Products > Test Equipment
open source GPIB adapter
dazz1:
Hi
Looking at the detail of the gpib cable standard to apply to my breakout board, the pin-out includes a shield ground plus signal grounds.
My question is: Should the shield ground be connected to the signal grounds?
The IEEE 488.1 mechanical specification costs a $squillion ( = $USD391.00) so not looking there.
caiser01:
--- Quote from: dazz1 on April 07, 2024, 08:19:10 am ---Hi
Looking at the detail of the gpib cable standard to apply to my breakout board, the pin-out includes a shield ground plus signal grounds.
My question is: Should the shield ground be connected to the signal grounds?
The IEEE 488.1 mechanical specification costs a $squillion ( = $USD391.00) so not looking there.
--- End quote ---
I do have access to a copy of IEEE 488.1 and regarding grounding, it says:
--- Quote ---5.6 Ground requirements
The overall shield of the interconnecting cable shall be connected through one contact of the connector to frame (safety earth) to minimize susceptibility to and generation of external noise.
WARNING—Devices should not be operated at significantly different frame potentials. The interface connection system may not be capable of handling excessive ground currents.
It is recommended that the ground returns of the individual control and status signal lines be connected to logic ground at the logic circuit driver or receiver to minimize cross-talk interference transients.
--- End quote ---
Based on that, I would answer your question with a 'No'. It sounds like the intent is to connect the cable shield to the safety earth ground of an instrument or controller (assuming it has a safety earth ground).
In your case, since this is a breakout board for debugging purposes and is essentially passive, like a cable, I would say just make sure the shield ground connects through the breakout from one GPIB connector to another. Do not permanently connect any grounds together on the breakout that would normally run separately inside a cable.
caiser01:
For completeness, it may also be worth noting the maximum resistance per meter for each conductor and insuring the design of your breakout doesn't cause a violation of these requirements:
--- Quote ---5.7 Cable characteristics
5.7.1 Conductor requirements
The maximum resistance for the cable conductors shall be, per meter of length
a) Each signal line (for example, DIO1, ATN) 0.14 Ω
b) Each individual signal line ground return 0.14 Ω
c) Common logic ground return 0.085 Ω
d) Overall shield 0.0085 Ω
--- End quote ---
dazz1:
--- Quote from: caiser01 on April 08, 2024, 12:24:03 am ---
Based on that, I would answer your question with a 'No'. It sounds like the intent is to connect the cable shield to the safety earth ground of an instrument or controller (assuming it has a safety earth ground).
In your case, since this is a breakout board for debugging purposes and is essentially passive, like a cable, I would say just make sure the shield ground connects through the breakout from one GPIB connector to another. Do not permanently connect any grounds together on the breakout that would normally run separately inside a cable.
--- End quote ---
The board by itself is passive, but it will be connected to the logic analyser I don't have, or my HP 54654D logic input. I am including a header that will allow the cable to be plugged into the board, no probes. The probe adapter for the cable has one earth. So I can separately route all of the individual earths that are supposed to be separate in the cable, but I still need an earth for the scope. The most logical gnd being the Logic gnd at pin 24. Does that sound right?
dazz1:
After a little more thought:
The intended use is to plug-in directly to the subject equipment. It is very likely that the equipment will have all earths connected. Repeating that connection at the gpib panel connector is probably not going to be enough of an issue to stop the breakout board doing the job it needs to do.
Also I suspect the average 1980's/90's engineer would not have paid for the standard and would likely simply connected all of the earths together. The exception might be the shield. Somewhere I read this should be linked to the other earths by a 1M resistor. Maybe to keep the dc floating volts within reasonable limits but that is not a convincing argument.
I could include a DIL2x8 header to selectively link earths to a main ground but it would actually be less work to produce a new Mod 2 pcb design.
So the plan is to connect all the grounds together, except the shield. I will run that straight through with a Fit-For-But-Not-With option for a smd resistor, just in case.
If there is a problem, I will design a new pcb.
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