Cost Effective Communications in a distributed I/O environment

The RS232 serial interface protocol is the only connection standard adhered to by all computer manufacturers. However RS232 is only able to operate effectively where the computer and the devices to be connected to it are situated within a few metres of each other.

Where information needs to be transmitted between multiple devices at distances of up to 1.2 kilometers, remote from the computer, the RS485 protocol is able to achieve this economically using a 2 wire 'twisted pair' circuit. However it is necessary to convert the signal from RS485 protocol back to RS232 at the computer end. This is easily and economically achieved by the use of a Control-it 5002 protocol converter.

The Control-it 5002 is a simple non-automatic converter designed for use in laboratory or educational environments. With a simple converter the computer has to control the "Request to Send " (RTS) line. A positive voltage causes the converter to transmit onto the RS485 line. A negative voltage causes the converter to listen.

Fig. 1: Computer using RTS line as transmit control for simple converter

To communicate with one remote device the following needs to happen:

1. The computer puts a +ve voltage on the RTS line in order to switch the 5002 to transmit.
2. Data is then sent from the computer to the remote device.
3. The computer lowers RTS to switch the 5002 to receive.
4. The remote device returns a response after a very short time (e.g. 0.5mSec)

What makes the 5002 so good

• In laboratory or educational environments all that is required is a simple conversion process. This unit provides exactly what is needed.
• It is very economical.
• It requires no technical set-up. Can be installed by anyone who is able to identify the difference between the two twisted-pair wires and the shield in a twisted-pair cable.
• The 9 way RS232 connector plugs directly into the PC. The RS485 side is simply wired using a screw driver.
• A compact unit which requires minimal space
• 3 year warranty