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additionally the large custom connector was expensive these drove up costs of the sio system and decuir blames this for sinking the system 1 2 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 3 in common usage sio refers to the entire system from the operating system to the bus and even the physical connectors history edit emphasizing that he was not the only contributor to sio joe decuir attributed its creation to atari s desire for productivity software and peripherals for its atari 8 bit home computers 4 fcc problem edit 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 5 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 5 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 5 ti diversion edit 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 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 5 when decuir explained his concept the salesman s eyes almost popped out 5 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 when decuir later explained the idea to his boss wade tuma tuma replied that no the fcc would never let us get away with that stunt 5 this proved to be true ti used decuir s idea and when they took it to the fcc in 1979 the fcc rejected it out of hand ti had to redesign their system and the resulting delay meant the atari s reached the market first 5 sio edit with this path to allowing card slots stymied decuir returned to the problem of providing expansion through an external system of some sort 5 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 to produce a ttl digital bus the sio system used two of the pokey s four sound channels to produce steady tones that represented clock signals of a given frequency a single byte buffer was used to send and receive data every time the clock signal toggled one bit from the buffer would be read or written when all eight bits were read or written the system generated an interrupt that triggered the operating system to read or write more data 6 unlike a cassette interface where only a single device would normally be used an external expansion port would need to be able to support more than one device to support this a simple protocol was developed and several new pins added to the original simple cassette port most important among these was the command pin which triggered the devices to listen for a 5 byte message that activated one of the devices on the bus and asked it for data or sent it commands 7 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 likewise the timing signals generated by the pokey were sent on the clockout and clockin pins although the asynchronous protocol did not use these 8 legacy edit sio was not patented by atari decuir said that when a patent troll sued the usb implementers forum microsoft and other companies cited sio as prior art 4 description edit hardware edit the sio connector used sprung metal leaves to make a firm connection to the pins in the devices the slots cut into the connector give the leaves room to move the atari 850 provided centronics printer and rs 232 serial ports to connect to 3rd party devices typical of sio devices it has both in and out ports to allow daisy chaining the sio bus was implemented using a custom 13 pin d connector arrangement although not d subminiature with the male connectors on the devices and the female connectors on either end of the cables 9 the connectors were physically robust to allow repeated use with very strong pins in the device socket and sprung connectors in the cables as opposed to friction fit as in a typical d connector most devices had in and out ports to allow daisy chaining peripherals although the atari 410 program recorder had to be placed at the end of the chain and thus did not include an out port communications edit sio was controlled by the atari s pokey chip which included a number of general purpose timers 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 these were re purposed as the basis of the sio system used as clocks in some modes or to produce the output signals directly in others the system included a single shift register that was used to semi automate most data transfers this consisted of a single 8 bit value lsb first 10 that was used to buffer reads and writes the user accesses these through two memory locations known as serout for writing and serin for reading these were shadow registers locations in the ram that mirrored registers in the various support chips like pokey the data bits were framed with a single zero start bit and a single one stop bit and no parity was used 10 to write data in synchronous mode the pokey s main timer channels were set to an appropriate clock rate say 9600 bit s any data written to the serout register was then sent one bit at a time every time the signal went high it was timed so the signal returned low in the middle of the bit when all 10 bits including the start and stop had been sent pokey sent a maskable interrupt to the cpu to indicate it was ready for another byte on reading if another byte of data was received before the serin was read the 3rd bit of the skstat was set to true to indicate the overflow individual bits being read were also sent to the 4th bit of skstat as they arrived allowing direct reading of the data without waiting for the framing to complete although the system had clockout and clockin pins that could in theory be used for synchronous communications in practice only the asynchronous system was used in this case a base speed was set in as above in the pokey which would follow changes of up to 5 from this base rate this made it much easier to work with real devices where mechanical or electrical issues caused the slight variation in the rates over time one example was the cassette deck where tape stretch could alter the speed another is a