National Semiconductor NS405 and NS455 Terminal Management Processor
The NS455 has firmware in the internal ROM; an input pin allows selection of either this, or code in an external ROM. The NS405 is the same chip, but for use with external ROM only. (The internal ROM is still present, but the contents are unknown.)
The CPU is based on the 8048-series microcontrollers. The chip includes most of what is needed for a terminal: e.g. the CRT controller, a character generator, clock, UART, etc. External level converters for EIA signal levels are required, as is RAM to hold the screen contents, though.