How is SDLC different from async communications?
The Synchronous Data Link Control, SDLC, protocol often used by 3770 SNA/RJE terminals is a specific form of synchronous communications. Although both synchronous and async (asynchronous) communications are used to send data over modems, the similarity stops there. Asynchronous communications sends and received each data byte individually; synchronous communications establishes a timing pattern between the transmitter and receiver and then data streams from one to the other without the overhead of start and stop bits — and usually without a parity bit — which are required by, and add overhead to, asynchronous communications. Almost all computers sold today have asynchronous communications capability. They are called by different names: COM ports, serial ports, and TTY ports. Almost no computers have built-in synchronous ports — that’s why the purchase of a synchronous communications product, like a 3780 emulator, almost always involves acquiring both hardware and software. Synchrono
