Echelon LonWorks Router User Manual

Page 76

The FTT-10A transceiver is compatible with Echelon’s LPT-11 Link Power

Transceiver, and these transceivers can communicate with each other on a single

twisted pair cable. This capability provides an inexpensive means of interfacing

to nodes whose current or voltage requirements would otherwise exceed the

capacity of the link power segment. When equipped with an FTT-10A

transceiver, these nodes can be operated from a local power supply without the

need for additional electrical isolation from the link power network.
The FTT-10A transceiver also provides electrical isolation for I/O devices that are

grounded, allowing such devices to be used on a link power network segment. In

many applications, some I/O devices are grounded, either to meet functional

requirements or safety regulations. The transformer of the FTT-10A transceiver

electrically isolates the node from the segment, allowing devices connected to the

node to be grounded without impairing communications.
The FTT-10A transceiver receives its clock input from the Router 5000 through

its CMOS input CLK pin. This pin is driven by the XOUT output of the Router

5000, buffered with a standard bus buffer/line driver that supports TTL-

compatible input and 5V CMOS output. Clock traces should be kept short (≤2

cm) to minimize noise coupling. In addition, a logic ground guard must be added

for the CLK trace to minimize clock noise and to help keep EMI levels low.

However, this ground guard should not be used as a ground source for digital

circuitry.
Figure 34 shows the basic configuration for connecting a Router 5000 Processor

to an FTT-10A Free Topology Twisted Pair Transceiver.
The major differences between connecting a Series 3100 Neuron Chip to an FTT-

10A transceiver (see the L

ORKS

FTT-10A Free Topology Transceiver User’s

Guide) and connecting a Router 5000 to an FTT-10A transceiver include:

•

The connection between the FTT-10A VCC pin and the Router 5000

VDD3V3 pin requires the addition of a low drop-out linear regulator to

convert the +5 V output from the LPT-11 transceiver to the +3.3 V input

for the Router 5000.

•

The connection between the FTT-10A TXD pin and the Router 5000 CP1

pin requires the addition of a non-inverting bus buffer/line driver that

supports TTL-compatible input and 5V CMOS output. The output of the

Router 5000 RST~ pin is also connected to the buffer/line driver to allow

the Router 5000 to propagate a device reset to the FTT-10A transceiver

by setting the buffer/line driver to a tri-state impedance state. An

example part for the buffer/line driver is an NXP 74AHCT1G126 bus

buffer/line driver.

•

The connection between the FTT-10A CLK pin and the Router 5000

XOUT pin requires the addition of a standard (inverting or non-

inverting) bus buffer/line driver that supports TTL-compatible input and

5V CMOS output.

Developing a LONWORKS Router