19/09/18
Page 21 of 67
ASR650 Operational Manual
3.5
The RS232 Interface
One of the several possible interfaces of the ASR650 reader is the RS232. Although it might be
a little bit old-fashioned in the consumer-electronic market meanwhile (lots of computers do not
even have such an interface anymore), it is still used in agricultural applications very often. One
of the major advantages compared to USB is that the RS232 does not require drivers to be
installed manually.
An RS232 interface needs at least three wires: TxD, RxD and GND.
TxD:
the line on which a device transmits data
RxD:
the line on which a device receives data
GND:
Ground (as reference to TxD and RxD)
In order to use the RS232 interface, connect all three lines to the host device. Please note that
the RxD and TxD lines have to be crossed. The line on which the ASR650 transmits data is the
line on which the host receives data and vice versa.
For connecting the RS232 of the reader to a 9-pole D-SUB connector, please use the following
wiring:
ASR550
– ST2
Connection
PC SUB-D 9 pole
Pin 3 - TxD
Pin 2 RxD
Pin 4 - RxD
Pin 3 TxD
Pin 5 - GND
Pin 5 GND
Please do not forget to connect the Ground line as this builds the reference
for RxD and TxD. The “–V” Pin (ST5-Pin2) is not the same as Ground! The
RS232 will not work properly without connecting Ground.
In addition, you always have to connect both lines: RxD and TxD
– even if
you do not want to send data to the reader. The reason is that the ASR650
uses an RS232 circuit which disables itself, if it does not detect valid RS232
signals.
The maximum allowed cable length for RS232 is 12 meters according to
the RS232 specification. This is because the maximum allowed cable
capacitance at 19200 Baud is 2500pF. Using low capacitance cable might
allow longer cables but please keep in mind that the critical cable length
gets smaller as the baud rate is rising!
The RS232 baud rate is configurable between 9600 and 115200 baud. You have to ensure that
both ends are using the same settings; otherwise the communication will not work at all.
ASR650
– ST2
TxD
RxD
GND
RxD
TxD
GND
Serial Connector