RS-485: Imported from Wikipedia (overwrite)

2026-05-03T12:23:38Z

Imported from Wikipedia (overwrite)

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The '''6551''' '''Asynchronous Communications Interface Adapter''' ('''ACIA''') is an [[integrated circuit]] made by [[MOS Technology]].<ref>{{Cite web |title=MOS Technology 6551 {{!}} Semantic Scholar |url=https://www.semanticscholar.org/topic/MOS-Technology-6551/1425152 |access-date=2025-03-15 |website=www.semanticscholar.org |language=en}}</ref> It served as a companion [[Universal asynchronous receiver-transmitter|UART]] chip for the widely popular [[MOS Technology 6502|6502]] [[microprocessor]]. Intended to implement [[RS-232]], its specifications called for a maximum speed of 19,200 [[bits per second]] with its onboard baud-rate generator, or 125 kbit/s using an external 16x clock. The 6551 was used in several computers of the 1970s and 1980s, including the [[Commodore PET]] and [[Commodore Plus/4|Plus/4]]. It was also used by [[Apple Computer]] on the [[Apple II serial cards|Apple II Super Serial Card]], and by [[RadioShack|Radio Shack]] on the Deluxe RS-232 Program Pak for their [[TRS-80 Color Computer]].

Several companies, including Dr. Evil Labs and [[Creative Micro Designs]], marketed an add-on [[ROM cartridge|cartridge]] containing a 6551 and an industry-standard RS-232 port to allow the C64 and 128 to use high-speed modems from companies such as [[U.S. Robotics]] and [[Hayes Communications]]. The Dr. Evil and CMD cartridges pushed the 6551 to 38,400 baud and, with a faster-still clock crystal, some end users reported getting 115,200 bit/s from the 6551. The ADTPro file transfer program disables the baud rate generator in the 6551, allowing 115,200 bit/s transfers with an unmodified clock crystal.

== Variants ==
The [[Rockwell Semiconductor|Rockwell]] 65C52 combines two [[CMOS]] 6551s on a chip.

== Similar chips ==
The [[Motorola 6800#Peripherals|Motorola 6850]] is a similar chip to the MOS Technology 6551, but without an onboard bit rate generator. The 6850 is often used for [[MIDI]].

The [[WDC 65C51|Western Design Center WDC 65C51]] is designed as a drop in replacement for the original MOS 6551, electrically, physically and programming- compatible with most 6551 and 6850 derivatives from most other suppliers. The WDC 65C51 has errata, in which the transmitter “ready” bit is “stuck” in the ready state.

==References==
{{Reflist}}

==External links==
* [http://archive.6502.org/datasheets/mos_6551_acia.pdf Datasheet]
{{MOS Interface}}

[[Category:MOS Technology integrated circuits]]
[[Category:Input/output integrated circuits]]

MOS Technology 6551 - Revision history

RS-485: Imported from Wikipedia (overwrite)