Dataplex DPX-213, User Manual

Page: 59 / 93

Section 6
DATA TRANSFER
Copyright (C) Dataplex Pty Ltd
6-3
There may be instances when the Constant Speed Interface is used without MNP/V42 and
where the DTE issues Xoff (or drops RTS) to the modem. In this case, data may be lost
unless AT&U is enabled.
For example, assume that the modems have Constant Speed Interface enabled
(AT&I1) and MNP/V42 disabled (AT&E0), the local terminal is set for 9600 bps
with Xon/Xoff flow control and the remote terminal is set for 1200 bps with
RTS/CTS flow control. In this case, AT&U2 or AT&U3 must be used. Binary files
cannot be transmitted unless Zmodem or a similar protocol is used.
Data from the local terminal (DTE A) flows into the local modem (Modem A)
faster than it can be sent to line and buffer 1 will soon need to stop DTE A or data
will be lost. Buffer 1 issues Xoff to DTE A by placing the Xoff character in the
outbound data stream of buffer 3 and the terminal stops with no loss of data.
The data is now moved down the line from buffer 2 to buffer 5. It arrives at buffer
5 faster than buffer 6 can take it since the output speed to the terminal is only 1200
bps. Buffer 6 is in danger of an overrun and so issues Xoff to buffer 5.
Buffer 5 may now overrun and so it places an Xoff in the output stream of buffer 7
to halt the flow from the sending modem. The Xoff is received at buffer 4 and
passed across to buffer 2 to halt the data flow. If AT&U3 is enabled (pass through),
the Xoff character is output to buffer 3 and the DTE A. If AT&U2 is enabled (no
pass through) the Xoff character is passed to buffer 2 but not output via buffer 3.
Consider another example, where both terminals operate at 4800 bps and the line
speed is 2400 bps. Xon/off at the local end and RTS/CTS at the remote. The remote
DTE in this example is slow and frequently drops RTS to keep up. If AT&U0
(default) is invoked in this example, data will be lost.
In this case data flows into buffer 1 until an Xoff stops it. Data is passed to buffer 2
then to 3 and finally flows to buffer 6. The DTE has RTS low quite often and
eventually buffer 6 fills. Dropping RTS at the remote end suspends output from
buffer 8 causing buffers 5 and 2 to eventually overrun and lose data.