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 5001 RS232 To RS485 converter.

The RS485 communications protocol operates like a telephone party line. One device communicates at a time. The diagram below shows all units interconnected via a single twisted pair of wires.

Fig. 1: Two RS485 devices are connected to the computer via a Control-it 5001 protocol converter

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

1. Data is sent from the computer to the remote device. The 5001 sees the data coming from the computer and automatically goes into transmit mode.

2. As soon as the computer stops sending data, the 5001 stops transmitting which leaves the line free.

3. The remote device is then able to respond after a very short time (e.g. 0.5mSec) and this data is passed back to the computer.

How the 5001 optimises data transmission

As described above, RS485 protocol operates like a telephone party line, where one person speaks at a time. For efficiency each party should keep their conversations as brief as possible. The Control-it 5001 converter is designed to take advantage of this cost effective RS485 protocol whilst minimising line turnaround time, in order to increase overall data throughput by up to 80% over similar converters.

The following screen shots compare a character sent at 9600 bps, showing a conventional automatic converter versus the Control-it 5001.

Fig. 2: A standard automatic converter, Total time wasted is 1300uSec

Fig. 3: The Control-it 5001 converter, Time wasted is less than 200uSec

The picture in Fig 2 shows both the wires in the RS485 cable. The bottom trace contains the same data as the top trace, but it is inverted. Each section of the trace is of interest.

The danger with a standard converter and its wasted 1300uSec of time, is that a remote device will respond before the RS485 line is available. Some remote devices can be programmed to wait for more time before they respond, but this slows down the amount of data that can be transferred each second. This potential problem is eliminated with the 5001 converter.

What makes the 5001 automatic converter so good

Many competing units have some of the features of the Technman unit. The advantage of the 5001 is that it has them all in one small package. The 5001 converter does it all!

• It is an automatic converter, which prevents data being lost if the multitasking PC operating system is temporarily diverted to another task.
• The quicker line turnaround time offered by this automatic converter, provides the fast response required by complex industrial or scientific processes.
• Automatic baud rate adjustment. No jumpers to set up.
• The 5001 converter optically isolates the computer from the other devices in the system, eliminating any possible damage resulting from electrical wiring faults.
• Transient protectors prevent damage to components of the converter. (Lightning strikes or arcs in an industrial environment can cause transients.)
• Fast. Works with data transfer rates of between 300 and 115,000 baud - many other converters cannot handle transmission speeds greater than 38,400 baud.
• No technical set-up required. 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.
• The RS485 line has built in termination, which removes the need for external resistors at the 5001 end of the line.
• The 5001 automatically nudges the line to a neutral state, which protects all devices in the system from spurious data received between transmissions.
• Your transmitted data from the computer is never echoed back to the computer. (This greatly simplifies software writing.)
• Runs on voltages between 9 and 24 volts DC
• A compact unit which requires minimal space
• 3 year warranty