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From: whit@milton.u.washington.edu (John Whitmore)
Newsgroups: sci.electronics
Subject: Re: Termination of TTL <-> TTL cable
Summary: Typical termination is 120 ohms to +3VDC
Message-ID: <5728@milton.u.washington.edu>
Date: 31 Jul 90 00:39:47 GMT
References: <26b48046.147d@petunia.CalPoly.EDU>
Distribution: na
Organization: University of Washington, Seattle
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In article <26b48046.147d@petunia.CalPoly.EDU> jburch@polyslo.CalPoly.EDU (Jerry Burch) writes:
>
>
>I am working on a project which needs to communicate over a ten foot cable
>with TTL gates on either end. Simply connecting the outputs on one end to
>the inputs on the other end has resulted in glitches on some signal lines
>when others switch. Is there a proper method to terminate such a cable?
>
	If your transmitting end can drive the load, use .050" spacing
ribbon cable (the kind that crimp-connects to two-row 0.100" pin
headers), ground every other wire, and pull up the driven (as opposed
to the DRIVING) end of the wire with 220 ohms to +5V and 330 ohms 
to GND.  Resistor packs with this termination pair are available 
(from six to fourteen resistor pairs in one pack).  
	You may have difficulty if the wires are not driven by 
bus-driving gates; high-current open collector gates are ideal,
(like 74LS38), but 24 mA sink current is the only requirement.
Typical 24 mA gates include 74LS240, 74LS241 ...
	If you have any bidirectional wires (driven at either
of the two ends), you will have to (1) use the 220/330 ohm
terminator at both ends of that wire, and (2) use gates with 48 mA
sink capability (because you're driving TWO terminators).


		John Whitmore