Model 65A Application Notes
Current loop technology is a signaling technique that has been in
existence for many years - actually going back to the earliest teletype
printers. Here data is represented by electrical current being sent in
either one direction around a loop or the other direction. It has
significant advantages when it comes to distance. Data sent by a current
loop connection can go much further than data sent by connecting the
common RS-232 interfaces. This signaling technique also provides much
greater protection against electrical interference than the common
RS-232. It is particularly suited to protecting data communication being
carried out in environments where the terminating equipment has high
voltage sources. Here there are many deleterious effects which current
loop signaling can ameliorate. Finally, many current loop systems have
the benefits of optical isolation and the protection that this affords
from the inaccuracies of ground loops.
There are then many situations where you would want to bring the
benefits of current loop signaling to devices, like PCs, where the data
is presented through the RS-232 interface. To do this you have to
convert RS-232 interface communications to current loop communications.
The Model 65A does just this type of conversion. It will accept RS-232
data at up to 9.6 KBPS and generate a 20mA current loop representation.
On the other side it can accept loop currents of either 20mA or 60mA and
convert them to RS-232 data.
The illustration above shows the Model 65A in a typical application.
Here a PC is set up to control a large air conditioning unit in an
office building. Data from the PC is coming from an RS-232 interface.
However, the distance to the air conditioning unit is large, 500 feet.
Current loop signaling can reliably make the connection while RS-232
communications cannot. Furthermore, current loop signaling can provide
better protection against the electrical interference generated by the
air conditioning unit. Current loop communication is the attractive
choice for control signals to the air conditioning unit. In fact, the
air conditioning unit has a current loop interface built-in. The Model
65A converts the control data from the PC's RS-232 interface and sends
it by current loop to the air conditioning unit.