VOGONS


First post, by donkom

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Hey everyone,

I'm trying to wrap my head around a few concepts I never fully understood before - the difference between 64-bit PCI and PCI-X, including bus frequencies and voltages.

Here's what I've learned in the past few days, PLEASE correct anything incorrect and offer additional information. There isn't a lot to go on.

- 64-bit PCI was an extension of the 32-bit PCI bus, and made available in 33mhz and 66mhz variants, sometimes on the same board.
- 66mhz versions were only 3.3 volt, 33mhz versions were either 5v or 3.3v (though I cannot currently find evidence of 33mhz 3.3v slots)
- PCI-X started at 66mhz and went up from there, offering 3.3v

That's where things get murky. The Wikipedia page for PCI-X seems to use 64-bit PCI interchangeably with PCI-X ( https://en.wikipedia.org/wiki/PCI-X ), as do most resources online. I have questions.

There's a paragraph here ( https://www.meinbergglobal.com/english/info/pci.htm ) that I found interesting:
"However, due to electrical limitations on the length of the wires between the slots the PCI-X specs allow only up to two slots to be connected to a single bus. So if there are more PCI-X slots available then they are normally connected to several independent buses, each bus with 2 slots."

And yet with 64-bit PCI, we have motherboards like the Tyan Thunder 2500 that was equipped with SIX 64-bit PCI slots, some 3.3v 66mhz and some 5v 33mhz. I don't believe they would have made three separate buses just for this. Here's the manual for that motherboard: ftp://ftp.tyan.com/manuals/m_s1867_100.pdf
(side note: anyone have one of these boards they are willing to sell? I'll give it a good home. Seriously.)

So, there are differences. But people seem to throw the terms together under the same umbrella as if they are the same. Yet, no one has been able to answer this question: Can I use a PCI-X card in a 3.3v 66mhz 64-bit PCI card slot? Do these standards overlap with an identical communication protocol?

Many PCI-X cards can operate in a standard 32-bit PCI slot with some reduced bandwidth/functionality, which makes me believe the underlying standard/protocol is similar - but no one ever talks about 64-bit PCI and it's successor PCI-X, and where the line is drawn between them. There's even weirder connections like this one which I deemed to be incompatible: https://www.ebay.com/itm/333655665804 (it looks similar to the ASUS Media Bus, but was used for a proprietary RAID card. Note the larger spacing for the notch at the end).

Furthermore, there's this weird beast: http://www.pridopia.co.uk/8114.html
Does that actually mean I can plug in a PCIe card in some bastardized fashion into a 64-bit PCI slot? What a monstrosity. I want it.

Reply 1 of 6, by eisapc

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Dont mess up the physical 64 bit PCI slot with the communication standards PCI 64 and PCI-X.
It should be possible to run a PCI-X card in a 64 Bit (3.3V) PCI 66 MHz slot if it it even working in a 32 bit slot.
Just bandwith will be reduced to 66 MHz bus clock compared to up to 533 MHz on PCI-X.

The Tyan board has no PCI-X slots so a single PCI bus is sufficient.
The PCI-X boards I own (mainly Compaq Proliants) really have only two PCI-X slots on each bus.
Some have additional 64 bit PCI 66 MHz slots on a different bus and other high bandwith onboard peripherals like RAID controllers or gigabit ethernet ports on a third.

Typical PCI slot configurations for a 64 BIT PCI machine like the mentionen Tyan or the Proliant DL 580 G1 are as followed
64-bit/66-MHz PCI Hot Plug2, 1 available3.3 Volt Only
64-bit/33-MHz PCI Hot Plug2, 2 available5 Volt or 3.3 Volt
64-bit/33-MHz PCI Non-HotPlug1, 1 available5 Volt or 3.3 Volt
32-bit/33-MHz PCI Non-HotPlug1, 1 available5 Volt Only

The Proliant DL 760 which was one of the first PCI-X machines has 8 64 Bit PCI slots, but only two PCI-X

Here is a link describing PCI-X more detailed:
https://m.eet.com/media/1148240/23667-46884.pdf

The attached picture shows the I/O board of an IBM 7026-H80 with 64 Bit 66 MHz slots.
This is the box with the most PCI slots I own, but still no PCI-X.

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Reply 2 of 6, by donkom

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Thanks for the response, eisapc!

"Dont mess up the physical 64 bit PCI slot with the communication standards PCI 64 and PCI-X." - exactly what I'm trying to clarify.

"communication standards" could be many things. Rev. 1 of the PCI-X standard includes 66mhz and 3.3v, which I suppose is the overlap and allows for backwards compatibility.

What's the deal with the coloured PCI-X slots on some boards, though? Do they signify a slot with a higher bus frequency?

Reply 3 of 6, by shamino

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The distinction between "64 bit PCI" and "PCI-X" is unclear to me also. It's being implied that it's a difference of signaling protocol, but some degree of compatibility was retained. There's also apparently some overlap in the clock speeds that each can run at.
They did clock PCI-X up to 133MHz but apparently it's very limited at that speed.

My file server use a Tyan S2882-D which is an AMD64 board. It uses 3 PCI buses, 2 of them being 3.3V PCI-X buses connected to the northbridge.

