USRobotics V.34 user manual Transmit Data Flow Control

COURIER HIGH SPEED MODEMS

Transmit Data Buffer Sizes

The Transmit Data refers to the data from the computer, which the modem is to transmit over the phone line.

The size of the Transmit data buffer depends on whether the connection is under error control or not, as follows.

•ARQ connections: 3.25K bytes.

•Non-ARQ connections: 1.5K bytes, allowing use of error control file transfer protocols such as Xmodem and Ymodem without flow control.

If bit 3 of Register S15 is turned on, the non-ARQ buffer size is reduced to 128 bytes, for the convenience of BBS operators taking calls from remote users of slower modems. See S-Register Summary, S15, in Appendix B.

Received Data Buffer Size

Received Data refers to the data the modem receives over the phone link, which the modem passes on to the attached computer.

The size of this buffer remains constant at 2K bytes.

Transmit Data Flow Control

&Hn This type of flow control is for data transmitted to the modem by its attached computer or terminal. The modem monitors its buffer as data comes from the computer or modem. If the buffer approaches 90% capacity, the modem signals the computer or terminal to stop transmitting. When the modem has sent enough data over the link to half empty the buffer, it signals the computer or terminal to resume transmitting.

&H0	Transmit Data flow control disabled.
&H1	Hardware flow control. Default. Requires that your
	computer or terminal and software support Clear to Send
	(CTS) at the RS-232 interface.
&H2	Software flow control. Requires that your software
	support XON/XOFF signaling.

4-26 Data Mode Operations