Epson Multi-mode Data Controller MFJ-1278B - Manual (Page 128)

MFJ-1278B MULTI-MODE ADVANCED OPERATION
A computer file transfer program may be unable to process data fast enough to keep up with
output from the MFJ-1278B. In order to be sure of reading every character, a computer must
respond to interrupts from its I/O devices. Some simple programs may poll the input register
for new data. If the polling is not done often enough, data may be lost. Some computers
disable interrupts during disk accesses. If the program enters a routine which will not allow it
to check for data or respond to it, it should signal the MFJ-1278B to stop sending data.
There are two methods of providing flow control which are supported by the MFJ-1278B.
XON/XOFF flow control, sometimes called "software flow control," is accomplished by
sending a special character (usually <CTRL-S>) to request that the output stop and another
special character (usually <CTRL-Q>) to restart output. Hardware flow control may be used
if both computers use the Clear To Send (CTS) and Data Terminal Ready (DTR) lines of the
RS-232C standard. Data is also halted if you press the space bar while in command mode.
Press <CR> to resume.
Some commonly used terminal programs and file transfer programs for home computers do
not implement flow control in software, and many serial ports do not support hardware flow
control. Although the DTR and CTS lines appear at the connector, they may not be used on
some computers unless the software reads the state of the CTS line. If you find that the MFJ-
1278B seems to lose data during file transfers, you should immediately suspect a flow control
problem.
XON/XOFF Flow Control
If you are using a terminal (rather than a computer) or if your computer does not support
DTR/CTS flow control, you should use XON/XOFF flow control, which 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 set the characters which
will be sent to the terminal by the MFJ-1278B, and the commands START and STOP set the
characters to be sent to the MFJ-1278B by the terminal. Your computer may receive as many
as 4 characters from the MFJ-1278B after sending a STOP character, since some characters
may already be "in route" through serial I/O chips.
If you send a STOP (START) character to the MFJ-1278B when it is already stopped
(started), the character will be ignored. If the STOP and START character are the same
character, this character will "toggle" the output, turning it off if it is on, and on if it is off.
You can disable XON/XOFF flow control in one direction only by setting the appropriate
flow control characters to 0. If you do this, the MFJ-1278B will automatically use CTS flow
control to stop input from the terminal. XON/XOFF flow control is normally disabled in
Transparent Mode, since all characters are treated as data. If you cannot use DTR/CTS flow
control, you may enable the XON and XOFF characters, the commands from the MFJ-1278B
to the terminal, by setting TXFLOW ON and XFLOW ON. The START and STOP