8
RS-232 Serial Port
(MiniPlex-2S, MiniPlex-2S/BT)
The RS-232 serial port is galvanically isolated from the multiplexer to prevent ground loops when
connected to a computer. Ground loops can result in excessive currents in ground connections, which
could destroy the multiplexer or the serial port of the connected computer.
The default speed of the serial port is 38400 Baud. It can be set to any speed in the range from 4800 to
115200 Baud using the MPX-Config utility. It is always best to choose the highest speed possible to
prevent any overflows in the multiplexer.
A rule of thumb is to add up the speeds of the inputs and set the output to the nearest higher value. If
the inputs are for example set to 38400, 4800, 4800 and 4800 Baud, the sum of these speeds is 52800
Baud. The nearest possible value is 57600 Baud.
In reality, a lower speed is possible because NMEA data is mostly sent in short bursts, resulting in a lower
effective speed. It is perfectly possible to leave the serial port at 38400 Baud in the above configuration.
Connect the serial port to a Sub-D9 connector as shown in Figure 5 below. Connections to TxD, RxD and
Com are required for proper operation of the multiplexer while the RTS connection is optional.
Figure 5
The serial port supports flow control through the RTS connection. Flow control is needed when waypoints
and routes are sent from the computer to a GPS through the multiplexer. While normal NMEA sentences
are sent at regular intervals, waypoints and routes are sent in one go without any pause between these
NMEA sentences. This fills up the queue in the multiplexer immediately and the multiplexer discards the
remaining waypoints and routes. The GPS now only receives a few waypoints and partial routes.
Flow control prevents this from happening. When the queue in the multiplexer is almost filled, the
multiplexer deactivates RTS to signal the computer to stop sending data. When the queue is sufficiently
emptied, the multiplexer activates RTS to signal the computer to continue. This requires a special setting
in your navigation software, which is called “flow control” or “handshake”. This setting can mostly be
found in the port settings of your software.
Set the flow control to “Hardware” or “CTS/RTS”. Do not use “Xon/Xoff” flow control since this uses
special characters instead of a signal. These characters are not part of the NMEA standard and therefore
ignored by the multiplexer.
TxD
RxD
RTS
Com
8
2
3
5
Sub-D9
connector
MiniPlex
terminals
