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{{short description|Serial bus of the home computers series of Commodore}}
{{Infobox connector
|name               = Commodore serial [[IEEE-488]]
|type               = Peripheral [[Engineering:Bus (computing)|bus]]
|image              = [[Image:Commodore-64-Back-serial-IEEE488.jpg|300px]]
|logo               = <!--[[Image:]]-->
|caption            =
|designer           = [[Company:Commodore International|Commodore International]]
|design_date        = {{start date and age|1980}}<ref>{{Cite web |url=http://www.commodore.ca/products/vic20/commodore_vic-20.htm |title=Commodore VIC-20 History |access-date=2016-04-02 |archive-url=https://web.archive.org/web/20150214045926/http://commodore.ca/products/vic20/commodore_vic-20.htm |archive-date=2015-02-14 |url-status=dead }}</ref><!--first present on VIC-20-->
|manufacturer       = Various
|production_date    = 1980–present
|superseded         =
|superseded_by      =
|superseded_by_date =
|weight             =
|length             = 1.8 meters maximum<ref name=schxe>{{Cite web|access-date=2016-04-02 |title=Building the XE1541 serial cable |url=https://ist.uwaterloo.ca/~schepers/xe1541.html |quote=(1) 6' or 1.8 meters (max) 4 conductor shielded cable (for serial connection) }}</ref>
|diameter           =
|width              = 
|height             =
|hotplug            = No
|daisy_chain        = Yes, up to 31 devices<ref name=cb64/>
|external           = Yes
|electrical         = [[Engineering:Open collector|Open collector]] 5&nbsp;V
|earth              =
|maximum_voltage    = {{nowrap|5 V}}
|maximum_current    = {{nowrap|3.2 mA}}<ref name=ds6526>{{Cite web |access-date=2016-04-02 |publication-date=2016-03-04 |title=6526 complex interface adapter (CIA) |url=http://archive.6502.org/datasheets/mos_6526_cia.pdf |quote=Output Low Current (Sinking); VOL <  .4 v (PA0-PA7, PC, PB0-PB7, DB0-DB7) IOL 3.2 mA |archive-url=https://web.archive.org/web/20160410073735/http://archive.6502.org/datasheets/mos_6526_cia.pdf |archive-date=2016-04-10 |url-status=dead }}</ref>
|audio_signal       =
|video_signal       =
|data_signal        = Yes
|data_bit_width     =
|data_bandwidth     = {{nowrap|3.2–41.6 [[Kilobit|kbit]]/s}}<ref name=ieee85/><ref name=zm71p01/>
|data_devices       =
|data_style         = Serial
|cable              =
|physical_connector = [[Engineering:DIN connector|DIN connector]]
|num_pins           = 6
|pin1=Service Request
|pin1_name=SRQ
|pin2=Ground
|pin2_name=GND
|pin3=Attention
|pin3_name=ATN
|pin4=Clock
|pin4_name=CLK
|pin5=Data
|pin5_name=DATA
|pin6=Reset
|pin6_name=RESET
|pinout_image       = [[File:Commodore serial bus DIN socket pinout.png]]
|pinout_caption     = Female socket from the front.
}}

The '''Commodore serial IEEE-488 bus''' ('''IEC Bus'''), is [[Company:Commodore International|Commodore]]'s interface for primarily [[Computer data storage|magnetic disk data storage]] and [[Engineering:Printer (computing)|printer]]s for Commodore [[8-bit|8-bit]] [[Engineering:Home computer|home computer]]s: the VIC-20, [[Engineering:Commodore 64|Commodore 64]], Commodore 128, Plus/4,<ref>{{cite web|title=Using Commodore 1541 disk drive on a Commodore + 4 |date=2005-10-03 |access-date=2016-05-15 |url=https://plus4world.powweb.com/forum/9314 |quote=The 1541 works with the Plus/4 just fine.}}</ref> Commodore 16, and [[Engineering:Commodore 65|Commodore 65]].

== Description and history ==
The parallel [[IEEE-488]] interface used on the Commodore PET (1977) computer line was too costly, so a cost reduced version was developed, which consisted of a stripped down, serial version of the IEEE-488 interface, with only a few signals remaining; however, the general protocol layout was kept. Commodore began using this bus with the VIC-20 (1980). Connection to the computer uses a [[Engineering:DIN connector|DIN-6 connector]] (DIN 45322).

