40
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
.
4.4.9 Flow 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.
RS-232/V.24
Modem-Initiated Flow Control
Computer
or
Terminal
Modem Data Flow
Computer-Initiated Flow Control Pacing
RS-232/V.24 Computer
or
Terminal
Modem Data Flow
Figure 4-1. Flow Control and Pacing