> 1.10 | Serial Interface Flow Control

> Flow control is essential to prevent loss of data or avoid errors when, in a data or fax call, the sending device is transferring data faster than the receiving side is ready to accept. When the receiving buffer reaches its capacity, the receiving device should be capable to cause the sending device to pause until it catches up.
> There are basically two approaches to regulate data flow: Software flow control and hardware flow control. The High Watermark (HWM) of the input/output buffer should be set to approximately 60% of the total buffer size, the Low Watermark (LWM) is recommended to be about 30%. The data flow should be stopped when the buffer capacity rises close to the High Watermark and resumed when it drops below the Low Watermark. The time required to cause stop and go results in a hysteresis between the High and Low Watermarks.