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Ribbon Cable Cross Talk - Is there a fix after the fact?

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1

$begingroup$

I'm involved in a project where the customer defined pins in a ribbon cable, without considering possible cross-talk issues. The signals are 1 MHz data signals with no ground wire separating them. I've never had experience with cross-talk and was amazed at the size of the induced glitches (0.5 to 0.65 volts). The receiving side was using 74HCxx line drivers (CMOS switching levels) which resulted in pure garbage on the data stream. The customer is switching to 74HCT drivers in an attempt to move the input "high" switching level below the glitch level, but I have my concerns.

Is the anything that can be done, besides switching to HCT parts or just properly redesigning the board to possibly salvage what we have?

Any comments welcome.

Thanks.

digital-logic switching crosstalk

share|improve this question

edited 29 mins ago

SamGibson

12k41840

asked 2 hours ago

JHinkleJHinkle

615

$endgroup$

• 
  
  
  

  
  

  
  
  
  
  
  

  
  $begingroup$
  What are the rise times required and cable length? HCT offers worse noise margin with CMOS drivers but better for TTL drivers, unless the data is normally high.
  $endgroup$
  – Sunnyskyguy EE75
  2 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  $begingroup$
  Your are a little sloppy with your terms driver/receiver. CMOS drivers with CMOS receivers have a good noise margin. The drivers will drive to GND + 0.5V and Vcc - 0.5V with a load and near the rails without a load. The guaranteed receiver thresholds are usually 30% and 70% of Vcc, and typically near 50%. You should have >= 1V of margin. HCT receivers have a logic low input threshold of 0.8V, the margin is only 0.3V. Switching to HCT will make it worse for logic 0.
  $endgroup$
  – Mattman944
  2 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  $begingroup$
  how wide are the glitches?
  $endgroup$
  – Sascha
  2 hours ago
  

  
  
  
  


• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  $begingroup$
  What is the rise/falltime of the signals? If you can add series resistance at the source to slow the edge times, that's likely to be your best fix.
  $endgroup$
  – The Photon
  2 hours ago
  

  
  
  
  

add a comment | 

1

$begingroup$

I'm involved in a project where the customer defined pins in a ribbon cable, without considering possible cross-talk issues. The signals are 1 MHz data signals with no ground wire separating them. I've never had experience with cross-talk and was amazed at the size of the induced glitches (0.5 to 0.65 volts). The receiving side was using 74HCxx line drivers (CMOS switching levels) which resulted in pure garbage on the data stream. The customer is switching to 74HCT drivers in an attempt to move the input "high" switching level below the glitch level, but I have my concerns.

Is the anything that can be done, besides switching to HCT parts or just properly redesigning the board to possibly salvage what we have?

Any comments welcome.

Thanks.

digital-logic switching crosstalk

share|improve this question

edited 29 mins ago

SamGibson

12k41840

asked 2 hours ago

JHinkleJHinkle

615

$endgroup$

• 
  
  
  

  
  

  
  
  
  
  
  

  
  $begingroup$
  What are the rise times required and cable length? HCT offers worse noise margin with CMOS drivers but better for TTL drivers, unless the data is normally high.
  $endgroup$
  – Sunnyskyguy EE75
  2 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  $begingroup$
  Your are a little sloppy with your terms driver/receiver. CMOS drivers with CMOS receivers have a good noise margin. The drivers will drive to GND + 0.5V and Vcc - 0.5V with a load and near the rails without a load. The guaranteed receiver thresholds are usually 30% and 70% of Vcc, and typically near 50%. You should have >= 1V of margin. HCT receivers have a logic low input threshold of 0.8V, the margin is only 0.3V. Switching to HCT will make it worse for logic 0.
  $endgroup$
  – Mattman944
  2 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  $begingroup$
  how wide are the glitches?
  $endgroup$
  – Sascha
  2 hours ago
  

  
  
  
  


• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  $begingroup$
  What is the rise/falltime of the signals? If you can add series resistance at the source to slow the edge times, that's likely to be your best fix.
  $endgroup$
  – The Photon
  2 hours ago
  

  
  
  
  

add a comment | 

$begingroup$

I'm involved in a project where the customer defined pins in a ribbon cable, without considering possible cross-talk issues. The signals are 1 MHz data signals with no ground wire separating them. I've never had experience with cross-talk and was amazed at the size of the induced glitches (0.5 to 0.65 volts). The receiving side was using 74HCxx line drivers (CMOS switching levels) which resulted in pure garbage on the data stream. The customer is switching to 74HCT drivers in an attempt to move the input "high" switching level below the glitch level, but I have my concerns.

Is the anything that can be done, besides switching to HCT parts or just properly redesigning the board to possibly salvage what we have?

Any comments welcome.

