RS232 - RS485 Dual Converter
An industrial grade RS232 - RS485 converter with 2 independent serial channels, designed to be used with the HeatMon or other serial communication systems. It offer high noise immunity and is designed to be used with twisted pair cables of 120 or 150 Ω loaded with matching terminating resistors. It is externally supplied with 5V from one USB port, include a 150 Ω terminating resistor and idle level polarization resistors on both channels. Another cable impedance matching resistor is required to the other end of the cable.
The converter can be used to create up to 2 separate RS485 half-duplex serial buses. On every RS485 serial bus it have an internal terminating equivalent resistance of 152 Ω (174 Ω resistor together with 2 polarization resistors of 620 Ω on D+ and D-). These resistors maintain a differential voltage over 200mV in the presence of a 120/150 Ω terminating resistor at the other end of the cable (around 300mV for 150 Ω cable). To have over 200mV differential voltage at the far end of the cable, the loop resistance (the added resistance of both wires) must be bellow 50 Ω for a 120 Ω cable and bellow 100 Ω for a 150 Ω cable (presuming a matching resistor at the cable end). The maximum cable length is 1200 m. The converter has been designed primarily to be used with 150 Ω Profibus DP twisted pair cable, which is easy to find, cost effective, and offer good performance. However it will also work well with any 120 Ω twisted pair cable. One end of the cable should be connected to the conveter and the other end should be terminated with a resistor equal with the cable impedance. The cable ground(shield) should be connected to the common ground.
Converter
The converter need to be installed close to the computer case, being connecter to the RS232 ports through 2 short cables (channel 1 and 2). The USB cable only provide the 5V supply for the converter circuits. The embedded DE-9 male connector expose the RS485 signals for both channels. The entire circuit board with the cables are encapsulated in silicone rubber for enhanced mechanical and environmental protection. Two integrated transceivers handle the RS485 signals, the RTS line is used to switch between the transmission and reception mode, this is controlled by the Windows serial driver. This serial driver will take up to 15 ms to disable the transmitter and enable the receiver. So in this case the device must have a response delay time of at least 16 ms.
RS485 Connector
The channel 1 signals are on pins 1 and 2, while the channel 2 signals are on pins 4 and 5. The pins 6 to 9 are GND and are connected to the computer GND. The twisted pair cable screen or shield (if any) should be connected the converter GND (pins 6 - 9) and also can be connected to the GND of devices (temperature controllers). Usually the devices have an isolated power supply so that any ground loops are avoided.