User`s guide

SLTA- 70 Adapter to Host Link Fails Partially
This can be observed as retries at the link layer when certain downlink or uplink transfers
are attempted. This can be due to any of the following conditions (in order of likelihood):
l Extreme-case delays in either the modem or the connection. In this case the DOS
network driver’s calculated timeout values are too short. Increase the basic timeout
value for the driver using the lRnn option. Start with 10 for nn and go up.
l Use with Microsoft Windows 3.1x, particularly at higher serial bit rates (9600 or
greater). This is always a problematic case. The priority of the serial I/O interrupts
for PC/ATs is always lower than the DOS tick interrupt, which is used by Windows
to perform many multi-tasking services. During these services, the serial I/O
interrupts may not be serviced in time, resulting in lost uplink data.
One solution is
to lower the serial bit rate. Another solution may be to replace the PC/AT’s UART
with the 16550 UART, which has a 16 byte FIFO buffer built into it. This only
works for external modem configurations, and will add about 16ms of interrupt
headroom at 9600 bps, because of the hardware FIFO buffer. Another suspect in
this area can be a disk caching program. These programs also perform services
under the DOS tick interrupt, such as flushing data onto the disk drive, which can
postpone serial I/O interrupts for lengthy periods.
l Modem serial bit rate overrun. For example, if the SLTA-10 Adapter serial bit rate
is set to 38,400 bps and the modem telephone line speed is set to 2,400 bps, the
modem will likely be overrun by sending it data faster than it can transmit it. This
can occur since no flow control schemes can be used to restrict the rate that data is
sent to the modem. In general, set the modem link rate equal to the telephone line
speed. In certain cases it will be acceptable to exceed the telephone line speed - for
instance, with a 14,400 bps V.32bis modem with data compression enabled, it may
be possible to run the modem link at 19,200 bps.
l Full duplex FIFO overrun. This is caused by excessive full duplex traffic when
using the buffered link protocol. The ALERT/ACK link protocol should be used
instead.
SLTA- 70 Adapter Sends Modem Configuration String, But It Has No Effect
Most modems will determine the serial bit rate based on the assumption that the first two
characters sent to them while in the command mode are the characters “AT”. This means
that a type of bit rate detection is being performed when these characters are sent to the
modem. If the modem has been power cycled it will need to repeat this process. Some
modems cannot handle being sent an entire configuration string following these first two
characters during this link rate sensing phase.
One way to accommodate these modems is
to add an extra AT command, followed by a delay, to the front of the configuration string.
For example:
"AT!-ATEO&Cl&D2SO=l!"
This will allow the modem time to become fully synchronized with the new link bit rate
before sending any actual command strings to the modem.
SLTA-10 Adapter User’s Guide