Although the adeptness to aggrandize a home computer with added RAM, accumulator and added appearance has been about for as continued as home computers exist, it wasn’t until the IBM PC that the abstraction of a absolutely accessible and modular computer arrangement became mainstream. Instead of actuality bound to a arrangement agreement provided by the architect and a few add-ons that absolutely didn’t accommodate well, the abstraction of amplification cards opened up accomplished industries as able-bodied as a big hobbyist market.
The aboriginal IBM PC had bristles 8-bit amplification slots that were affiliated anon to the 8088 CPU. With the IBM PC/AT these amplification slots became 16-bit abode of the 80286 CPU it was congenital around. These slots could be acclimated for annihilation from cartoon cards to networking, broadcast anamnesis or custom I/O. Admitting there was no audible aboriginal name for this agenda bend interface, about the PC/AT era it got referred to as PC bus, as able-bodied as AT bus. The name Industry Accepted Architectonics (ISA) bus is a retronym created by PC carbon makers.
With such artlessness came the adeptness to almost accessible and cheaply accomplish your own cards for the ISA bus, and the consecutive and appropriately accessible PCI bus. To this day this artlessness allows for a active ecosystem, whether one wishes to body a custom ISA or PCI soundcard, or add USB abutment to a 1981 IBM PC system.
But what does it booty to get started with ISA or PCI amplification cards today?
An important affair to agenda about ISA and the aboriginal PC/AT bus is that it isn’t so abundant a all-encompassing bus as it describes accessories blind off an 8088 or 80286 acclamation and abstracts bus. This agency that for archetype that originally the bus is as fast as the alarm acceleration of the CPU in question: 4.77 MHz for the aboriginal PC bus and 6-8 MHz for the PC/AT. Although 8-bit cards could be acclimated in 16-bit slots best of the time, there was no agreement that they would assignment properly.
As PC carbon vendors began to acquaint faster CPUs in their models, the AT bus concluded up actuality clocked at anywhere from 10 to 16 MHz. Understandably, this led to abounding absolute AT (ISA) bus cards not alive appropriately in those systems. Eventually, the alarm for the bus was decoupled from the processor alarm by best manufacturers, but admitting what the acronym ‘ISA’ suggests, at no point in time was ISA absolutely standardized.
It was about attempted to assimilate a backup for ISA in the anatomy of Extended ISA (EISA). Created in 1988, this featured a 32-bit bus, active at 8.33 MHz. Although it didn’t booty off in customer PCs, EISA saw some uptake in the server market, abnormally as a cheaper another to IBM’s proprietary Micro Channel architectonics (MCA) bus. MCA itself was envisioned by IBM as the backup of ISA.
Ultimately, ISA survives to this day in mostly automated accessories and anchored applications (e.g. the LPC bus), while the blow of the industry confused on to PCI and to PCIe abundant later. Cartoon cards saw a few detours in the anatomy of VESA Local Bus (VLB) and Accelerated Cartoon Anchorage (AGP), which were specialized interfaces aimed at the needs of GPUs.
The aftereffect of this agitated history of ISA in accurate is that one has to be accurate aback designing a new ‘ISA amplification card’. For absolutely advanced compatibility, one could architecture an 8-bit agenda that can assignment with a bus acceleration from anywhere from 4.77 to 20 MHz. Going beeline to a 16-bit agenda would be an advantage if one has no charge to abutment 8088-based PCs. Aback designing a PC/104 card, there should be no affinity issues, as it follows appealing abundant the best accepted anatomy of the ISA bus.
The concrete interface is not a botheration with either ISA or PCI, as both use bend connectors. These were best mostly because they were bargain yet reliable, which hasn’t afflicted today. On the PCB end, no concrete adapter exists, alone the conductive ‘fingers’ that acquaintance the contacts of the bend connector. One can use a arrangement for this part, to get acceptable alignment with the contacts. Additionally accumulate in apperception the array of the PCB as the agenda has to accomplish acceptable contact. Here the accepted 1.6 mm seems to be a acceptable match.
One can calmly acquisition assets for ISA and PCI architecture rules online if one wishes to actualize the bend adapter themselves, such as this accomplished overview on the Multi-CB (PCB manufacturer, no affiliation) site. This shows the feel spacing, and the 45 degrees abate on the edge, forth with feel thickness and ambit requirements.
Useful for the electrical ambit architecture allotment is to apperceive that ISA uses 5 V akin signaling, admitting PCI can use 5 V, 3.3 V or both. For the latter, this aberration is adumbrated application the adjustment of the cleft in the PCI slot, as abstinent from the IO plate: at 56.21 mm for 3.3 V cards and 104.47 mm for 5 V. PCI cards themselves will accept either one of these notches, or both if they abutment both voltages (Universal card).
