How to connect ground lines? 1

[raivo]

I have three microcontroller boards connected between two devices. Data flows through these boards from one device to another. Some calculations and transformations are done with data flow.

I do not go to details as this probably is not important. For testing i used LPT port of PC 1 as sending device and LPT port of PC2 as receiving device. (Boards between them.)

All works but with soft errors. Error rate depends greatly on placing ground wires and sometimes i can send more than 200Mb from one PC to another with no errors at all but when changing placement of boards and resoldering some power or ground wires to other points in board, error rate may increase to tens errors for one megabyte. Communication speed is 30 kilobytes to 300 kilobytes per secound and it seems that error rate does not depend on communication speed.
For constant amount of time there will be aproximately same amount of errors if ground and power line placement is not changed.

I am well familir with digital electronics where only ones and nuls exist but here i do not grasp the rules. Can anyone expalin me how ground and power lines must be connected to get multichip / multiboard system to run most reliably? What other things i have to consider? I currently use only trial and error for best placement and if i once get it working it works well as long as i do not change something in boards.

I suspect that there are more professional ways to solve such problems. Any tips and ideas are welcome!

 

[IRstuff]

You MUST maintain a signal ground reference for each signal that crosses a device boundary.

Different circuits can result in different ground potentials, wreaking havoc with signal noise margins.

You might have to replace your existing transceivers if they've been damaged by any common-mode voltage differences between the two devices. Typical 422 transceiver can only tolerate a 7VDC common-mode, and are extremely susceptible to power-on transients that affect the ground potentials which are then equalized through the transceivers, if you have no signal reference grounds connected.



TTFN

 

[rodar]

Hi
Every data line should be twisted with its own ground line.
At 200 MB/sec may have to go to coax for each line.

Buy "High Speed Digital Design a Handbook of Black Magic"
by Howard Johnson.

The book is worth every penny even if you don't do high speed design.

Rodar

 

[BrianR]

In general connect the power lines so they radiate like octopus arms from the PS to each load. Do not daisy-chain load after load. Use twisted pair power/return for each voltage and if there is a ground connection needed then that should preferrably be at the power supply end only.

As Rodar said, signal lines should be twisted pair too and if serious shielding is needed then a twisted pair inside a shield is very good. For high frequency signals with this connection, ground the shield at both ends but do not use the shield for either power or signal return. It's function is to prevent pickup of external magnetic interference which is the most common cause of interference.

If there are still issues related to where you connect to a controller board, then consider each location in the light of avoiding common mode power and signal connections. Don't tempt the power to travel on the same return as the signal as you are sure of interference then.