Editing Atari interface (section)

====Sega 16-bit====
The Sega 16-bit Sega Mega Drive/Genesis is mostly backwards-compatible with the 8-bit machines, and its controller port is the same, except that the controllers use special protocols over existing pins for more buttons.
Sega 16-bit gamepads draw power on pin 5, using pin 7 instead to select between groups of inputs on the other pins.<ref name="megadrive">Pinouts.ru—[http://pinouts.ru/Game/genesiscontroller_pinout.shtml Sega Genesis Joystick controller pinout]. Retrieved 2014-10-04</ref>
This works with many Atari-compatible hosts that keep pin 5 high at all times, using it as input for paddles or as a button input with a pull-up resistor.

However, unlike the Atari standard which has pull-up resistors on each input line on the host side, Sega 16-bit systems have them in the controller. This means that lines are high when not active, and this could damage some hardware.
For instance, the [[Commodore 64]] and 128 computers reuse the same physical lines for ports and the [[keyboard matrix]], which could lead to excess current into the I/O chip (CIA #1) if a key is pressed while a Sega gamepad is plugged in.
The host could however be protected with a simple adaptor with diodes on the input pins.
<ref name="cw5">Doug Cotton, Mark Fellows: Hard Tips, Building a Sega 'Game Pad' Adapter. [https://www.scribd.com/document/8945979/Commodore-World-Issue-05 Commodore World, Volume 1, Issue 05 (at Scribd)]. Retrieved 2018-05-24.</ref>

Sega 16-bit gamepads should be safe for use on most Amiga computers, where buttons B and C work as Fire/Left mouse and Secondary fire/Right mouse.
However, if a game tries to talk to a Sega 16-bit gamepad as a [[#Amiga CD32 gamepad|Amiga CD32 gamepad]], power would be intermittent and in this case, an adaptor would be needed.
A small number of Amiga games (Hired Guns, Flashback, ADoom...) are able to talk Sega's 3-button or even 6-button protocol but those require a modified gamepad or special adaptor that ''crosses'' pins 5 and 7. Some guides recommend also using diodes and putting a 470ohm resistor in-between pins 5 and 7 for extra protection.{{citation needed}}

The three-button controller for the Sega Mega Drive/Genesis uses a 74157 selector to change between two sets of inputs. Setting the "Select" pin high selects Left/Right/B/C, while setting it low selects GND/GND/A/Start.
Because Left and Right are opposing directions and ''should'' not be active at once, the host should be able to detect the controller as having three buttons.<ref group="footnote">There are third-party Sega controller where the [[D-pad]] lacks a central pivot, thus allowing opposite directional inputs to be active at once...</ref>
The normal state for a Sega host is to have the select pin high, and to pulse it low for a short time during each video frame period when it polls the inputs.
<ref name="megadrive6">Charlie Rosenberg's home page—
[http://www.cs.cmu.edu/~chuck/infopg/segasix.txt Sega Six Button Controller Hardware Info]. Dated 1996-09-09. Retrieved 2017-11-17</ref>

Sega's six-button controller for the Mega Drive/Genesis has a microcontroller instead of a selector chip.
A game supporting a six-button controller pulses the Select line low at least four times per video frame in quick intervals.
During the third pulse, lines 1 through 4 all read low and during the fourth pulse, lines 1 through 4 all read high, but in-between those two pulses they read the values of Z, Y, X and the mode switch respectively.
Within that special period, pins 6 and 9 always read high.
For six-button reporting to kick in, the pulses must be short enough with a long enough interval until the next time. There is also a mode-switch for disabling six-button behaviour in the controller for older games that use different timing for the select-line.
<ref name="md6bpade">Ein Terakawa. 
[http://applause.elfmimi.jp/md6bpad-e.html Interface Protocol of SEGA MegaDrive's 6-Button-Controller]. Retrieved 2017-11-17.
</ref>
<ref name="megadrive6"/>

Other Sega 16-bit peripherals that used the DE-9 ports included a keyboard, keypad, mice, light guns and multiplayer adaptors.
A keyboard was made to be used with Internet multiplayer services from XBAND (US) and Teclado Mega Net (Brazil). It was connected to port #2.<ref name="segaretroxbandkb">Sega Retro—[https://segaretro.org/XB%E2%88%80ND_Keyboard XB∀ND Keyboard]. Version from 2017-07-22 04:32. Retrieved 2018-11-16</ref>. The ports did not have any serial hardware, so the protocol was probably [https://en.wikipedia.org/wiki/Bit_banging "bit-banged"] by the CPU.