PCI slots abide in 32-bit and 64-bit versions, of which alone the above fabricated a burst in the customer market. On the flip-side of PCI we acquisition PCI-X: an change of PCI, which saw best use in servers in its 64-bit version. PCI-X about doubles the best abundance of PCI (66 to 133 MHz), while removing 5V signaling support. PCI-X cards will generally assignment in 3.3V PCI slots for this reason, as able-bodied as vice-versa. A 64-bit agenda can abatement aback to 32-bit approach if it is amid into a shorter, 32-bit slot, whether PCI or PCI-X.
Every accessory on a bus adds a amount which a signaling accessory has to overcome. In addition, on a bus with aggregate lines, it’s important that alone accessories can abstract themselves from these aggregate curve aback they are not application them. The accepted way to accord with this is to use a tri-state buffer, such as the accepted 74LS244. Not alone does it accommodate the abreast provided by a accepted agenda absorber circuit, it can additionally about-face to a Hi-Z (high-impedance) state, in which it is finer disconnected.
In the case of our ISA card, we charge to accept commodity like the 74LS244 or its bi-directional affinity 74LS245 to appropriately interface with the bus. Each bus arresting affiliation needs to accept an adapted absorber or latch placed on it, which for the ISA bus is covered in detail in this commodity by Abhishek Dutta. A acceptable archetype of a modern-day ISA agenda is the ‘Snark Barker’ SoundBlaster clone.
PCI could conceivably be done in such a detached abode as well, but best frequently bartering PCI cards acclimated I/O accelerator ASICs, which accommodate a simple, ISA-like interface to the card’s circuitry. These ICs are about far from bargain today (barring demography a accident with commodity like the WCH CH365), so a acceptable another is to apparatus the PCI ambassador in an FPGA. The MCA adaptation of the above ‘Snark Barker’ (as ahead covered by us) uses a CPLD to interface with the MCA bus. Sites like OpenCores affection absolute PCI ambition projects one could use as a starting point.
After creating a agleam PCB with gold bend acquaintance fingers and soldering some bus absorber ICs or an FPGA assimilate it, one still has to be able to absolutely allocution the absolute ISA or PCI protocol. Fortunately, a lot of assets abide for the ISA protocol, such as this one for ISA. The PCI agreement is, like the PCIe protocol, a ‘trade secret’, and alone clearly accessible via the PCI-SIG website for a price. This hasn’t kept copies from the blueprint to aperture over the accomplished decades, however.
It’s absolutely accessible to use absolute ISA and PCI projects as a arrangement or advertence for one’s own projects. The above CPLD/FPGA projects are a way to abstain implementing the agreement oneself and aloof accepting to the acceptable bits. Either way, one has to use the arrest (IRQ) arrangement for the corresponding bus (dedicated arresting lines, as able-bodied as message-based in afterwards PCI versions), with the advantage to use DMA (DRQn & DACKn on ISA). Covering the intricacies of the ISA and PCI bus would about booty a accomplished commodity by itself. For those of us who accept had ISA cards with toggle switches or (worse), ISA PnP (Plug’n’Pray) inflicted on them, a lot of this should already be familiar, however.
As with any aggregate bus, the capital agreement aback autograph or account involves requesting bus admission from the bus master, or triggering the bus adjudication agreement with assorted bus masters in PCI. An amplification agenda can additionally be addressed anon application its bus address, as Abhishek Dutta covered in his ISA article, which on Linux involves application atom routines (sys/io.h) to admission admission permissions afore one can accelerate abstracts to a specific IO anchorage on which the agenda can be addressed. Essentially:
With ISA, the IO abode is set in the card, and the abode decoder on the abode arresting curve acclimated to actuate a match. Generally toggle switches or jumpers were acclimated to acquiesce a specific address, IRQ and DMA line. ISA PnP approved to advance on this process, but finer acquired added trouble. For PCI, PnP is allotment of the standard: the PCI bus is scanned for accessories on boot, and the onboard ROM (BIOS) queried for the card’s needs afterwards which the abode and added ambit are set up automatically.
Obviously, this commodity has almost alike covered the essentials aback it comes to developing one’s own custom ISA or PCI amplification cards, but hopefully it has at atomic accustomed a ample overview of the topic. A lot of what one needs depends on the blazon of agenda one wishes to develop, whether it’s a basal 8-bit ISA (PC/XT) card, or a 64-bit PCI-X one.
A lot of the fun with buses such as ISA and PCI, however, is that they are actual approachable. Their bus speeds are able-bodied aural the ability of hobbyist accouterments and oscilloscopes in case of debugging/analysis. The use of a slower alongside abstracts bus agency that no cogwheel signaling is acclimated which simplifies the acquisition of traces.
Even admitting these bequest buses are not arena in the aforementioned alliance as PCIe, their affection set and accessibility agency that it can accord old systems a new charter on life, alike if it is for commodity as simple as abacus Flash-based accumulator to an aboriginal IBM PC.
[Heading image: Snark Barker ISA SoundBlaster carbon board. Credit: Tube Time]
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