8

4749D–AUTO–10/07

B10011S

Figure 6-3.

Implementation of a Power Filter and Over Clamps

7.

Application Hints

As an interface between CAN controllers and a two-wire data bus system for serial data inter-

change, this device is adapted to a special high-level, low-speed transmission system, which is

useful in harsh environments. High immunity against ground offset and interference voltages on

the bus have been the design goals for this device, rather than low power consumption or a min-

imum of external components. An error detection scheme is implemented in the receiver part to

give quick information to the controller in case of faults occurring on the bus. Thus, the controller

is able to start a search cycle in order to look for the possibility of single-wire operation or to dis-

able the station from the bus.

An automatic error-signal end is not feasible because parts of the system are disabled during

single-wire operation. Therefore, the controller has to carry out short tests by switching to the

two-wire state and checking, whether the error signal is still present or not. Errors due to dirty

contacts, shorts between high and low line, or interruptions may not be recognized at all,

because this device does not contain a complete fault computer.

Two control inputs A

page 4’). Depending on the nature of the error, the error signal ER is internally generated partly

in the recessive or partly in the dominant transmission state. In order to avoid watching the error

bits bitwise, an open-collector output driver (with a 1-k

Ω series resistor) discharges a storage

capacitor, which is charged by a time constant, long enough to hold the 0 state for, e.g., 200 µs,

thus, giving the controller enough time to recognize this status during idle times. Only the charg-

ing resistor may be changed and not the 2.2-nF capacitor. In order to perform a faster error-end

test, the charging resistor may be shorted by an NPN emitter follower or by a tristate output high

for approximately 1 to 2 µs.

The pinout of the device shows a controller side (pins 1 to 8) and a bus side (pins 9 to 16). The

application circuit utilizes an input filter section which is necessary for every station and a bias

section which is needed in two master stations only. Additional slave stations only contain the

driving resistors at pins 11 and 12 (270

Ω and 220Ω), the choke coil, and capacitor between pins

13 and 10.

A power filter and overvoltage clamp is highly recommended in order to avoid transmission

errors due to spikes on the 24-V battery voltage.

The input filter is designed as an 2-RC filter for 125 kbit/s and may be changed to 250 kbit/s. Its

good pulse response and good suppression of high frequencies should not be weakened by

omitting one of the capacitors.

From battery

(cl. 15)

Ground

10

33V

To V

To pin 10

+

22 µF