Thanks.

digital-logic switching crosstalk

share|improve this question

edited 29 mins ago

SamGibson

12k41840

asked 2 hours ago

JHinkleJHinkle

615

$endgroup$

share|improve this question

edited 29 mins ago

SamGibson

12k41840

asked 2 hours ago

JHinkleJHinkle

615

share|improve this question

edited 29 mins ago

SamGibson

12k41840

asked 2 hours ago

JHinkleJHinkle

615

edited 29 mins ago

SamGibson

12k41840

edited 29 mins ago

SamGibson

12k41840

asked 2 hours ago

JHinkleJHinkle

615

asked 2 hours ago

JHinkleJHinkle

615

3 Answers
3

active

oldest

votes

2

$begingroup$

You can leave the board design as-is, but make a short adapter on both ends of the cable, and make the actual cable either as a non-ribbon cable (micro coax, this will be the best), or use proper grounding between signal wires. Essentially you need to make a different cable to fit the IDC plugs (or whatever they selected as board-to-cable connector). Something like this:

share|improve this answer

answered 1 hour ago

Ale..chenskiAle..chenski

30.2k11967

$endgroup$

add a comment | 

1

$begingroup$

After the fact, you have a few choices:

1. Use Schmitt trigger input receivers


2. use shielded foil ribbon cable


3. terminate with 470 pF as a starting value


4. terminate with cable impedance 110-120 Ohms to ground


5. terminate with driver impedance ~ 50 Ohms to Vcc/2 ore equiv pull/down

Increasing the source resistance reduces risetime but won't reduce crosstalk , because the impedance ratio of crosstalk capacitance Xc/Rs rises as slew rate of current reduces.

edit

Proof of ideas using 1m ribbon cable estimate ESL and C

Here using 5 different signals near 1MHz square wave but different to get alias crosstalk with different source and load impedances. Normally I recall, ribbon cables are 120 Ohm single ended which translates into a lump inductance and capacitance per meter but depends on AWG and dielectric spacing.

share|improve this answer

edited 1 hour ago

answered 1 hour ago

Sunnyskyguy EE75Sunnyskyguy EE75

74k228104

$endgroup$

add a comment | 

1

$begingroup$

Can you change the ribbon cable, or insert an adapter to a higher pin-count cable? Consider what IDE/ATA did to increase bandwidth -- it was switched from a 40-wire cable to an 80-wire cable, with every other wire inside the cable tied to ground within the connector. A similar solution could apply here.

Alternatively, can you reduce the slew rate? At 1 MHz, your problem is likely to be less about the frequency of the signals themselves and more about their fast edges. A filter network on the transmit side may help.

share|improve this answer

answered 1 hour ago

duskwuffduskwuff

18.4k32853

$endgroup$

add a comment | 

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2

$begingroup$

You can leave the board design as-is, but make a short adapter on both ends of the cable, and make the actual cable either as a non-ribbon cable (micro coax, this will be the best), or use proper grounding between signal wires. Essentially you need to make a different cable to fit the IDC plugs (or whatever they selected as board-to-cable connector). Something like this:

share|improve this answer

answered 1 hour ago

Ale..chenskiAle..chenski

30.2k11967

$endgroup$

add a comment | 

2

$begingroup$

You can leave the board design as-is, but make a short adapter on both ends of the cable, and make the actual cable either as a non-ribbon cable (micro coax, this will be the best), or use proper grounding between signal wires. Essentially you need to make a different cable to fit the IDC plugs (or whatever they selected as board-to-cable connector). Something like this:

share|improve this answer

answered 1 hour ago

Ale..chenskiAle..chenski

30.2k11967

$endgroup$

add a comment | 

$begingroup$

You can leave the board design as-is, but make a short adapter on both ends of the cable, and make the actual cable either as a non-ribbon cable (micro coax, this will be the best), or use proper grounding between signal wires. Essentially you need to make a different cable to fit the IDC plugs (or whatever they selected as board-to-cable connector). Something like this:

share|improve this answer

answered 1 hour ago

Ale..chenskiAle..chenski

30.2k11967

$endgroup$

share|improve this answer

answered 1 hour ago

Ale..chenskiAle..chenski

30.2k11967

answered 1 hour ago

Ale..chenskiAle..chenski

30.2k11967

answered 1 hour ago

Ale..chenskiAle..chenski

30.2k11967

1

$begingroup$

After the fact, you have a few choices:

1. Use Schmitt trigger input receivers


2. use shielded foil ribbon cable


3. terminate with 470 pF as a starting value


4. terminate with cable impedance 110-120 Ohms to ground


5. terminate with driver impedance ~ 50 Ohms to Vcc/2 ore equiv pull/down

Increasing the source resistance reduces risetime but won't reduce crosstalk , because the impedance ratio of crosstalk capacitance Xc/Rs rises as slew rate of current reduces.

