DEC STD 111

This document, DEC STD 111 REV. A, titled "DEC STANDARD FOR TERMINAL SYNCHRONIZATION," defines a mechanism for software to interface with terminal devices, serving as both a hardware and software standard.

The primary purpose of the standard is to manage data flow between computers and terminals to prevent issues such as:

1. Unreadable displays at high baud rates due to rapid scrolling.
2. Overwhelming slower peripheral devices (like hard copy printers).
3. Allowing time for terminal-specific functions (e.g., screen erase) to complete without interruption by new data.

The core of the synchronization mechanism relies on two control characters:

• DC3 (formerly XOFF): Transmitted by either the terminal or the host to request the receiving party to temporarily cease sending data.
• DC1 (formerly XON): Transmitted by either the terminal or the host to signal that the receiving party is ready to resume sending data.

Specific flow control applications include:

• Terminal to Host: A terminal sends DC3 to tell the host to stop sending data (e.g., because its internal buffer, called a "silo," is full or a user needs to pause input). When ready, the terminal sends DC1.
• Host to Terminal: The host sends DC3 to tell the terminal to stop transmitting data (e.g., to prevent host buffer overflow). When ready, the host sends DC1.

The standard applies to Digital's CRT (Cathode Ray Tube) terminals (like the VT50 and future designs) and requires conformance from Digital's operating systems with central terminal support. It operates in full-duplex mode and is designed to be compatible with existing Teletype synchronization specifications. Additionally, DC2 and DC4 characters are reserved for future synchronization uses.