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.