User`s guide

LONWORKS FTT-10A Transceiver User’s Guide A-5
Replacement in Existing PC Board Designs
In general, the FTT-10A transceiver may be used in place of the FTT-10 transceiver
in any node design, provided that it is done in accordance with the LONWORKS FTT-
10 Free Topology Transceiver User's Guide. Echelon has conducted a variety of tests
on a typical node using a 2-layer PC board originally designed for the FTT-10, with
the FTT-10A transceiver replacing the FTT-10 transceiver. These tests include
radiated emissions and IEC 1000-4-2, 1000-4-3, 1000-4-4, and 1000-4-5 (formerly IEC
801-2, 3, 4, and 5) tests described in the user's guide. The FTT-10A transceiver
performed as well as, or better than, the FTT-10 transceiver in all tests. Like the
FTT-10 transceiver, the FTT-10A transceiver is a component rather than a system,
and users should choose an appropriate qualification plan for designs incorporating
the FTT-10A transceiver based on specific system requirements.
There are certain cases where the FTT-10A transceiver will require changes to a PC
board designed for the FTT-10 transceiver:
1. As documented above, if the FTT-10A transceiver is placed on a PC board
designed for the FTT-10 transceiver, there may be mechanical interference with
components located adjacent to the NET_A/ NET_B edge of the FTT-10A
transceiver.
2. The FTT-10A transceiver cannot be used as a physical layer repeater using
schematics from the LONWORKS FTT-10 Free Topology Transceiver User's Guide.
It is very important to use only the repeater schematics shown Chapter 2 of this
FTT-10A User’s Guide. In particular, note that the FTT-10A repeater uses a
5MHz rather than a 2.5MHz clock source, that there is no logical inversion in the
path between RXD and TXD signals, and that the external circuitry immediately
around the transceiver is somewhat different.
EXTERNAL COMPONENT REMOVAL FROM AN EXISTING BOARD LAYOUT
1. To prevent a possible, brief transient on the network when the Neuron Chip goes
through its reset process, the 47kΩ CLKSEL1 resistor (R1 in the LONWORKS
FTT-10 Free Topology Transceiver User's Guide figure 2.1) should be removed.
2. The following components can be removed from an existing PC board for simplification:
Rr, C4, and C5. The LVI may be removed if not required by a Neuron Chip or the
application circuitry.
EMI Comparison Scans for the FTT-10 and FTT-10A
Transceivers
This section presents EMI scans of both the FTT-10 and FTT-10A transceivers. EMI
test data for Echelon’s SLTA-10 Serial LonTalk Adapter which utilizes an FTT-10A
transceiver operating at10MHz is also presented.
The net result is that devices using the FTT-10 and FTT-10A transceivers have
comparable EMI performance, and products using the FTT-10A transceiver at both