Chapter 6 - System Operation and Control
38
packet is ready when the unit begins to transmit.
MAXframe
in combination with
Paclen
deter-
mines how much Information can be sent in a single transmission. The best combination for effi-
cient data transfer is determined partly by the channel quality and partly by the rate at which the
terminal can process data. For a 1200 baud terminal data rate, you should start with a combina-
tion that produces about 300 characters outstanding at one time.
The radio data transmission rate is set at 300 baud. On noisy HF channels a
MAXframe
of 1
with reduced
Paclen
of 50-60 characters usually works best.
FLOW CONTROL
Whenever data is transferred to Computers, there is a chance that the data will be received
faster than the Computer can handle it. Some programs try to prevent this problem by providing
data buffers for storing incoming data until the program is ready for it. However, this merely post-
pones the problem, since there is a finite amount of room in any buffer. In order to prevent loss of
data, the Computer must be able to make whatever is sending data to stop sending, and later tell
it to resume sending. If you are a home Computer user, you are probably already familiar with one
type of flow control, which allows you to stop the Output from the Computer while you read it and
restart it when you have finished.
There are two methods of providing flow control which are supported by the KAM. XON/OFF
flow control, sometimes called "Software flow control", is accomplished by sending a special char-
acter (usually CTRL-S) to request that the out-put stop and another special character (usually
CTRL-Q) to restart output. Hardware flow control may be used if the Computer or terminal can use
the request-to-send (RTS) and clear-to-send (CTS) lines of the RS-232 Standard for hand-shaking.
The single greatest difficulty experienced during our testing was correct implementation of flow
control on the user's Computer. Many inexpensive and commonly used terminal programs and file
transfer programs for home Computers DO NOT implement flow control in Software, and many
RS-232 ports do not support hardware flow control. Even if the RTS and CTS lines appear at the
connector, software which directly reads the CTS line may be required in order for flow control to
be implemented. If you find that the KAM seems to lose data during file transfers, you should sus-
pect a flow control error.
XON/XOFF FLOW CONTROL
If your terminal or Computer does not support RTS/CTS flow control, you should use
XON/XOFF flow control. This method of flow control is enabled by setting
Xflow
ON. The special
flow control characters are set to CTRL-S and CTRL-Q by default, but they may be changed. The
commands XON and XOff select the characters which will be sent to the terminal and the com-
mands
STARt
and
STOp
select the characters which will be sent to the KAM by the terminal. If you
set the codes for these characters to zero, you will disable that function. If you set
STARt
or XON
to zero,
STOp
or XOff will be automatically disabled as well.
The KAMs input buffer may fill up in command mode if you try to type too long a command. In
data mode, the buffer may fill up if you are using your computer to transfer data at a rate faster
than the data rate for radio transmission or if radio data transmits ion has slowed down because of
noise (on AMTOR). The KAM will send the terminal an
XOff
character when there is room remain-
ing for about ten characters in the buffer. If you continue sending data until there are only five
spaces left, the unit will send an
XOff
character after each character received. When the buffer fills
up entirely, data will be lost. When the buffer empties out, the KAM will send a single
XON
charac-
ter to the terminal.
A computer file transfer program can very easily be unable to process data fast enough to keep
up with output from the KAM. In order to be sure of reading every character, a computer must re-
spond to interrupts from its devices. Some simple programs, especially those written in BASIC,
may poll the input register for new data. If the polling is not done fast enough, data may be lost.
If the program enters a routine which will not allow it to check for data often, it should send a
STOp
character to the KAM. Some disk operating systems, such as Apple's, disable all interrupts
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