This bi-directional small converter transforms the PC RS232 signal to RS485 and the RS485 signal level to RS232 in half duplex mode. It can directly be plugged into the PC RS232 output/input connector.

The converter takes its low power supply from the RS232 PC output port and does not need extern power. A low power sensor can also be powered (few mA) from the PC through the RS485 connector via pin 4 and/or 9.

An extra cable can be plugged between the PC and the converter if needed. The cable female/male must be an 1 to 1 type and must have all connections wired to insure suply to the converter.

If only the RX, TX and GND lines are used on the RS232 side, 6 to 32Vdc can be supplied on the RS485 side on pen 4 and/or 9 to feed to the converter.

RS232 female connector:

1    DCT            Data Carrier Detect
2    RX              Receive Data
3    TX              Transmit Data
4    DTR            Data Terminal Ready
5    GND           Ground
6    DSR            Data Set Ready
7    RTS            Request To Send
8    CTS            Clear To Send
9    RI               Ring indicator   (Not Connected)

RS485 male connector:

1    nc             Not Connected
2    A              RS485 line A, internally connected to pin 8
3    B              RS485 line B, internally connected to pin 7
4    V+            Output or input supply or not used, internally connected to pin 9.
5    0V            Ground through 100 Ohm resistor
6    nc             Not Connected
7    B              RS485 line B, internally connected to pin 3
8    A              RS485 line A, internally connected to pin 2
9    V+            Output or input supply or not used, internally connected to pin 4.

There is no termination resistor (120R) installed. Line A and B are internally connected to respectivelly GND and +5V, both through an 1K resistor. This can be suppressed by opening the enclosure and removing jumper RA and RB.

Application examples:

One RS485 sensor or new RS485 hardware design can be connected to one of the PC RS232 com port.
Start a communication terminal or your own written software application, select then the correct baud rate (matching the RS485 one), the right communication port, hardware handshake (data transport control) if the converted needs to be powered via the PC or no handshake if the power is externally applied (via pin 4 and/or 9 (both are internally connected)). No special sofware protocol needs to be implemented on the RS485 side with one device. Just as simple as the usal RS232 protocol.
More than one PC can be connected to the same RS485 device. They each need a converter on their RS232 comport to be able to communicate with the RS485 device, in turns.

Be aware that strange enough, C or VB software applications do not drive the RS232 control lines right the same way on different Window platform 95/98/ME/XP. So if the converter power needs to come from the PC, one has to do some checking on this. Especially RTS, CTS and DTR.

Multiple RS485 connections on one PC.
First same as above, connect all lines A and all lines B together. With short connections and upto 32 RS485 devices it should not give any trouble even with no termination resistor. This time a kind of protocol needs to be implemented to be able to address specifically one of the RS485 devices.

Two or more PC's connected together over a long distance.
This funny application does for sure not arise the quality of a good network, but is a simple way to connect PC's together over several hundreds of meters in the half duplex mode. Just plug a converter to each PC and connect the two lines A and B and for a long distance the two GND together.

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