Multi-Tech Systems MT5634SMI V.34, MT5634SMI V.92 Interface Configuration Commands, At+Vbt=?, 119

Chapter 4 – Voice Commands

Interface Configuration Commands

The commands in this section are used to define the interface between the PC and the modem.

Command:	+VBT=<deassert>,<assert> Set Modem Flow Control Assert and Deassert Points
Values:	<assert> and <deassert> are buffer offsets from the start of the buffer. The buffer's first
	position is 0. The offset units are octets.
Result Codes:	OK if the modem accepts the command; ERROR if either the <assert> or <deassert>
	parameter is greater than the buffer size, or if the <de-assert> parameter is greater than
	or equal to the <assert> value.
Description:	The +VBT= command is used to set the flow control assert and deassert points inside the
	modem's internal transmit buffer. As data issent from the PC to the modem and is stored
	in the modem's buffer, when the number of octets in the buffer equals the <assert> value,
	the modem asserts flow control to the PC (e.g., turns off CTS circuits, or sends an XOFF
	character). As the modem removes data from the buffer and processes the data, when
	the number of octets in the buffer equals the <deassert> value, the modem de-asserts
	flow control (e.g., turns on CTS circuits, or sends an XON character). The modem may
	inform the PC (using the +VBT=? command) that the PC does not permit the modifying of
	the flow control assert and deassert points by returning a single value, not in the range of
	values, for each control point.
	The +VBT= command controls the amount of "skid" in the modem's voice buffer, where
	"skid" is the amount of octets that the modem could accept before losing data after the
	modem asserts an off flow control signal to the PC.
	You can use the +VBT= command to balance performance versus robustness. For
	example, if the PC knows there are only 16450 UARTs present, a small "skid" is probably
	sufficient. If there are 16550 UARTs present, a larger "skid" is probably required.

Command:	+VBT=?	Report Modem Flow Control Assert and Deassert Points
Description:	The +VBT=? command displays the possible <assert> and <deassert> values set by the
	+VBT= command, followed by the OK result code.
Example:	In the lines below, the +VBT=? command is used to ask about the modem's flow control
	and buffer size ranges. The modem reports the deassert point is adjustable between 20
	and 100 octets, the assert point is adjustable between 150 and 180 octets, and the
	transmit buffer size is 200 octets.
	AT+VBT=?
	(20-100) , (150-180) , (200
	OK

Command:	+VPP=<enable>	Enable or Disable Voice Mode Packet Protocol
Values:	0, 1; (2–255 reserved for future standards)
Result Codes:	OK if the modem accepts the command; ERROR if the <enable> value is out of range.
Description:	The +VPP= command enables and disables the Packet protocol for Voice mode
	operation, and handles the new unsolicited Voice mode result codes. The Packet

protocol is used to detect lost octets on the modem-to-PC serial link, and to recover the lost octets by requesting retransmission. The Packet protocol assumes that the data corruption is not a problem on the communications link, the last octet sent will never be lost due to data overrun (i.e., that the newer octets always overwrite previous octets in the communications input buffer, a common UART design feature).

Several PC processes can cause serial input channel neglect for longer than the Protocol time between asynchronous characters (typically less than 521 microseconds), and data loss can occur. If a character is lost in the received data, the playback of the voice data may be impaired or lost. If a character is lost in the final result code, the connection may fail. The Packet protocol permits recovery from such data loss.

Multi-Tech Systems, Inc. SocketModem MT5634SMIxx Reference Guide (S000263C) 119