Model 245 Application Notes

A common problem in many data communications environments is the need for both interface conversion and isolation facilities.

The first need, that of interface conversion, arises because often there is the need to connect data equipment with different interfaces. Usually there is the need to connect a data device having the common RS-232 interface with some instrument using the RS-422 or RS-485 interface. The reason being, the second device needs the extra interference protection of differential signaling by RS-422, or it is using a polling protocol consistent with RS-485.

The second need, that of isolation, arises because in many instances the equipment being connected must be grounded at different points. Different grounds imply different reference levels for voltages and this can lead to inaccuracies in the physical quantities being measured by the different instruments. It can also lead to the generation of currents between the grounds - currents that attempt to equalize the ground voltage levels and bring them into equilibrium. These 'ground currents' can damage equipment and possibly hurt personnel handling the equipment. Isolation provides a barrier between different ground points. This barrier does not allow ground currents to be generated. Isolation allows equipment being grounded at different points to be connected together without the need to worry about inaccuracies or equipment/personnel damage due to ground currents.

The Model 245 provides the facilities of both interface conversion and isolation in one convenient product. Moreover, it allows conversion of RS-232 to either RS-422 or RS-485.

One interesting application of the Model 245 is shown in the illustration above. This is a medical application. Here we have a patient in a hospital environment, say suffering from hypertension, who is having his blood pressure monitored. The monitoring device sends the data out from the RS-422 interface because of its extra protection from common mode interference. The destination of the monitoring data is a PC. Here the data will be logged into a file and possibly remotely accessed by a physician. The PC can only accept the data from its RS-232 interface. Consequently, to connect the monitoring instrument and the PC interface conversion must be done. The Model 245 can do this. However, there is more to be done.

Both the PC and the blood pressure monitoring instrument are grounded at different points. Both may be grounded at the wall socket. But the different sockets may be tied into different cold water pipes going into 'earth ground.' In this hospital environment the resulting ground currents could give medical personnel or the patient electric shocks. The currents may also force the blood pressure monitoring equipment to give errant readings. However, the isolation feature of the Model 245 provides a barrier between these grounds. There need be no fear of inaccurate readings or safety when the interface conversion is being carried out with it.


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