== Transfer speed ==
{|class=wikitable
! Setup !! Speed !! Effective bitrate
|-
| [[Engineering:Commodore 64|Commodore 64]] + 1541 || 400 bytes/s || 3 200 bit/s
|-
| [[Engineering:Commodore 64|Commodore 64]] + 1541 with fast loader || {{nowrap|2560 bytes/s<ref name=ieee85>{{Cite web|access-date=2016-04-02 |date=1985 |publication-date=2012-04-16 |title=Design case history: the Commodore 64 |url=https://spectrum.ieee.org/ns/pdfs/commodore64_mar1985.pdf |quote=Programming tricks used by [[Company:Epyx|Epyx]] have yielded transfer rates of up to 2.5 kilobytes per second. }}</ref>}} || 20 480 bit/s
|-
| Commodore 128 + 1571 || {{nowrap|5200 bytes/s<ref name=zm71p01>{{Cite web|title=Commodore 1571 disk drive specifications |access-date=2016-04-27 |publication-date=October 1986 |publisher=Commodore Business Machines, Inc. |url=ftp://www.zimmers.net/pub/cbm/schematics/drives/new/1571/manual/01.html}}</ref>}} || 41 600 bit/s
|-
| Theoretical 20&nbsp;µs<ref name=cb64/> || {{nowrap|6250 bytes/s}} || {{nowrap|50 000 bit/s}}
|}

== Interface ==
[[Image:Commodore-64-Back-serial-IEEE488 pinout.jpg|300px|thumb]]

{| class="wikitable"
|+ Pinout<ref name=rupin>{{Cite web|access-date=2016-04-02 |date=2013-12-16 |title=C64 Serial I/O pinout and wiring @ old.pinouts.ru |url=http://old.pinouts.ru/SerialPorts/SerialIOC64_pinout.shtml }}</ref>
|-
! Pin || Name || Function
|-
| 1  || /SRQIN || Service request input to let peripherals request being served by the host (C64).  Used by the C128 for fast transfers.<ref name=sch>{{Cite web|access-date=2016-04-02 |date=2005-12-10 |title=C-64 Workshop / Pin Connections |url=https://ist.uwaterloo.ca/~schepers/MJK/serialbus.html }}</ref>
|-
| 2 || GND    || Ground<ref name=rupin/>
|-
| 3 || /ATN   || Serial ATN In/out. Set low by the host (C64) to indicate the beginning of a serial data transfer.<ref name=sch/>
|-
| 4 || /CLK   || Serial CLK In/out. Used for software handshaking.<ref name=sch/>
|-
| 5 || /DATA  || Serial DATA In/out. Data bit transfer.<ref name=sch/>
|-
| 6 || /RESET || Resets peripherals and also resets an older C64.<ref name=sch/>
|}

== Protocol description ==
The bus signals are digital [[Engineering:Single-ended signaling|single-ended]] [[Engineering:Open collector|open collector]] {{nowrap|5 volt}} [[Transistor–transistor logic|TTL]] and active when low. Bus devices have to provide their own power.

Because the bus lines are electrically open collector it works like a long [[OR gate]] between all device line drivers. The logical value for ground is true and vice versa. Any device may set a line "true". A line only becomes "false" if all devices signal false.

Transmission begins with the bus '''talker''' holding the Clock line true, and the listener(s) holding the Data line true. To begin the talker releases the Clock line to false. When all bus '''listeners''' are ready to receive they release the Data line to false.  If the talker waits more than 200&nbsp;µs without the Clock line going true (idle state), listeners have to perform End-or-Identify (EOI).<ref name=cb64>{{cite web|title=How the VIC/64 Serial Bus works |date=2015-04-17 |access-date=2016-04-28 |url=https://codebase64.org/doku.php?id=base:how_the_vic_64_serial_bus_works}}</ref>

If the Data line being false (released) isn't acknowledged by the talker within 200&nbsp;µs, the listener knows that the talker is in the process of  EOI that means "this character will be the last one."  When the listener detects the 200&nbsp;µs timeout, it must acknowledge this by pulling the Data line true for at least 60&nbsp;µs, and then release it.  The talker can revert to transmitting again within 60&nbsp;µs by pulling the Clock line true.<ref name=cb64/>

Data is eight bits starting with the least significant bit. The Data line is set according to the bit to send (1=true=ground). Once the Data line is set, the Clock line is released to false. The Clock and Data lines will be held steady for at least 20&nbsp;µs (except for Commodore 64 that needs 60&nbsp;µs). After 8 bits has been sent, the talker releases the Data line to false and the listener then acknowledge the talker by pulling the Data line true within 1000&nbsp;µs. After this the talker sets the Clock line true and listener sets the Data line true thus back where the transmission begun. If an EOI is signaled by holding the Clock line false the transmission is ended and the listener acknowledge this by pulling the Data line true for 200&nbsp;µs.<ref name=cb64/>

The ATN line is set to true and bytes are sent like above to all devices, but the byte is interpreted as one of the commands "Talk," "Listen," "Untalk," and "Unlisten". That tell a specific device to become a talker or listener.  Only devices with matching device numbers switch into talk and listen mode. A secondary address may also follow.<ref name=cb64/>

