Roman said:
Hi All!
I am going to connect FastEthernet LAN adapter via twisted-pair to my device
with PECL logics. Does anybody know, how can I negotiate PECL input with twisted
pair TX and PECL output with twisted pair RX ?
PECL is just ECL run between +5V and 0V rather than 0V and -4.5V.
Because ECL works with current sources, the package runs only 11%
hotter, so you don't lose much of your temperature range.
ECL outputs are designed to drive terminating resistors - as low as 50R
- returned to a -2V rail, from the logic high of -0.8V and low of -1.6V
and can source the necessary 24mA.
PECL lines thus need to be terminated at the receiver to +3V with
resistors that match the characteristic impedance of your twisted pair.
Typical telecoms twisted pairs have an impedance of between 100R and
120R. Your will be driving the pair differentially, so you'd need two
resistors that sum to the characteristic impedance - say 56R. You can
set the 3V with a zener diode, or just join both 56R resistors to a 68R
resistor which is returned to 0V - this third resistor would be
dissipating close to 150mW, so don't use a tiny SMD part.
The bad thing about PECL is that the logic outputs can be instantly
destroyed by shorting them to 0V (not a problem when the same parts are
used with regular ECL power supplies) so make sure that the outputs are
insulated against ground clips and other grounded metal-work - one of
HPs interferometers used a bit of PECL and more than half the boards
returned for repair had lost their PECL outputs.
