Power distribution to multiple propeller PCBs using ribbon cable - QUESTIONS
Thomas Taylor
Posts: 27
I am seeking advice concerning the use of ribbon cable for power distribution for multiple propeller PCBs.
My application uses SPI communications between one SPI MASTER propeller and 4 SPI SLAVE propellers – each on their own small PCB. Ribbon cable provides the four signal lines required for SPI. I have been considering expanding the ribbon conductor count to 6 (or even 8 – doubling up) to carry Vdd and Vss. The cable lengths are 3 and 6 inches long (2 each).
Is this a reasonable approach? I have never dealt with multiple boards and power distribution for propellers (or any other microcontroller).
Doing a little research, I am not sure how to interpret the information in some tables. I am hoping someone with more experience might be able to provide some guidance.
Here are the specs I am working with followed by my thoughts and questions.
Wire gauge: 28
cir mils = 159 (from: www.interfacebus.com/Reference_Cable_AWG_Sizes.html)
"Maximum amps for chassis wiring" = 1.4
"Maximum amps for power transmission" = 0.226
The above two currents were extracted from the table located at:
http://www.powerstream.com/Wire_Size.htm
The follow paragraph accompanied the table providing some explanation of what is meant by “air” and “bundled”. (Here is where I have a question as to how the term "bundled" relates to ribbon cable.)
“Load Carrying Capacities (see table below)
The following chart is a guideline of ampacity or copper wire current carrying capacity following the Handbook of Electronic Tables and Formulas for American Wire Gauge. As you might guess, the rated ampacities are just a rule of thumb. In careful engineering the voltage drop, insulation temperature limit, thickness, thermal conductivity, and air convection and temperature should all be taken into account. The Maximum Amps for Power Transmission uses the 700 circular mils per amp rule, which is very very conservative. The Maximum Amps for Chassis Wiring is also a conservative rating, but is meant for wiring in air, and not in a bundle. For short lengths of wire, such as is used in battery packs you should trade off the resistance and load with size, weight, and flexibility. NOTE: For installations that need to conform to the National Electrical Code, you must use their guidelines. Contact your local electrician to find out what is legal!”
Assuming the max current for the propeller (300 ma.), I find myself BETWEEN these two specifications and it would seem that much depends on how you interpret “bundle”.
Because ribbon cable conductors share heat dissipation properties with adjacent conductors, the ribbon appears (to me) somewhat like a bundle (thus lowering the ampacity rating).
Also, one of my concerns is with the bypass capacitors being “starved”. I am using two X7R MLC capacitors on the foil side directly under the propeller DIPs. Even though the conductors may meet an ampacity rating, at high switching frequencies might there be problems if the replenishing current requirement can not be met? (This is how I think of these things. Am I way off on cloud 9?)
In addition, how would you select capacitors for these lines? I am thinking I would need a tantalum at the entry point on the slave PCB (which would be right by the end of the propeller DIP. Also, are 6 inch power distribution lines something to be specially treated? I do not have a gut feel for how critical the lengths of conductors are when we jump off board going to another board.
I would greatly appreciate the thoughts of anyone who can shine some light on these issues. Thanks in advance.
Tom
My application uses SPI communications between one SPI MASTER propeller and 4 SPI SLAVE propellers – each on their own small PCB. Ribbon cable provides the four signal lines required for SPI. I have been considering expanding the ribbon conductor count to 6 (or even 8 – doubling up) to carry Vdd and Vss. The cable lengths are 3 and 6 inches long (2 each).
Is this a reasonable approach? I have never dealt with multiple boards and power distribution for propellers (or any other microcontroller).
Doing a little research, I am not sure how to interpret the information in some tables. I am hoping someone with more experience might be able to provide some guidance.
Here are the specs I am working with followed by my thoughts and questions.
Wire gauge: 28
cir mils = 159 (from: www.interfacebus.com/Reference_Cable_AWG_Sizes.html)
"Maximum amps for chassis wiring" = 1.4
"Maximum amps for power transmission" = 0.226
The above two currents were extracted from the table located at:
http://www.powerstream.com/Wire_Size.htm
The follow paragraph accompanied the table providing some explanation of what is meant by “air” and “bundled”. (Here is where I have a question as to how the term "bundled" relates to ribbon cable.)
“Load Carrying Capacities (see table below)
The following chart is a guideline of ampacity or copper wire current carrying capacity following the Handbook of Electronic Tables and Formulas for American Wire Gauge. As you might guess, the rated ampacities are just a rule of thumb. In careful engineering the voltage drop, insulation temperature limit, thickness, thermal conductivity, and air convection and temperature should all be taken into account. The Maximum Amps for Power Transmission uses the 700 circular mils per amp rule, which is very very conservative. The Maximum Amps for Chassis Wiring is also a conservative rating, but is meant for wiring in air, and not in a bundle. For short lengths of wire, such as is used in battery packs you should trade off the resistance and load with size, weight, and flexibility. NOTE: For installations that need to conform to the National Electrical Code, you must use their guidelines. Contact your local electrician to find out what is legal!”
Assuming the max current for the propeller (300 ma.), I find myself BETWEEN these two specifications and it would seem that much depends on how you interpret “bundle”.
Because ribbon cable conductors share heat dissipation properties with adjacent conductors, the ribbon appears (to me) somewhat like a bundle (thus lowering the ampacity rating).
Also, one of my concerns is with the bypass capacitors being “starved”. I am using two X7R MLC capacitors on the foil side directly under the propeller DIPs. Even though the conductors may meet an ampacity rating, at high switching frequencies might there be problems if the replenishing current requirement can not be met? (This is how I think of these things. Am I way off on cloud 9?)
In addition, how would you select capacitors for these lines? I am thinking I would need a tantalum at the entry point on the slave PCB (which would be right by the end of the propeller DIP. Also, are 6 inch power distribution lines something to be specially treated? I do not have a gut feel for how critical the lengths of conductors are when we jump off board going to another board.
I would greatly appreciate the thoughts of anyone who can shine some light on these issues. Thanks in advance.
Tom
Comments
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Links to other interesting threads:
· Home of the MultiBladeProps: TriBlade,·RamBlade,·SixBlade, website
· Single Board Computer:·3 Propeller ICs·and a·TriBladeProp board (ZiCog Z80 Emulator)
· Prop Tools under Development or Completed (Index)
· Emulators: CPUs Z80 etc; Micros Altair etc;· Terminals·VT100 etc; (Index) ZiCog (Z80) , MoCog (6809)·
· Prop OS: SphinxOS·, PropDos , PropCmd··· Search the Propeller forums·(uses advanced Google search)
My cruising website is: ·www.bluemagic.biz·· MultiBlade Props: www.cluso.bluemagic.biz
Having said that, I've got hooked on insulation displacement headers for ribbon cable. Pop them in the vice and squeeze and the connections are all done. Polarised so no worries about plugging in the wrong way. I've also got hooked on the great prices from futurlec, and the smallest header futurlec sell is 10 pins.
So for SPI, you could parallel up lots of wires if you used a 10 way header.
But for 6 inches, single wires would be fine.
Are you using SIL headers (plugs and sockets)?
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www.smarthome.viviti.com/propeller
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April, 2008: when I discovered the answers to all my micro-computational-botherations!
Some of my objects:
MCP3X0X ADC Driver - Programmable Schmitt inputs, frequency reading, and more!
Simple Propeller-based Database - Making life easier and more readable for all your EEPROM storage needs.
String Manipulation Library - Don't allow strings to be the bane of the Propeller, bend them to your will!
Fast Inter-Propeller Comm - Fast communication between two propellers (1.37MB/s @100MHz)!