AMIS-492X0 Datasheet, PDF (20/22 Pages) ON Semiconductor – Fieldbus MAU

AMIS-492x0
There is one other minor difference in the recommended external circuitry between the μSAA22Q and the AMIS-492x0. Figure 14
shows the start-up circuits recommended for the μSAA22Q and the AMIS-492x0. The circuit shown for the AMIS-492x0 is different from
that shown for the μSAA22Q but either one will work. Both are current sources that turn on when power is applied to the H1 segment
terminals so that the AMIS-492x0 can turn on without any turn-on transients on the network.
μ^{SAA 22Q}
Startup Circuit
Loop +
AMIS 49200
Startup Circuit
R5
100 k _R6
1k
Loop +
Q1
V Shunt
D3
5.1 V
V Shunt
Figure 14: Recommended Start-up Circuits
5.4 Active Components
Transistors Q1 – Q4 are ordinary small signal transistors. Diodes D1 and D2 are similarly ordinary small signal diodes. Users desiring
to replace a μSAA22Q with the AMIS-49200 in an existing design should be able to use whatever transistors and diodes were used with
the μSAA22Q. For new designs, the specified transistors can be used or other devices may be chosen.
5.5 Alternative Designs
Some users of the Yokogawa μSAA22Q did not use the exact recommended external circuit for the media interface circuit (see Figure
13). Using the AMIS-492x0 without the Yokogawa recommended external circuit may result in some compatibility problems. There are
many alternative designs and it is beyond the scope of this document to identify all possible configurations and their associated design
implications. Please refer to the AMIS-492x0 Fieldbus MAU Reference Design Application Note for a recommended, FOUNDATION
Fieldbus certifiable board design.
5.6 Verification
All designs using the AMIS-492x0 should re-run the entire physical layer conformance test as defined in FOUNDATION Fieldbus
document FF-830, FOUNDATION™ Specification 31.25 kbit/s Physical Layer Conformance Test. Board layout can alter the behavior of
all circuit implementations, even designs that follow the recommended implementation.
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