NM232DDC Datasheet, PDF (2/5 Pages) C&D Technologies – Isolated Dual EIA-232-D Transmitter and Receiver

NM232DDC
Isolated Dual EIA-232-D Transmitter and Receiver
APPLICATION NOTES
The NM232DDC is an isolated interface device providing EIA-232-D compatibility.
A single supply from the logic side provides all necessary power for the device.
The isolation feature allows the protective or frame ground of DTEs or DCEs to
be isolated from each other, eliminating ground loop currents and inherently long
noise paths. The voltage level between different ground points can be up to ±5V
or ±6V and this level flucuates as the power taken varies. This voltage appears, in
non-isolated equipment, as normal mode noise on the signals. With such high noise
levels present it is clearly very difficult with single ended signals to have reliable
communication. Isolating the input from output so that one side floats relative to
the other allows reliable communication in environments with considerable degrees
of noise between equipments.
Figure 1 shows a typical pcb track layout with a clear separation between the logic
pin connections (pins 1-12) and the EIA-232-D connections (pins 13-24). Between
the two sides of the layout is a track which can be connected to frame or equip-
ment ground, this can be seen as the boundary between the logic and EIA-232-D
side of the device. No other rules or speed conditions need to be applied other than
to keep the two halves’ separation to a maximum.
An example of a DTE application circuit is shown in figure 2. The schematic is
illustrative of the way the NM232DDC may be connected to implement an isolated
EIA‑232‑D interface with a typical UART and processor bus. Precise circuitry will
depend on the application and in particular the use of the control lines shown will
need to be altered to suit the situation.
The ENABLE control line (pin 11), puts the receiver output pins 1 and 12 into a high
impedance state when set to a high level. The shutdown control line (pin 3) sets the
device into a shutdown power mode when it is at high logic level. The maximum
power drawn in shutdown mode is 50µW, a low enough level for consideration in
battery powered equipment.
Where to use the NM232DDC
The function of the NM232DDC is to provide an isolated signal path between a
UART or similar device and the EIA-232-D interface ‘D’ connector, providing two
receivers and two transmitters which can be used in a number of handshake
modes. One receiver and transmitter will normally be used for the signal lines and
the remaining pair will act as handshake lines. Figure 3 shows simply where the
device will be connected.
More complicated schemes for controlled signal interchange will require two or
more such devices. As the devices are self contained no external DC/DC converter
is required. The enable line provides a means for creating a communications bus
where each NM232DDC can have a different logical address via a decoder. This
would allow one UART to talk with many EIA-232-D connected interfaces (see
figure 4).
Figure 1
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