modem there the remote system may not be exactly clocked to a given speed the system officially supported speeds up to 19 200 bit s but this rate was chosen only because the atari engineer s protocol analyzer topped out at that speed the system was actually capable of much higher performance 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 device control edit the sio system allowed devices to be daisy chained and thus required some way of identifying that information on the various data pins was intended for a specific device on the chain this was accomplished with the command pin 10 the command pin was normally held high 9 and when it was pulled low devices on the bus were required to listen for a command frame this consisted of a 5 byte packet the first byte was the device id the second was a device specific command number and then two auxiliary bytes of data that could be used by the driver for any purpose these four were followed by a checksum byte 7 the command pin went high again when the frame was complete 11 on reception of the packet the device specified in the first byte was expected to reply 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 for commands that exchanged data the command frame would be followed by a data frame from or to the selected device this frame would then be acknowledged by the receiver with a c for complete or e for error 12 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 13 devices were enumerated mechanically typically using small dip switches 14 each class of device was given a different set of 16 potential numbers based on hexadecimal numbers the 30 range for disk drives and 40 for printers for instance however each driver could support as many or as few devices as it wanted the atari 820 printer driver supported only a single printer numbered 40 15 while the disk drivers could support four drives numbered 31 to 34 16 cassette use edit the original 410 was replaced by the much smaller 1010 during the xl era design of what became the sio had started as a system for interfacing to cassette recorders using the pokey chip s sound capabilities to generate the appropriate tones this capability was retained in the production versions allowing the atari 410 and its successors to be relatively simple devices when set to operate the cassette the outputs from channel 1 and 2 of the pokey were sent to the dataout rather than the clock pins the two channels were set to produce tones that were safe to record on the tape 3995 hz for a zero was in pokey channel 2 and 5326 hz for a one was in channel 1 in this mode when the pokey read bits from the serin any 1 s resulted in channel 1 playing into the data pin and 0 s played channel 2 in this fashion a byte of data was converted into tones on the tape reading however used a different system as there was no a to d converter in the computer instead the cassette decks included two narrow band filters tuned to the two frequencies during a read the output of one or the other of these filters would be asserted as the bits were read off the tape these were sent as digital data back to the host computer 17 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 data was written in blocks of 132 bytes per record with the first two bytes being the bit pattern 01010101 01010101 an inter record gap between the blocks with no tones allowed the operating system to know when a new record was starting by looking for the leading zero it then rapidly read the port and timed the transitions of the timing bits from 0 to 1 and back to determine the precise data rate the next byte was a control byte specifying if this was a normal record of 128 data bytes a short block or an end of file up to 128 bytes of data followed itself followed by a checksum byte including everything before the checksum 17 the operation was further controlled by the motor pin in the sio port dedicated to this purpose when this pin was low the motor in the deck was turned off this allowed the user to press play or play and record without the tape beginning to move when the appropriate command was entered on the computer motor would be asserted and the cassette would begin to turn 17 another dedicated pin was the audioin which was connected directly to the sound output circuits between the pokey s d to a converters and the final output so that any signal on the pin mixed with the sound from the pokey if any and was then sent to the television speaker this was connected to the left sound channel in the cassette while the right channel was connected to the data pins this allowed users to record normal sounds on the left channel and then have them play through the television this was often combined with direct motor control to produce interactive language learning tapes and similar programs some software companies would record sounds or music on this channel to make the loading process more enjoyable 18 see also edit special input output references edit interview with joe decuir atari museum atari computer systems atari inc archived from the original on 2015 12 08 retrieved 2015 01 14 sio 1980 p 1 1 2 vcf east 2019 joe decuir atari 800 series computers 40 years vintage computer federation may 6 2019 presentation on 2019 05 05 event occurs at 37 15 retrieved 2026 05 24 via youtube 1 2 3 4 5 6 7 8 decuir sio 1980 pp 6 7 1 2 sio 1980 p 13 sio 1980 p 8 1 2 sio 1980 p 7 1 2 3 sio 1980 p 6 sio 1980 p 18 sio 1980 p 16 atari 810 disk drive operator s manual pdf technical report 1979 p 6 indus gt installation sio 1980 p 25 sio 1980 p 28 1 2 3 dere 1981 p appendix c victor john october 1982 audio while you cload antic bibliography edit atari 400 800 operating system users manual atari 1980 atari 400 800 hardware manual pdf atari 1980 atari home computer system serial input output interface pdf 1980 de re atari atari 198...