Zilog SCC

The SCC, short for Serial Communication Controller, is a family of serial port driver integrated circuits made by Zilog. The primary members of the family are the Z8030/Z8530, and the Z85233. Developed from the earlier Zilog SIO devices (Z8443), the SCC added a number of serial-to-parallel modes that allowed internal implementation of a variety of data link layer protocols like Bisync, HDLC and SDLC. The SCC could be set up as a conventional RS-232 port for driving legacy systems, or alternately as a RS-422 port for much higher performance, up to 10 Mbit/s. Implementation details generally limited performance to 5 Mbit/s or less.

One of the most famous users of the SCC was the Apple Macintosh computer line, which used the SCC to implement two serial ports on the back of the early designs, labeled "modem" and "printer". AppleTalk was developed to use the RS-422 mode to produce a low-cost medium-performance local area network system called LocalTalk, running at 230.4 kbit/s. AppleTalk evolved from earlier work that intended to produce a peripheral bus similar to Universal Serial Bus, which is why the relatively expensive SCC was selected for the Mac.

Traditional serial communications are normally implemented using a device known as a UART, which translates data from the computer bus's internal parallel format to serial and back. This allows the computer to send data serially simply by placing data in memory in its own internal format, say 16-bit little-endian, and the UART will convert this to serial form and send it. Generally there were different UARTs for each computer architecture, with the goal of being as low-cost as possible. A good example is the Zilog Z-80 SIO from 1977, designed to work with the widely used Zilog Z-80 to provide two serial ports with relatively high speeds up to 800 kbit/s.

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