Editing PDP-8 (section)

==Input/output==
The I/O systems underwent huge changes during the PDP-8 era. Early PDP-8 models use a [[front panel]] interface, a [[punched tape|paper-tape]] reader and a [[Teleprinter|teletype]] printer with an optional paper-tape punch. Over time, I/O systems such as [[DECtape|magnetic tape]], [[RS-232]] and [[current loop]] [[computer terminal|dumb terminals]], [[punched card]] readers, and [[drum memory|fixed-head disks]] were added. Toward the end of the PDP-8 era, [[floppy disk]]s and moving-head [[disk pack|cartridge]] [[hard disk drive|disk drives]] were popular I/O devices. Modern enthusiasts have created standard [[IBM PC compatible|PC]] style [[Integrated Drive Electronics|IDE]] hard disk adapters for real and simulated PDP-8 computers.

Several types of I/O are supported:

*In-[[backplane]] dedicated slots for I/O controllers
*A "Negative" I/O bus (using negative voltage signalling)
*A "Positive" I/O bus (the same architecture using TTL signalling)
*The Omnibus (a backplane of undedicated [[system bus]] slots) introduced in the PDP-8/E. (Details are described in the referenced IEEE article listed below.)

A simplified, inexpensive form of [[direct memory access|DMA]] called "three-cycle data break" is supported; this requires the assistance of the processor. The "data break" method moves some of common logic needed to implement DMA I/O from each I/O device into one common copy of the logic within the processor. "Data break" places the processor in charge of maintaining the DMA address and word count registers. In three successive memory cycles, the processor updates the word count, updates the transfer address, and stores or retrieves the actual I/O data word.

One-cycle data break effectively triples the DMA transfer rate because only the target data needed to be transferred to and from the core memory. However, the I/O devices need more electronic logic to manage their own word count and transfer address registers. By the time the PDP-8/E was introduced, electronic logic had become less expensive and "one-cycle data break" became more popular.