A full manual has been not written yet. Briefly and with apologies for the
bad english:

The "EXT" button starts program execution and connection with the EKD500.
Before clicking on it, be sure to select in the "Options" menu the correct
RS232 port and speed, the number of receivers and the receivers addresses.
If multiple receivers are to be controlled by the program, they must be
daisy-chained as per manual, with the control PC serial port connected to
EXT connector of the first EKD500, EXP of the first EKD500 connected to EXT
of the second EKD500, and so on. A maximum of 99 receiivers can be
configured. The chosen configuration will be saved automatically. For serial
ports other than the first two, or USB, select the "Custom" serial port
option and write the name of the port. Don't forget to set the same
parameters on the receivers and to activate the remote mode (LED EXT
must be lit). Check also that your user has serial port access privileges;
under Linux usually it must be a member of the "dialout" group.

To switch the control from one receiver to another, select the number of 
the wanted EKD500 with the "Receiver#" combo box. Only the configured
number of receivers can be selected. If a receiver not switched on or not
properly configured is selected and no response comes in, an appropriate
error message is written to the message display and, if the S-meter is
enabled, it will be disabled until a properly configured receiver is selected.

When you start the program, the RX state (frequency, mode, filters, etc)
will be read from RX itself. The state can also be saved to and restored
from a file. Default file name is <frequency>-<mode>.dat, e.g.
10100.8-F0.dat.

Option "Enable S-meter" starts polling the received signal level (using the
G command) every 0.5s and display a  bar-meter for the received signal
strenght. During the command duration, other keys are disabled, so selecting
this option slows down somewhat operations. This function can be enabled
only on higher serial port speeds (1200 and 2400 baud).

Frequency can be changed by clicking on the RX display, with the mouse wheel
or by writing the desired frequency in the spinedit at lower right of the
main window.

Left-clicking on a figure in the RX frequency display decrements that figure
by one, right-clicking increments that figure by one. This only happens if
the final frequency is in the range of the RX. Clicking on the decimal point
the displayed frequency will be rewritten. Smallest increment/decrement
available is 10 Hz.

If the mouse wheel is rotated with the pointer over one of the figures of
the RX frequency, that figure is incremented or decremented. This now works
also under Windows.

If the mouse wheel is rotated with the pointer over the frequency spinedit,
the frequency is incremented/decremented by 10 Hz.

Last, if the mouse wheel is rotated with the pointer anywhere else, RX
frequency is incremented or decremented by the tuning step in effect. This
is the fastest tuning way since only one character (+ or -) must be sent to
receiver.

There are also some keyboard shortcuts:

    +: Tune up
    -: Tune down
    /: Change tune rate
    *: Set receive frequency
    ⏎: same as E key

allowing basic control of the receiver using only the numerical keypad.

All standard EKD500 remote commands are also accepted:

    f: set RX frequency in kHz down to 10 Hz (xxxxx.xx, e.g. 10100.80)
    d: set frequency step in kHz (e.g. 100 Hz is 0.1, min 10 Hz is 0.01)
    m: set RX Mode (1..9, see table on the RX front panel)
    b: set IF bandwidth (1..9, see table on the RX front panel) 
    g: set AGC mode (1..9, see table on the RX front panel)
    v: Preselector off/on  (0..1, 0=off, 1=on)
    c: Memory recall (1..99)
    s: Memory store (1..99)
    ": Memory 98 fast recall
    !: Memory 99 fast recall
    l: Scan start/stop
    a: Set scan parameters
    e: Execute key
    .: Decimal separator
    ;: Arrow key (; not j as written in the user manual on page 57)
 
The above commands can be issued either upper or lower case. The frequency
value for commands f and d (or F and ΔF buttons) can be entered either with
the keyboard or with the on-screen numeric keys.

OTHER COMMANDS & CONTROLS
The message display shows informations about the command being executed or
status/info/error messages. Double-clicking on it clears display.

RX parameters (frequency, step, AGC, etc.) can be set also by selecting the
desired value with the controls at the lower right of the main window.

Since the "EXT FCT" key (remote interface configuration) has no equivalent
remote command, that key is used to show briefly on the display the actual
program address and RS232 speed, in the same format used by the "EXT FCT"
RX key. A reminder on the lack of equivalent remote command is also issued.

The program tries to disable illegal keys for the actual state (e. g.
pressing the "F" key, all other function keys are disabled and after the 5th
figure the only enabled key is the "." key). If this behaviour is not
wanted, then check the "Option>Enable all controls" menu entry.

NOTE ON CONFIGURATION AND STATE FILES.
All configuration files are kept in the .EKD500Control directory of your
home directory. In case the program configuration has messed up, delete (or
move) the "Config.dat" files and the program at next start will recreate it
using the default configuration.

Files saved in the Channels subdirectory (using the "File>Save Channel" menu
entry) will be read at program startup and shown in the Channels menu entry
and can be recalled using only one mouse click. To allow to rename and
delete the saved state files from within the program, a very minimal file
manager is built in (menu entries "File>Manage states" and "File>Manage
channels").


TODO.
Since the EKD500 interface has the RTS and CTS handshake lines, see if it is
possible to get rid of the fixed delays now inserted in the program code. In
command modes B and C, the commands must be sent with a 75 ms 
inter-character delay, otherwise a NAK or ? error message is triggered.

In command mode D the accepted speed of the commands is notably higher,
but the RX sends back no answer or info at all. Mode D is used actually
only to restore a saved state and to issue frequency and step commands.
If the program recreates the EKD500 state machine for each command, 
then will be possible to use only that mode, speeding up things.

Check for bugs and/or timing/synchronization errors and/or missed delays.