One of those PCI-X buses has 2 slots and is also connected to the onboard GbE controller and the onboard SCSI controller (if present - mine isn't). So at maximum that's 4 devices, but only 2 of them in slots. That bus can be set to run at either 66MHz or 100MHz. The manual seems to say (oddly phrased so I could be misinterpreting) that if any cards are added to that bus then you should clock it down to 66MHz. So that implies it has a 2 device limit at 100Mhz.
That bus also has a separate jumper, independent of the speed setting, to select between "PCI" and "PCI-X" mode - so there's that confusion again. Apparently at least 66MHz 64-bit "PCI" is valid. Is there any validity to running 64-bit "PCI" at 100Mhz as well?
The presence of this jumper implies that a "PCI" card won't work in a PCI-X slot, thus the jumper provided for compatibility. But we've seen the reverse, where you can use a PCI-X card in a "PCI" slot.

The photo in the manual shows one of the slots on that bus is colored. I don't see a reason for it, there's nothing special about that slot that I can tell. It might make sense if they were both colored, but I don't know why it's only 1 of them.

The other PCI-X bus has 2 slots but no onboard devices attached. Both slots are white. If you use only 1 slot, then you can set a jumper to run it at 133MHz. If you want to use both slots then they advise 100MHz. So at 133MHz it can only handle 1 device.

What's the deal with the coloured PCI-X slots on some boards, though? Do they signify a slot with a higher bus frequency?

Not in the case of the S2282, at least. The manual recommends one specific slot for 133MHz, but it's not the one they colored.

My old file server, retired a few years ago, was an Intel L440GX+. It has a single 66MHz 32-bit PCI slot which is still keyed for 5V signaling. I'm not sure that abides by any official standard, but it does perform as advertised.
I think that slot is just repurposing the AGP support of the 440GX chipset.

There's even weirder connections like this one which I deemed to be incompatible: https://www.ebay.com/itm/333655665804 (it looks similar to the ASUS Media Bus, but was used for a proprietary RAID card. Note the larger spacing for the notch at the end).

Just noticed that's the same board (L440GX+). I have an HP board from the same time that uses that same slot, and an IBM that I think has that slot as well. That slot bridges some RAID functionality (on the card) to the onboard SCSI controller I believe, not sure about the specifics. Actually, I think all 3 examples I have of that slot use onboard Adaptec controllers, so maybe it's a slot that Adaptec came up with.
It can be used as a standard PCI slot, I think the only signal difference is in the extension which probably routes to the onboard Adaptec chip. I've used those slots with 32-bit 33MHz PCI cards and it works fine.

Last edited by shamino on 2020-08-05, 15:50. Edited 1 time in total.

Reply 4 of 6, by LightStruk

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donkom wrote on 2020-08-04, 19:07:

Here's what I've learned in the past few days, PLEASE correct anything incorrect and offer additional information. There isn't a lot to go on.

- 66mhz versions were only 3.3 volt, 33mhz versions were either 5v or 3.3v (though I cannot currently find evidence of 33mhz 3.3v slots)

I am the unfortunate owner of just such a board with a 32-bit, 33 MHz, 3.3V only PCI slot. The alix1e. Had I realized that it's impossible to find a PCI sound card that actually supports 3.3V only operation (not just having the required notch), I never would have bought this board. It's nice otherwise.

alix1b.jpg

Reply 5 of 6, by Hanamichi

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I'm navigating the same issues at the moment. I didn't realise just PCI 64bit was it's own thing before PCI-X.

This old post seems to explain it quite well:
https://arstechnica.com/civis/viewtopic.php?f=8&t=308217
Quoting the 4th post:

PCI, PCI-64 and PCI-X are all similar, but different. It should be noted that not all PCI cards with universal notching will function in 3.3V slots. I've had systems hang in POST because of this problem.

PCI - 32-bit with a 33MHz or 66MHz (rare) clock speed. Signal levels are 3.3V (notch in card-edge near the back of the chassis - PCI slots of this type are rare, but it's fully compatible with PCI-X) or 5V (notch in card-edge further from back of chassis). A universal card will fit in the slot, as will a card of the proper voltage.

PCI-64 - 64-bit with a 33MHz or 66MHz clock speed. Signal levels are 5V (same notch as above). All 5V PCI cards and universal cards will fit in the slot. A 3.3V card won't fit in this slot. EDIT this bit seems a tad wrong. PCI-64 66Mhz is 3.3v while 33Mhz is 5v. This "standard" was short-lived, and quickly replaced by PCI-X. Intel's 840 and 860 chipsets used this. (Tyan Thunder 2500)

PCI-X - 64-bit with a 33MHz, 66MHz, 100MHz or 133MHz clock (the PCI-X standard covers higher clocks, but I've never seen one). Signal levels are 3.3V. All 3.3V PCI cards and universal cards will fit in the slot. A 5V card won't fit in this slot. A PCI-64 card isn't likely to fit, as it probably won't have the 3.3V notch. Edit PCI-64 66Mhz card will fit.

Another catch is that if you mix bus speeds say 33Mhz and 66mhz on the same PCI bus/lane everything runs at the slower speed. Which explains the need for independent PCI buses.

So i own a Tyan Tiger 2500, looking to do my slotket tualatin mod soon. You are probably missing the general chipset diagram that explains a bit more. Although it's missing the AGP slot which I believe is running off another PCI-64Bit 66MHz bus with a converter chip.

03fig19.gif

In the end you get:
One 2x / Pro AGP 2.0 slot (questionably connected)
Two fully independent 64-bit PCI buses
Two 64-bit / 66 MHz PCI slots (3.3V)
Four 64-bit / 33 MHz PCI slots (I guess 5V)
One 16-bit ISA slot (shared w/ one PCI slot)

Last edited by Hanamichi on 2020-10-12, 01:06. Edited 2 times in total.