Multi-Tech Systems BA-Series manual Flow Control Commands, Retransmit Count $R

MultiModemBA User Guide

Retransmit Count $R

If errors are received during a Reliable connection, the modem re-sends the block of data which contained an error. With the $R0 command, if another error occurs, the block will be re-sent again. The modem counts the number of times that a data block is re-sent. If the same block of data is resent 12 times and still has not been received properly, the modem assumes that the transmission line is unsuitable for transmission, and abort the connection.

This retransmit counter is disabled by the $R1 command. When the retransmit counter is disabled, the modem keeps trying to send data and will not abort, no matter how many times the same block is resent.

AT$R0 = Disconnect if retransmit count is exceeded.

AT$R1 = Do not disconnect due to retransmits.

The factory default setting is $R0.

V.42 Error Correction/300bps $E

At 300 bps, error correction is not typically used. $E1 lets the modem function at 300 bps in either Normal (&E0), Auto-Reliable (&E1) or Reliable (&E2) mode. $E0, which is the modem’s default, disables 300 bps/V.42 error correction altogether.

AT$E0 = No V.42 Error correction at 300 bps.

AT$E1 = V.42 Error Correction at 300 bps.

The factory default setting is $E0.

4.4.9Flow Control Commands

Flow control refers to the techniques used by computer devices to stop and restart the flow of data to and from each other. Flow control is necessary so that a device does not receive more data than it can handle. In the case of the MultiModem, there is a need for flow control in both directions. As illustrated on the next page, flow control for data passing from your computer to the modem is called Modem-Initiated Flow Control and flow control for data passing from the modem to your computer is called Computer/Terminal-Initiated Pacing.

The MultiModem supports both hardware and software Modem Initiated Flow Control. On the Computer/Terminal-Initiated Pacing side, it supports hardware and software flow control, and a special version used by Hewlett-Packard compatible systems called ENQ/ACK Pacing. The modem allows hardware and software pacing to be passed through the modem to the other end of the link so that your computer or terminal can control data start/stop activity through your modem.

This is called Xon/Xoff Pass-Through. To state it simply, “Flow Control” is something the modem does to the computer, while “Pacing” is something the computer does to the modem.

Modem-Initiated Flow Control

Modem	RS-232/V.24
	Data Flow

Computer

or

Terminal

Computer-Initiated Flow Control Pacing

Modem

RS-232/V.24

Data Flow

Computer

or

Terminal

Figure 4-1. Flow Control and Pacing

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