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Ribbon cable termination

N

Noway2

Jan 1, 1970
0
In a project, I am planning to use a flat ribbon cable, Z0 ~ 100 Ohms,
with a length of about 3 feet to connect a couple of logic boards. I
am wondering what is the best way to terminate the cable. From my
readings, I am thinking that I will want to terminate near the
receiving connector with a 100 Ohm resistor and a small capacitor for a
typical AC termination scheme. From that point, my plan would be to
pick up the signal with a schmitt trigger buffer / inverter to drive
the logic that receives this signal. This goes against the concept of
terminating the line past the last receiver, but it does terminate it
at the point where the 100 Ohm cable hits the PCB traces. From the
point of ribbon / pcb interface, the connection length to the schmitt
trigger will be short, under a couple of inches.

I am also thinking that it should be suitable to drive the circuit with
standard logic gates as they have a much lower output impedance than
the cable.

Does this approach sound correct or is there a better way?

In case it matters, there will be various address, data, control and a
clk signal corresponding to the addresses and data transmitted by
ribbon cable.
 
J

Jon

Jan 1, 1970
0
Noway2 said:
In a project, I am planning to use a flat ribbon cable, Z0 ~ 100 Ohms,
with a length of about 3 feet to connect a couple of logic boards. I
am wondering what is the best way to terminate the cable. From my
readings, I am thinking that I will want to terminate near the
receiving connector with a 100 Ohm resistor and a small capacitor for a
typical AC termination scheme. From that point, my plan would be to
pick up the signal with a schmitt trigger buffer / inverter to drive
the logic that receives this signal. This goes against the concept of
terminating the line past the last receiver, but it does terminate it
at the point where the 100 Ohm cable hits the PCB traces. From the
point of ribbon / pcb interface, the connection length to the schmitt
trigger will be short, under a couple of inches.

I am also thinking that it should be suitable to drive the circuit with
standard logic gates as they have a much lower output impedance than
the cable.

Does this approach sound correct or is there a better way?

In case it matters, there will be various address, data, control and a
clk signal corresponding to the addresses and data transmitted by
ribbon cable.

Noway2,
The best way to terminate the cable is to use a differential line
receiver. Most of them give you the option of using a buit-in
termination resistor (usually approximately 125 Ohms) or using your own
termination resistor.
 
N

Noway2

Jan 1, 1970
0
Would I then need a differential transmitter too or is the differential
input simply the signal and ground?
 
J

Jon

Jan 1, 1970
0
You can use a differential reciever in a single ended fashion, with one
input connected to ground. A differential transmitter-receiver pair
will give you the best common mode noise immunity.
 
N

Noway2

Jan 1, 1970
0
Thank you for the suggestion. Initially I was thinking about using a
34 pin ribbon connection and running a parallel bus. I am starting to
consider using a smaller, perhaps 10 pin, ribbon connection and running
a few lines of signal. I want to use a mass termination (ribbon or
similar) cable because of manufacturing considerations.
 
J

John B

Jan 1, 1970
0
In a project, I am planning to use a flat ribbon cable, Z0 ~ 100 Ohms,
with a length of about 3 feet to connect a couple of logic boards. I
am wondering what is the best way to terminate the cable. From my
readings, I am thinking that I will want to terminate near the
receiving connector with a 100 Ohm resistor and a small capacitor for a
typical AC termination scheme. From that point, my plan would be to
pick up the signal with a schmitt trigger buffer / inverter to drive
the logic that receives this signal. This goes against the concept of
terminating the line past the last receiver, but it does terminate it
at the point where the 100 Ohm cable hits the PCB traces. From the
point of ribbon / pcb interface, the connection length to the schmitt
trigger will be short, under a couple of inches.

I am also thinking that it should be suitable to drive the circuit with
standard logic gates as they have a much lower output impedance than
the cable.

Does this approach sound correct or is there a better way?

In case it matters, there will be various address, data, control and a
clk signal corresponding to the addresses and data transmitted by
ribbon cable.

The 'old-fashioned' way of terminating logic lines was 220R to Ground and 330R to +5V. I think that
SIL terminators are still available.
 
R

Rich Grise

Jan 1, 1970
0
The 'old-fashioned' way of terminating logic lines was 220R to Ground and 330R to +5V. I think that
SIL terminators are still available.

If you're terminating TTL, you've got it upside down. Also, an "active
termination" is one where you have that 330/220 voltage divider connected
to the base of an NPN transistor, with an additional 330R (or so) to each
line.

Cheers!
Rich
 
J

Joerg

Jan 1, 1970
0
Hello John,
The 'old-fashioned' way of terminating logic lines was 220R to Ground and 330R to +5V. I think that
SIL terminators are still available.

We called them space heaters. I usually prefer AC termination, much
lower dissipation.

Regards, Joerg
 
P

Paul Burke

Jan 1, 1970
0
Joerg said:
We called them space heaters. I usually prefer AC termination, much
lower dissipation.

Yes, frequency dependent of course, but 82 ohms in series with 220pF is
quite common.

Paul Burke
 
J

John B

Jan 1, 1970
0
On 20/01/2006 the venerable Rich Grise etched in runes:

..
..
..
If you're terminating TTL, you've got it upside down.

or ars* over t*t as some might say but I prefer head over heels.


..
..
..
Cheers!
Rich

Thanks Rich. Too much Merlot again (me not you).
 
R

Rene Tschaggelar

Jan 1, 1970
0
Noway2 said:
In a project, I am planning to use a flat ribbon cable, Z0 ~ 100 Ohms,
with a length of about 3 feet to connect a couple of logic boards. I
am wondering what is the best way to terminate the cable. From my
readings, I am thinking that I will want to terminate near the
receiving connector with a 100 Ohm resistor and a small capacitor for a
typical AC termination scheme. From that point, my plan would be to
pick up the signal with a schmitt trigger buffer / inverter to drive
the logic that receives this signal. This goes against the concept of
terminating the line past the last receiver, but it does terminate it
at the point where the 100 Ohm cable hits the PCB traces. From the
point of ribbon / pcb interface, the connection length to the schmitt
trigger will be short, under a couple of inches.

I am also thinking that it should be suitable to drive the circuit with
standard logic gates as they have a much lower output impedance than
the cable.

Does this approach sound correct or is there a better way?

Whatever standard logic gates mean to you.
If a common mode range of almost zero is acceptable,
then I'd use LVDS, dissipating just 3mA into 100 Ohms
is not much.

Rene
 
J

Joerg

Jan 1, 1970
0
Hello Paul,
Yes, frequency dependent of course, but 82 ohms in series with 220pF is
quite common.

220pF seems a bit high for a short cable. I try to use a value large
enough to muffle reflection spikes but small enough not to mess with the
load characteristics in the frequency range of the data.

Regards, Joerg
 
N

Noway2

Jan 1, 1970
0
I think I have talked myself into a varient of SPI using LVDS
transcievers over an 8 wire ribbon cable. I am thinking that I will
use a master clock, from the main controller, with a data in and data
out channel, like SPI. There are no addresses or enabled to worry
about since it is a single point to point channel. The varient comes
in to play in that I am going to have one line driven by the slave
board that is a INT to the computer board. That way, I shouldn't even
have to poll the board to see if it is ready. I am also planning on
implementing CRC in the messages as this is easy to do in hardware with
a serial transmission, though the electrical properties of the medium
should make errror occurances very low. Another nice thing about this
arrangement is that I will have two drivers and two receivers at both
ends, which is a common configuration for differential transcievers and
I can then use the same part in both places.

Again, thanks to everybody for their help,
 
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