On higher logical level the host will set the ATN line to true and transmit the bytes "Device number 8, listen", "Secondary address 2, open".  Next it will set the ATN line false and the host then becomes the talker, holding the Clock line true. The device will be the listener, holding the Data line true.  The host will transmit the specific open command and end it with an EOI signal sequence.  After this the host will set with ATN line true, "Device number 8, unlisten". Followed up by ATN line true and "Device number 8, listen", "Secondary address 2, data". Then the host sets the ATN line false and sends the data.  When the host has finished sending data the ATN line is set to true and "Device number 8, unlisten" is sent.<ref name=cb64/>

When it is necessary to switch roles and make the host a listener and the device a talker the occurs after a talk command has been sent to the device. The host sets the Data line true and releases the Clock line to false.  The device waits for the Clock line to go false and then pulls it to true and release the Data line to false. After this sequence the standard talk-listener interaction may follow.<ref name=cb64/>

{| class="wikitable"
|+ Bus commands<ref name=cb64/>
! Code !! Meaning
|-
| device &#124; 0x20  || Listen, device (0–30)
|-
| 0x3F 	              || Unlisten, all devices
|-
| device &#124; 0x40  || Talk, device
|-
| 0x5F                || Untalk, all devices
|-
| channel &#124; 0x60 || Reopen, channel (0–15)
|-
| channel &#124; 0xE0 || Close, channel
|-
| channel &#124; 0xF0 || Open, channel
|}

To read a normal file from the floppy device number 8 the command <code>LOAD "filename",8,1</code> is issued on a [[Engineering:Commodore 64|Commodore 64]]. That causes the following high level communication to take place:
{| class="wikitable"
|+ High level protocol<ref name=zsb/>
! Command !! Destination !! Meaning
|-
| /28 || Device  || Listen, device number 8
|-
| /F0 || Device  || Open channel 0
|-
|     || Device  || Send filename bytes
|-
| /3F || Devices || Unlisten all devices
|-
| /48 || Device  || Talk, Device number 8
|-
| /60 || Device  || Reopen channel 0
|-
|colspan=3| Device number 8 becomes the master of the bus
|-
|     || Host    ||  Receive byte data
|-
|colspan=3| The host becomes the master of the bus (normal operation)
|-
| /5F || Devices || Untalk all devices
|-
| /28 || Device  || Listen, device number 8
|-
| /E0 || Device  || Close channel 0
|-
| /3F || Devices || Unlisten all devices
|}

The Commodore 1541 floppy drive uses a slower Commodore 64 compatible mode which can be deactivated for faster speed by using the command <code>OPEN 15,8,15,"UI-":CLOSE 15</code>.<ref>{{cite web|title=VIC-1541 |website=C64-wiki.com
|date=2016-03-11
|access-date=2016-04-28
|url=https://www.c64-wiki.com/index.php?title=VIC-1541&oldid=25254}}</ref>

== Device numbering ==
{| class="wikitable"
|+ Device numbering per [[Engineering:Commodore 64|Commodore 64]] [[Software:KERNAL|KERNAL]] ROM<ref name=zsb>{{cite web
|title=IEC-bus documentation as used for the 1541-III IEC disected
|date=2008-02-24
|access-date=2016-04-28
|url=https://www.zimmers.net/anonftp/pub/cbm/programming/serial-bus.pdf
|url-status=dead
|archive-url=https://web.archive.org/web/20170116173410/https://www.zimmers.net/anonftp/pub/cbm/programming/serial-bus.pdf
|archive-date=2017-01-16
}}</ref>
! Device !! Type
|-
| 0     || Keyboard
|-
| 1     || Cassette port 
|-
| 2     || [[RS-232]] on the user port or second cassette on PETs
|-
| 3     || Screen 
|-
| 4–5   || Printer
|-
| 6     || Typically plotter device
|-
| 7     || Second plotter?
|-
| 8–15  || Disk (10{{snd}} used by some serial-to-parallel printer interfaces)
|-
| 16–30 || Unknown 
|-
| 31    || Reserved as a command to all devices 
|}
Device number 0–3 are not associated with the Commodore bus.<ref name=zsb/>

== Host implementation ==
The VIC-20 computer and the Commodore 1540 and 1541 floppy drives use the MOS Technology 6522 VIA to handle IEC Bus transmissions. The Commodore 64 and 128 computers and the Commodore 1571 drive use the [[Engineering:MOS Technology CIA|Complex Interface Adapter]].

