U-90E Data/Fax Modem
Basic Modem Operation
4-10
The serial port is driven by interrupts. Every interrupt needs a certain amount of overhead
processing time. Too many interrupts reduce the computer's efficiency. The UART 16450
is very commonly used in serial port devices. For every character (byte) received, it
generates an interrupt. If your hardware allows it and if your software supports it, replace
the 16450 UART with a 16550 model. This newer chip has an internal buffer and
generates an interrupt for up to every 16 characters (several trigger levels are available).
With this UART installed, you may drive your serial port at 57600 bps and above.
While data is written from the transfer-buffer to your hard-disk, characters may be lost at
the serial port. This is due to the fact that disk-access interrupts have a higher priority than
serial port interrupts. If you are running at a high serial speed, e.g. 230.4Kbps or
460.8Kbps, on your PC, be sure to enable the disk cache by including SMARTDRV
execution in your AUTOEXEC batch file.
4.5.1 Default Modem Settings for PC’s
The U-90E factory settings are configured for operation with PC type computers and
communication software. In most cases, no additional settings will be required. The
following are some of the default settings that are used for operation with PC computers
and software:
AT Command
Description
E1
Echoes command characters.
&C1
Carrier detect follows remote carrier.
&D2
Modem disconnects on DTR on-to-off transition.
&K4
Uses both V.42 and MNP 4 error correction, and V.42bis
and MNP 5 data compression.
&N0
Modem negotiates highest possible connection speed.
Table 4-6 Default Settings
