Asynchronous bus - a bus that does not rely on a clock to mark or synchronize bit/byte transfers.

An asynchronous bus relies on additional control/status signals to indicate that a valid value is on the address and/or data buses and that the information has been successfully transmitted and received.

State change occurs when target devices indicate that data successfully transferred. This type of bus is designed for a maximum throughput but actually may run at a much slower throughput often determined by slowest device on bus.

The GPIB (IEEE 488) bus uses this technique.

Configuration :

8-bit address/data bus
8 ground lines
5 interface management (control)
- ATN - asserts address/control data on data lines. (PCI has FRAME)
- EOI - end of identity. Used to indicate last byte of data (end of transmission). Or used to poll devices.
- IFC - interface clear - reset or initialize devices.
- REN - remote enable.
- SRQ - service request - interrupt from a device. Controller then uses EOI and address to pole each device to locate requesting device.
3 handshake lines
- !NRFD - not ready for data - by listeners. Talker must wait until all listeners signal that they are ready for data.
- !NDAC - not data accepted - by listeners. Talker must wait until all listeners signal that they have accepted data.
- !DAV - data valid - by talker. Talker indicates that data is valid.

Using the interface, handshake, and data lines, the initiator (arbiter), usually an actual person manipulating one of the devices interfaced on the device, selects a talker. The talker, in turn, selects a listener. One or the other of these is often also the initiator but does not necessarily have to be.

General procedure for transmitting data :

Start here

The talker begins by signaling that it wishes to transmit a byte of data.
As each listener becomes available to listen, it indicates this.
When all listeners indicate they are listening.
Talker puts data line and indicates that the data is valid.
Each listener indicates that it has seen the data and is accepting it.
Once data read, each listener indicates it has latched the data.
Only when all listeners have successfully latched data, can the talker start preparing the next byte transfer.
If talker has more data to more to transmit, go to 'Start here'

Data transmission speed is set by the slowest responding device. NRZ - used for signals.

What 'control data' could be processed by a device was highly dependent on the logic on the device.

While provided signal support for transmitting command data, there was no standardized definition of commands in early versions.