== Common devices ==
{|class=wikitable
! Device !! Info
|-
| Commodore 1541 || 5¼-inch 170&nbsp;KB floppy
|-
| Commodore 1570 || 5¼-inch 170&nbsp;KB floppy
|-
| Commodore 1571 || 5¼-inch 350–410&nbsp;KB floppy
|-
| Commodore 1581 || 3½-inch 800&nbsp;KB floppy
|-
| MSD SD-1/SD-2 || 5¼-inch single/dual 170&nbsp;KB floppy
|-
| Commodore MPS 801  || [[Dot matrix|Dot-matrix]] printer
|-
| Commodore 1515<ref>{{cite web|title=VIC-1515 User's manual Graphic printer by Commodore
|date=April 1981
|publication-date=2011-11-05
|access-date=2016-04-28
|url=http://www.classiccmp.org/cini/pdf/Commodore/VIC-1515%20Printer%20User's%20Manual.pdf}}</ref>          || [[Dot matrix|Dot-matrix]] printer
|-
| Commodore 1520          || [[Engineering:Ballpoint pen|Ballpoint pen]] printer
|-
| Commodore VIC-1525<ref>{{cite web|title=VIC-1525 Graphics Printer User Manual|url=http://www.classiccmp.org/cini/pdf/Commodore/VIC-1525%20Printer%20User%27s%20Manual.pdf|publisher=Commodore Computer|access-date=22 February 2015}}</ref>  || Dot-matrix printer
|-
| Commodore VIC-1526      || Rebranded MPS 802, dot-matrix printer
|-
| Okimate 10          || Dot-matrix printer
|-
| Commodore DPS-1101<ref>{{cite web|title=Commodore DPS-1101 Daisy Wheel Printer
|url=https://www.recycledgoods.com/commodore-dps-1101-daisy-wheel-printer.html}}</ref> || Daisy wheel printer
|-
| INTERPOD                || Standalone [[IEEE-488]] + [[RS-232]]<ref>{{cite web|title=Interpod IEEE-488 Interface
|date=April 2008<!--html code-->
|access-date=2016-04-28
|url=http://mikenaberezny.com/hardware/c64-128/interpod-ieee-488-interface/}}</ref>
|-
| VIC-20    || 1&nbsp;MHz 5&nbsp;KB computer
|-
| [[Engineering:Commodore 64|Commodore 64]]        || 1&nbsp;MHz 64&nbsp;KB computer
|-
| [[Engineering:Commodore SX-64|Commodore SX-64]]     || 1&nbsp;MHz 64&nbsp;KB computer
|-
| Commodore 128       || 2&nbsp;MHz 128&nbsp;KB computer
|-
| Commodore 16        || 1-2&nbsp;MHz 16&nbsp;KB computer
|-
| Commodore Plus/4    || 1.76&nbsp;MHz 64&nbsp;KB computer
|-
| [[Engineering:Commodore 65|Commodore 65]]        || 3.54&nbsp;MHz 128&nbsp;KB computer
|}

== Devices ==
<gallery>
File:Commodore-64-1541-Floppy-Drive-01.jpg|Commodore 1541 disk drive
File:VC1571.JPG|VC 1571 floppy drive
File:Commodore 1581 01.jpg|Commodore 1581 disk drive
File:Commodore Matrixdrucker MPS-802 (weißen hintergrund).jpg|Commodore MPS 802 printer
</gallery>

== See also ==
* [[Software:Commodore DOS|Commodore DOS]]
* Commodore 64 peripherals
* List of device bit rates
* Commodore 1541
* Fast loader
* [[Physics:Magnetic tape data storage|Magnetic tape data storage]]
* [[IEEE-488]], the original parallel version (originally HP-IB)
* [[HP-IL]], another serialisation of IEEE-488

== References ==
{{reflist}}

== External links ==
* {{cite web |url=https://www.lemon64.com/forum/viewtopic.php?t=8348 |website=lemon64.com |title=Saving with 64HDD / XE1541 cable length...}}
* {{cite web |url=https://spectrum.ieee.org/ns/pdfs/commodore64_mar1985.pdf |title=Design case history: the Commodore 64 |work=IEEE Spectrum |date=March 1985}}
* {{cite web |url=https://ist.uwaterloo.ca/~schepers/MJK/serialbus.html |website=uwaterloo.ca |title=Serial Bus signal description}}
* {{cite web |url=https://www.zimmers.net/anonftp/pub/cbm/programming/serial-bus.pdf |date=2008-02-24 |archive-date=2017-01-16 |archive-url=https://web.archive.org/web/20170116173410/https://www.zimmers.net/anonftp/pub/cbm/programming/serial-bus.pdf |website=zimmers.net |title=IEC disected}}{{snd}} IEC-bus documentation as used for the 1541-III IEC dissected

[[Category:Computer buses]]
[[Category:Computer connectors]]
[[Category:Computer storage buses]]
[[Category:IEEE standards]]
[[Category:Personal area networks]]
[[Category:Serial buses]]

{{Computer bus}}

{{Sourceattribution|Commodore bus}}