jburch@polyslo.CalPoly.EDU (Jerry Burch) (07/31/90)
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?
Preferably this would only involve one end of the cable as the other is off
of a port on a micro which uses a pair of LSTTL gates as buffers.
Any suggestions would be appreciated.
Jerry Burch
jburch@polyslo.calpoly.edu
--
+----------------------+------------------------------------+
| Jerry Burch | jburch@polyslo.CalPoly.EDU |
+----------------------+------------------------------------+grege@gold.GVG.TEK.COM (Greg Ebert) (07/31/90)
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. This one is 'compliments of the floppy drive manufacturers'. First, you alternate signal and ground lines (odd=GND/even=signal). Signals are driven by open-collector drivers, such as the 74XX38. At the other end of the cable, you pull-up w/150 ohms. If you parallel 2 drivers, you can terminate the line closer to its characteristic impedance, which is somewhere between 50 and 100 ohms; use a TDR scope and experiment.
brian@ucsd.Edu (Brian Kantor) (07/31/90)
One that works well for me: Drive the cable with an LS240 or LS244. Receive it with LS14s. Terminate the receiver end with 220/330 packs. Use twisted pairs or alternate ground wires, or use a ground-plane ribbon cable. Note that LS14s are slow, so you can't run blazingly fast signals down this cable. - Brian
whit@milton.u.washington.edu (John Whitmore) (07/31/90)
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
sukenick@sci.ccny.cuny.edu (SYG) (08/08/90)
>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 And, if your environment is noisy, two fast reversed bias diodes (one to ground, the other to +). We had the strangest problems with our counter until we added the diodes (our environment is very noisy, but very well shielded; still the glitches got through enough to lockup the counter. Then again, the counter was some HCATLSXXXZXZXZ nonsense).