Commodore bus
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| Type | Serial | ||
|---|---|---|---|
| Designer | Commodore International | ||
| Designed | 1980 | ||
| Manufacturer | Various | ||
| Produced | 1980–present | ||
| Length | 1.8 meters maximum | ||
| Hot pluggable | No | ||
| Daisy chain | Yes, up to 31 devices | ||
| External | Yes | ||
| Signal | Open collector 5 V | ||
| Max. voltage | 5 V | ||
| Max. current | 3.2 mA | ||
| Data signal | Yes | ||
| Bitrate | 3.2 - 41.6 kbit/s | ||
The Commodore serial IEEE-488 bus (IEC Bus), is Commodore's interface for primarily magnetic disk data storage and printers for the Commodore 8-bit home/personal computers, notably the VIC-20, C64, C128, Plus/4,C16 and C65.
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 utilizes a DIN-6 connector.
The bus signals are digital single-ended open collector 5 volt TTL and active when negative. 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 IEEE 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 IEEE listeners are ready to receive they release the Data line to false. If the talker waits more than 200 µs without the Clock line going true (idle state), listeners have to perform End-or-Identify (EOI).
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