edit

Proof of ideas using 1m ribbon cable estimate ESL and C

Here using 5 different signals near 1MHz square wave but different to get alias crosstalk with different source and load impedances. Normally I recall, ribbon cables are 120 Ohm single ended which translates into a lump inductance and capacitance per meter but depends on AWG and dielectric spacing.

share|improve this answer

edited 1 hour ago

answered 1 hour ago

Sunnyskyguy EE75Sunnyskyguy EE75

74k228104

$endgroup$

add a comment | 

1

$begingroup$

After the fact, you have a few choices:

1. Use Schmitt trigger input receivers


2. use shielded foil ribbon cable


3. terminate with 470 pF as a starting value


4. terminate with cable impedance 110-120 Ohms to ground


5. terminate with driver impedance ~ 50 Ohms to Vcc/2 ore equiv pull/down

Increasing the source resistance reduces risetime but won't reduce crosstalk , because the impedance ratio of crosstalk capacitance Xc/Rs rises as slew rate of current reduces.

edit

Proof of ideas using 1m ribbon cable estimate ESL and C

Here using 5 different signals near 1MHz square wave but different to get alias crosstalk with different source and load impedances. Normally I recall, ribbon cables are 120 Ohm single ended which translates into a lump inductance and capacitance per meter but depends on AWG and dielectric spacing.

share|improve this answer

edited 1 hour ago

answered 1 hour ago

Sunnyskyguy EE75Sunnyskyguy EE75

74k228104

$endgroup$

add a comment | 

$begingroup$

After the fact, you have a few choices:

1. Use Schmitt trigger input receivers


2. use shielded foil ribbon cable


3. terminate with 470 pF as a starting value


4. terminate with cable impedance 110-120 Ohms to ground


5. terminate with driver impedance ~ 50 Ohms to Vcc/2 ore equiv pull/down

Increasing the source resistance reduces risetime but won't reduce crosstalk , because the impedance ratio of crosstalk capacitance Xc/Rs rises as slew rate of current reduces.

edit

Proof of ideas using 1m ribbon cable estimate ESL and C

Here using 5 different signals near 1MHz square wave but different to get alias crosstalk with different source and load impedances. Normally I recall, ribbon cables are 120 Ohm single ended which translates into a lump inductance and capacitance per meter but depends on AWG and dielectric spacing.

share|improve this answer

edited 1 hour ago

answered 1 hour ago

Sunnyskyguy EE75Sunnyskyguy EE75

74k228104

$endgroup$

share|improve this answer

edited 1 hour ago

answered 1 hour ago

Sunnyskyguy EE75Sunnyskyguy EE75

74k228104

answered 1 hour ago

Sunnyskyguy EE75Sunnyskyguy EE75

74k228104

answered 1 hour ago

Sunnyskyguy EE75Sunnyskyguy EE75

74k228104

1

$begingroup$

Can you change the ribbon cable, or insert an adapter to a higher pin-count cable? Consider what IDE/ATA did to increase bandwidth -- it was switched from a 40-wire cable to an 80-wire cable, with every other wire inside the cable tied to ground within the connector. A similar solution could apply here.

Alternatively, can you reduce the slew rate? At 1 MHz, your problem is likely to be less about the frequency of the signals themselves and more about their fast edges. A filter network on the transmit side may help.

share|improve this answer

answered 1 hour ago

duskwuffduskwuff

18.4k32853

$endgroup$

add a comment | 

1

$begingroup$

Can you change the ribbon cable, or insert an adapter to a higher pin-count cable? Consider what IDE/ATA did to increase bandwidth -- it was switched from a 40-wire cable to an 80-wire cable, with every other wire inside the cable tied to ground within the connector. A similar solution could apply here.

Alternatively, can you reduce the slew rate? At 1 MHz, your problem is likely to be less about the frequency of the signals themselves and more about their fast edges. A filter network on the transmit side may help.

share|improve this answer

answered 1 hour ago

duskwuffduskwuff

18.4k32853

$endgroup$

add a comment | 

$begingroup$

Can you change the ribbon cable, or insert an adapter to a higher pin-count cable? Consider what IDE/ATA did to increase bandwidth -- it was switched from a 40-wire cable to an 80-wire cable, with every other wire inside the cable tied to ground within the connector. A similar solution could apply here.

Alternatively, can you reduce the slew rate? At 1 MHz, your problem is likely to be less about the frequency of the signals themselves and more about their fast edges. A filter network on the transmit side may help.

share|improve this answer

answered 1 hour ago

duskwuffduskwuff

18.4k32853

$endgroup$

share|improve this answer

answered 1 hour ago

duskwuffduskwuff

18.4k32853

answered 1 hour ago

duskwuffduskwuff

18.4k32853

answered 1 hour ago

duskwuffduskwuff

18.4k32853