PCI master vs. PCI slave

[i]

Hi,

Can someone explain the difference between "master" and "slave" with respect to the PCI bus?

I've tried to find an answer on the web, but I can only find passing references, with most saying something like, "in the old days, with some motherboards you had to worry about whether you had a PCI master or PCI slave card - these days you don't have to do that."

Well, great ... very helpful. In this case I happen to have one of those old motherboards. Bah!

Here are the details:

It's an old MSI motherboard. It has 5 PCI slots, and 2 ISA slots. It's currently runing with a P233 processor, so it's not too bad a system. I noticed that, in the BIOS, the setting for "PCI 2.1 Compliance" is Disabled. (Is that a good or bad thing?)

PCI slot 1 = Intel Pro100+ network card
PCI slot 2 = ATI Xpert 98 video card
PCI slot 3 = US Robotics 56K modem with voice capability
PCI slot 4 = Adaptec 2940 SCSI card
PCI slot 5 = empty

ISA slot 1 = used by a USB adapter direct to motherboard
ISA slot 2 = empty

I will quote directly from the manual for this motherboard:

All PCI slots can be used as master. But since the 1st and 5th PCI slots share the same bus master signal, only one of these slots can be used as a master at a time; which means that if a bus master card is installed in PCI slot 1, PCI Slot 5 can only accomodate a slave card, and vice versa.

Huh? Like I said, I tried to find an explanation as to what that really means, but I haven't had any luck. I want to put a soundcard into this system, and I happen to have a SoundBlaster Live PCI card lying around. Can I put that into PCI slot 5 and expect things to work ok?

I hope someone here can help.

Sincerely,

i

 

[Mercutio]

I believe that the difference is that "Master" slots can have a unique IRQ assigned to them, and a slave or slaves have to share one with a master... masters have some other abilities. I'm not an electrical engineer. the least technical info I can find is the followingL a "master" controls access to the bus for the slave, inserts waits betwen data transfers for both itself and the slave, and ends bus access for either the master or the slave.

In the end, what this means is that a bus-mastering card like a SCSI controller or network card is in your first slot - something that wants the full attention of the Bus, don't expect to be able to put anything your fifth slot, unless it doesn't use an IRQ at all.

 

[Barry K. Nathan]

http://www.pcguide.com/ref/mbsys/buses/types/pciMastering-c.html

...bus mastering is the capability of devices on the PCI bus (other than the system chipset, of course) to take control of the bus and perform transfers directly.

I believe this is a trick to improve performance, but that many (most?) devices nowadays will refuse to operate without the ability. I may be wrong though.

Your SCSI and network cards almost certainly require bus mastering, and I believe this is also true of the SB Live! (and probably most PCI sound cards for that matter, I would expect). I don't know one way or another about the modem. The video card would typically use bus mastering, but see if the drivers have an option for disabling it (I'm pretty sure Matrox drivers do, for example).

If I got anything wrong, please correct me.

 

[timwhit]

On my motherboard P3V4X the sixth slot is not bus mastering, but I have an Intel network card plugged in and it works fine. I don't think you will run into any problems. But if the device doesn't work, it won't break anything.

So plug it in and test it.

-Tim

 

[i]

Thanks everyone. I guess I will just have to try it to be certain. I still wish I fully understood the difference. It's weird because so many cards seem to say "supports bus mastering," implying that they're doing a favor for you somehow. Yet when you look at it from the perspective of the motherboard, that just doesn't seem to make sense. If the card only supports bus mastering, then you're kind of screwed if you need it to be a slave, aren't you? Hardly a situation they should be boasting about when they list "supports bus mastering" in their specifications.

Maybe those cards really mean to say, "supports but mastering and slaving."

Who knows.

The reason I asked about the SoundBlaster Live specifically was because I've heard so much bad stuff about it with respect to the PCI bus already, without working any of this master/slave stuff into the picture.

I see Multiwave has a basic SB 16 PCI card that's only $20, and I have to order some stuff from them anyway, so ... hmm.

Thinking a bit more about my other hidden question - I suspect I should enable the "PCI 2.1 Compliance" option in the BIOS shouldn't I?

 

[James]

May as well.

You could also move your USR card into the 5th slot. Of all your cards, it's the least likely to require bus mastering. Then you could put your SB whatever card into the 3rd slot, which given its finikyness (sp?) is probably the best place for it (other than back in the shop, in a bin, etc.).

Naaaaah, no bad experiences with Creative here! *sarcasm* YMMV, obviously.

 

[i]

Hah. That means you're probably one of the people in the old SR forums that have contributed to my fear of the SoundBlaster Live. Honestly, after noting so many complaints about it in the old SR forums, I have found myself opening the drawer that contains the card very slowly, and I cringe lest is jumps out at me.

Or something.

Seriously though, I've ordered a cheap SB 16 from Multiwave just to be on the safe side.

 

[James]

Serious audiophiles seem to sneer at them, but I've had good results with CMedia 8738-based cards. Their main advantage is that even the cheap ones tend to come with 6 channel output and SPDIF out, which is great. And when I say cheap, I mean cheap - if I can get them retail here for less than A$20, people in the US should be able to get 'em for less than US$10.

They're not as CPU-friendly as a Live! (when it works) - they use 2% CPU or something when playing a high grade MP3 - but we're not talking particularly noticeable here. I've had one or two blue screens in WinXP (IRQL_IS_LESS_OR_NOT_EQUAL, or whatever it is, lovely) which I think are the card's fault, but it was rock solid under Win2K and Win98.

To be fair it might also be the Detonator XP drivers for my Geforce 2GTS, but either way I'm sure N driver revisions in the future it'll be sorted out.

 

[Mercutio]

Those CMedia chips are the basis of the low-end Philips card, which retails for around $30 here... I can get the same card with the addition of a TOSlink out (audiophiles take note) for $7US.

Big step up from onboard audio, too.

 

[i]

Oh hey, I forgot to post what happened in the end.

I bought the cheap SoundBlaster PCI card, and installed it in the 5th PCI slot. The result? More excitement with a computer than I'd had in a long time.

The system would boot fine, but would freeze solid after some short, random interval once Windows had started.

I gave up and pulled an old ISA SoundBlaster card out of another system (not what I wanted to do - now I need to find a replacement for that system). Aside from it grabbing a lot of resources (it had a built in IDE controller along with all the usual SB garbage) it's been working ok.

Thanks for the help everyone.