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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  shows a typical pcb track layout with a clear separation between the logic

pin connections (pins -2) and the EIA-232-D connections (pins 3-24). 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 ), puts the receiver output pins  and 2 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 

Figure 2

Figure 3

Figure 4