The Intel MD8251A and MD8251B represent a pivotal series of Universal Asynchronous Receiver/Transmitter (UART) integrated circuits that were instrumental in shaping early digital communications. As a Programmable Communications Interface (PCI) chip, its primary function was to serve as a critical intermediary, converting parallel data from a computer's CPU into a serial stream for transmission, and vice-versa for incoming data. This capability made it a cornerstone technology for interfacing microprocessors like the Intel 8080/8085 with modems, terminals, and other peripheral devices.
Architecturally, the 8251 is a complex peripheral that is entirely software-configured. Upon initialization, the host processor loads a set of control words into the chip's internal registers. This setup defines the core operational parameters, most notably the asynchronous communication format, including data bits (5-8), stop bits (1, 1.5, 2), and parity (even, odd, or none). This flexibility allowed it to interface with a vast array of serial equipment with different protocol requirements. The chip supports standard baud rates up to 19.2K for the A-version and 64K for the higher-performance B-version, which was a significant speed for its era.
The internal structure of the 8251 can be divided into several key functional blocks: the Read/Write Control Logic, the Transmitter section, and the Receiver section. The Read/Write Control Logic manages the chip's interaction with the system data and control buses, decoding commands from the CPU. The Transmitter section accepts parallel data, automatically formats it with the configured start, parity, and stop bits, and outputs it serially via the `TxD` pin. A key feature here is its transmit buffer register, which allows for double-buffering, enabling the CPU to write the next byte to transmit while the current one is still being shifted out, improving overall efficiency.
Conversely, the Receiver section continuously monitors the `RxD` line for a start bit. Upon detecting one, it samples the incoming serial stream, strips away the framing bits, checks for parity errors, and converts the valid data into a parallel byte for the CPU to read. It also features double-buffering on reception, holding a fully received byte in its buffer while the next character is being assembled in the shift register. The chip provides crucial status information through its status register, flagging critical conditions such as parity error, framing error, and overrun error, which are essential for robust communication software.
Despite its advanced features for the time, the 8251 had a reputation for a somewhat complex and notoriously tricky initialization sequence. A specific "dummy" reset procedure was required to ensure the chip was in a known state before programming, a step that was often a source of frustration for engineers and hobbyists alike. While long since superseded by more advanced multi-channel UARTs (like the 16550) and integrated microcontroller peripherals, the Intel 8251 remains a historically significant device that provided a fundamental building block for the connected world.
ICGOODFIND: The Intel MD8251A/B was a foundational and highly configurable UART that standardized serial communications for early microprocessor systems, despite its complex initialization routine.
Keywords: UART, Serial Communication, Programmable Peripheral Interface, Asynchronous Data, Intel 8251
