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.

Devices that used the SIO interface included printers, floppy disk drives, cassette decks, modems and expansion boxes.

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.

[4] 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.

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.

The success of the Apple II led to the system being repositioned as a home computer, and this market required peripheral devices.

[4] 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.

"[4] Unknown to the Grass Valley team, TI was at that time in the midst of developing the TI-99/4 and was facing the same problem with RF output.

[4] With this path to allowing card slots stymied, Decuir returned to the problem of providing expansion through an external system of some sort.

[4] By this time, considerable work had been carried out on using the Atari's POKEY chip to run a cassette deck by directly outputting sounds that would be recorded to the tape.

It was realized that, with suitable modifications, the POKEY could bypass digital-to-analog conversion hardware and drive TTL output directly.

[6] They also added the PROCEED and INTERRUPT pins which could be used by the devices to set bits in control registers in the host, but these were not used in the deployed system.

Four of these allowed fine control over the timing rates, and were intended to be used for sound output by connecting them to a digital-to-analog converter (D-to-A) and then mixing them into the television signal before entering the RF modulator.

A number of 3rd party devices, especially floppy drives, used custom hardware and drivers to greatly increase the transmission speeds to as much as 72,000 bit/s.

This made it much easier to work with real devices where mechanical or electrical issues caused the slight variation in the rates over time.

This consisted of a single byte containing an ASCII character, "A" for Acknowledge if the packet was properly decoded and the checksum matched, "N" otherwise.

[11] Since every packet of 128 data bytes required another command frame before the next could be sent, throughput was affected by latency issues; the Atari 810 disk drive normally used a 19,200 bit/s speed, but was limited to about 6,000 bit/s as a result of the overhead.

[15] Design of what became the SIO had started as a system for interfacing to cassette recorders using the sound hardware to generate the appropriate tones.

[16] Because the tape was subject to stretching and other mechanical problems that could speed or slow transport across the heads, the system used asynchronous reads and writes.

This was often combined with direct motor control to produce interactive language learning tapes and similar programs.