single board today than you could in a whole set of boards 30 years ago, and it can still be made modular, just in a different way.

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BigDumbDinosaur
 Post subject: Re: Driving the bus
PostPosted: Sun Dec 12, 2021 8:46 pm 
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GARTHWILSON wrote:
BigDumbDinosaur wrote:
I would consider that hardware de rigueur and would incorporate all of it onto a single mainboard (although I wouldn't be using the 6551). If you think you will be adding to it in the future, set up an expansion port.

or just leave places for the extra ICs...you can put more on a single board today than you could in a whole set of boards 30 years ago, and it can still be made modular, just in a different way.

Plus the cost of decent-sized PCBs has come down a lot in recent years, so building on a larger board to accommodate extra features without breaking the bank is possible.

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BigEd
 Post subject: Re: Driving the bus
PostPosted: Mon Dec 13, 2021 1:28 pm 
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(Just to note, I wouldn't want anyone to be put off building a backplane design, just because BDD doesn't like to.)
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BillO
 Post subject: Re: Driving the bus
PostPosted: Mon Dec 13, 2021 2:34 pm 
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I'm a fan of backplanes on computers used for experiments. They are much easier and quicker to reconfigure due to the inherent modularity . I'm also a fan of buffering them to protect the more expensive chips like CPU and memory from damage due to to failed experiments. It's a lot cheaper to replace a 74XX245 than a W65C02.

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BigDumbDinosaur
 Post subject: Re: Driving the bus
PostPosted: Mon Dec 13, 2021 9:14 pm 
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BigEd wrote:
(Just to note, I wouldn't want anyone to be put off building a backplane design, just because BDD doesn't like to.)

I'm merely pointing out the issues a backplane can introduce, and don't understand, Ed, why any time I mention the “cons” as well as the “pros” of anything you feel compelled to interject a snide remark into the discussion.

The OP says in his first post that he's learning, which implies he's not at the expert level with hardware. While I don't claim expert level as well, I've scratch-designed and built enough hardware to know what's what and where things can go wrong. If it offends you, Ed, for me to point out that it's not always unicorns and rainbows when it comes to this stuff, may I suggest you heed your own, often-repeated, advice and move on to something else.

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