RS232 to RS485 Converter

  • Translates RS232 Rx & Tx to RS485 - direction controlled by data

  • Not RTS controlled - does not rely on PC handshake timing

  • Baud rate is dipswitch selectable from 1200 to 115,200 baud

RS232 to RS485 Converter
Project (Short name: 232to485)
RS232 to RS485 Converter
Project Status
Design work Complete
PCB Number
AB0603356
Version Date
30 Jun 2006
Circuit diagram

Circuit Diagram

Our new model of RS485 converter overcomes the RTS controlled problem experienced by our free design. However, to do that requires a microprocessor, so there is a greater complexity to the design. We sell this unit, although currently we are limiting our sales to customers who can pay by bank transfer - which in reality is local Australian companies. This design has been relatively trouble free, and if you require quantities we can adapt the connector pinout to suit your application.

The unit is line powered from the RS232 sise, with the RS485 signals coming out on a D9 male connector. There are indicator LEDs to show the communications traffic. The unit features Pulldown and pullup resistors to establish the line idle condition.

This is a 2-wire RS485 device - half duplex. In essence, 4 wire RS485 devices do exist, but they are rare - 4 wire RS485 will normally consist of one driving signal from the PC or other host that is continuously active, with a second pair bringing the COMs back from all the nodes - the nodes need to be able to disable their drivers and talk in party-line mode, the host does not. Such a 4 wire system can be full duplex, but frequently is not due to the way serial protocols generally behave, with a send / receive polling structure implemented by the host. 2 wire systems save the wiring and frequently perform as well as 4 wire.

The RS232 level shifting and conversion is accomplished here with a pair of common transistors. The bias resistor values used in this circuit are trimmed to get a typical propagation delay of 500ns - as good as most standard purpose built semiconductor RS232 solutions. However, the RS232 transmit back to the host does not swing to a negative voltage - the converter will however work with all common RS232 receivers, but not over long distances. Naturally, the RS485 side is capable of the long distances, so cable length should not be an issue.

The converter is line powered - it relies on DTR and RTS being at a +12V level active during operation, these are rectified and provide input power for VR1, which in turn powers the rest of the circuitry in the converter.

RS232 transmit levels are normally at -12V when idle or not transmit (also called the mark condition). This is equivalent, on the RS485 side, to the BX+ pin being close to +5V and the BX- pin being close to 0V - although what really matters is the differential voltage across the pair (+5 minus 0V = about 5V). The idle or mark condition is also the voltage level used to send '1' data bits and stop bits.

RS232 transmit levels are normally at +12V when transmiting start bits or 0 bits (also called the space condition). This is equivalent, on the RS485 side, to the BX+ pin being close to 0V and the BX- pin being close to %V - although again what matters is the differential voltage which in this case is (0V minus 5V = about -5V).

Bill of materials

Parts list and notes

RS232 to RS485 Converter Bill Of Materials

PCB design


PCB size H WH x W
Metric36.1mm38.1mm1374mm2
Inches1.420in1.500in2.13in2

PCB overlay image

Final board


RS232 to RS485 Converter Photo

Documentation package


RS232 to RS485 Converter Documentation (PDF)
Copyright
©2010 AirBorn Electronics Pty Ltd

©2010 AirBorn - Last updated 08 July 2010

Background Image Credit: The Catís Paw Nebula (NGC 6334): ESO, DSS2