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Atari SIO
The Serial Input/Output system, universally known as SIO, was a proprietary peripheral bus and related software protocol stacks used on the Atari 8-bit computers to provide most input/output duties for those computers. Unlike most I/O systems of the era, such as RS-232, SIO included a lightweight protocol that allowed multiple devices to be attached to a single daisy-chained port that supported dozens of devices. It also supported plug-and-play operations. SIO's designer, Joe Decuir, credits his work on the system as the basis of USB.
SIO was developed in order to allow expansion without using internal card slots as in the Apple II, due to problems with the FCC over radio interference. This required it to be fairly flexible in terms of device support. Devices that used the SIO interface included printers, floppy disk drives, cassette decks, modems and expansion boxes. Some devices had ROM based drivers that were copied to the host computer when booted allowing new devices to be supported without native support built into the computer itself.
SIO required logic in the peripherals to support the protocols, and in some cases a significant amount of processing power was required - the Atari 810 floppy disk drive included a MOS Technology 6507 for instance. Additionally, the large custom connector was expensive. These drove up costs of the SIO system, and Decuir blames this for "sinking the system". There were unsuccessful efforts to lower the cost of the system during the 8-bits' history.
The name "SIO" properly refers only to the sections of the operating system that handled the data exchange, in Atari documentation the bus itself is simply the "serial bus" or "interface bus", although this is also sometimes referred to as SIO. In common usage, SIO refers to the entire system from the operating system to the bus and even the physical connectors.
The SIO system ultimately owes its existence to the FCC's rules on the allowable amount of RF interference that could leak from any device that directly generated analog television signals. These rules demanded very low amounts of leakage and had to pass an extensive testing suite. These rules were undergoing revisions during the period when Atari's Grass Valley group was designing the Colleen machine that would become the Atari 800.
The Apple II, one of the few pre-built machines that connected to a television in that era, had avoided this problem by not including the RF modulator in the computer. Instead, Apple arranged a deal with a local electronics company, M&R Enterprises, to sell plug-in modulators under the name Sup'R'Mod. This meant the Apple did not, technically, generate television signals and did not have to undergo FCC testing. One of Atari's major vendors, Sears, felt this was not a suitable solution for their off-the-shelf sales, so to meet the interference requirements they encased the entire system in a cast-aluminum block 2 mm thick.
Colleen was originally intended to be a game console, the successor to the Atari 2600. The success of the Apple II led to the system being repositioned as a home computer, and this market required peripheral devices. On machines like the Apple II, peripherals were supported by placing an adapter card in one of the machine's internal card slots, running a cable through a hole in the case, and connecting the device to that cable. A hole large enough for such a cable would mean Colleen would fail the RF tests, which presented a serious problem. Additionally, convection cooling the cards would be very difficult.
During a visit in early 1978, a Texas Instruments (TI) salesman demonstrated a system consisting of a fibre optic cable with transceivers molded into both ends. Joe Decuir suggested they could use this to send the video signal to an external RF modulator, which would be as simple to use as the coaxial cable one needed to run the signal to the television anyway. Now the computer could have normal slots; like the Apple II, the RF portion would be entirely external and could be tested on its own separately from the computer.
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Atari SIO AI simulator
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Atari SIO
The Serial Input/Output system, universally known as SIO, was a proprietary peripheral bus and related software protocol stacks used on the Atari 8-bit computers to provide most input/output duties for those computers. Unlike most I/O systems of the era, such as RS-232, SIO included a lightweight protocol that allowed multiple devices to be attached to a single daisy-chained port that supported dozens of devices. It also supported plug-and-play operations. SIO's designer, Joe Decuir, credits his work on the system as the basis of USB.
SIO was developed in order to allow expansion without using internal card slots as in the Apple II, due to problems with the FCC over radio interference. This required it to be fairly flexible in terms of device support. Devices that used the SIO interface included printers, floppy disk drives, cassette decks, modems and expansion boxes. Some devices had ROM based drivers that were copied to the host computer when booted allowing new devices to be supported without native support built into the computer itself.
SIO required logic in the peripherals to support the protocols, and in some cases a significant amount of processing power was required - the Atari 810 floppy disk drive included a MOS Technology 6507 for instance. Additionally, the large custom connector was expensive. These drove up costs of the SIO system, and Decuir blames this for "sinking the system". There were unsuccessful efforts to lower the cost of the system during the 8-bits' history.
The name "SIO" properly refers only to the sections of the operating system that handled the data exchange, in Atari documentation the bus itself is simply the "serial bus" or "interface bus", although this is also sometimes referred to as SIO. In common usage, SIO refers to the entire system from the operating system to the bus and even the physical connectors.
The SIO system ultimately owes its existence to the FCC's rules on the allowable amount of RF interference that could leak from any device that directly generated analog television signals. These rules demanded very low amounts of leakage and had to pass an extensive testing suite. These rules were undergoing revisions during the period when Atari's Grass Valley group was designing the Colleen machine that would become the Atari 800.
The Apple II, one of the few pre-built machines that connected to a television in that era, had avoided this problem by not including the RF modulator in the computer. Instead, Apple arranged a deal with a local electronics company, M&R Enterprises, to sell plug-in modulators under the name Sup'R'Mod. This meant the Apple did not, technically, generate television signals and did not have to undergo FCC testing. One of Atari's major vendors, Sears, felt this was not a suitable solution for their off-the-shelf sales, so to meet the interference requirements they encased the entire system in a cast-aluminum block 2 mm thick.
Colleen was originally intended to be a game console, the successor to the Atari 2600. The success of the Apple II led to the system being repositioned as a home computer, and this market required peripheral devices. On machines like the Apple II, peripherals were supported by placing an adapter card in one of the machine's internal card slots, running a cable through a hole in the case, and connecting the device to that cable. A hole large enough for such a cable would mean Colleen would fail the RF tests, which presented a serious problem. Additionally, convection cooling the cards would be very difficult.
During a visit in early 1978, a Texas Instruments (TI) salesman demonstrated a system consisting of a fibre optic cable with transceivers molded into both ends. Joe Decuir suggested they could use this to send the video signal to an external RF modulator, which would be as simple to use as the coaxial cable one needed to run the signal to the television anyway. Now the computer could have normal slots; like the Apple II, the RF portion would be entirely external and could be tested on its own separately from the computer.