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termination for TTL driver

J

john

Jan 1, 1970
0
I have a ckt board outputing, thru a TTL driver, 8 bit data in parallel
to another board loacted 3 feet away. I was wondering is it necessary to
add some termination on the data line? If so, what type of termination
should be used and how to connect it ? Resistor connecting to V+ is ok ?
any help is very much appreciated
 
H

Henry

Jan 1, 1970
0
Series termination. Value is Z(0) of line minus Z(out) of TTL Driver. On the
receiving end a resistor is normally not required.
-Henry

john schrieb in Nachricht ...
 
R

Rene Tschaggelar

Jan 1, 1970
0
john said:
I have a ckt board outputing, thru a TTL driver, 8 bit data in parallel
to another board loacted 3 feet away. I was wondering is it necessary to
add some termination on the data line? If so, what type of termination
should be used and how to connect it ? Resistor connecting to V+ is ok ?

TTL was never made for transmission lines.
You should consider conversion to something else.
Eg LVDS, ECL.

Rene
 
N

Norm Dresner

Jan 1, 1970
0
john said:
I have a ckt board outputing, thru a TTL driver, 8 bit data in parallel
to another board loacted 3 feet away. I was wondering is it necessary to
add some termination on the data line? If so, what type of termination
should be used and how to connect it ? Resistor connecting to V+ is ok ?
any help is very much appreciated

There are TTL parts that were designed to drive lines of specific
impedances. IIRC, 74S140 is a (dual)? 50 ohm driver and 74128 is a (triple)
75 ohm driver. The important thing about the termination is that it has to
be matched to the characteristics of the lines as much as to the
characteristics of the drivers.

Here are some important considerations:

What kind of cable are you using?

What frequencies are the lines switching at?

What, if any, transition-time requirements do you have at the receivers?

What logic family are the receivers?

If I were doing this and had not-to-fast signals, I'd consider using some
differential drivers and receivers which are TTL-compatible. 26LS31/26LS32
pairs are probably reliable to 10 MHz or so, but only if you terminate the
lines properly. For twisted pairs, my first guess would be to assume the
line impedance was on the order of 100-150 ohms and I'd try to put two
resistors at each receiving point, one to ground and one to Vcc, each twice
the value of the cable impedance.

Norm
 
K

Keith R. Williams

Jan 1, 1970
0
Series termination. Value is Z(0) of line minus Z(out) of TTL Driver. On the
receiving end a resistor is normally not required.

As long as the data rate isn't too high, this is the right
answer. 30 ohms (one can play) is about right for normal TTL
drivers and .100" IDF cable with a ground inbetween each signal.
 
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