RC Time query and implementation
skylight
Posts: 1,915
I was thinking that it would be a cool project to create a Multicore cable tester using the BS2.
Any good·tester would need to test for continuity between pins, show pin configuration ie is the cable 1 to 1,·identify any shorts between pins and finally show resistance of each core to show up any bad joints or if cable length is a problem.
My initial thought is·of using two I/O's per core say P0 as an output and P1 as an input with the core under test between them (a BS2's 16 I/O's giving the capability of testing up to an 8 core cable), obviously as this is a wire the resistance would hopefully be very low and therefore i'd make sure of a 220 ohm resistor connected to each output pin or would it be better to have a 110 ohm resistor( is there such a thing?)·on each output and input pin being connected by the core under test in series to give 220 ohms?
This setup I believe would work ok for straightforward continuity and short testing and also cable pin configuration ie 1 to 1 or crossover.
Programming would make sure only one core is "live" at any one time in order to isolate cores for the testing of any shorts or even intentional connections·ie an output would show up on multiple inputs if more than one of the cores are connected and a confirmation on a debug screen or LCD would ask if the cable is wired that way or not.
The problem i forsee is when you come to reading resistance, the circuits shown in the Basic Stamp reference·Manual under the RCTime function page (pdf page 368 fig 5.33) show the RC circuit connected between one I/O pin and ground this would mean several cores would have to be grounded at one end and not good for short testing purposes so would it be ok to imitate the ground connection per each core using the ODD numbered pins ie in the above P0 was an output and P1 an input could P1 under a subroutine call called "resistance" change from an input to an output and made LOW and P0 made high for the Capacitor charging and then an input to read the change from 1 to 0 as discharge takes place.
Any reading would have to subtract the 220 ohms in circuit to gain the actual value of resistance, I presume·I'd do this by finding the RCTime for the 220 ohm resistor first and subtract that from the total RCTime. I'm not quite understanding how to convert RCTime back into Ohms though at the moment in order to display it and would be grateful for help on this.
Sorry for the long post but I would graciously accept any thoughts as to whether firstly its viable using the BS2 and debug screen , or if using an LCD display i may have to go for the BS2pe for the extra I/O's and to whether i have overlooked anything that may potentially cause problems with the concept.
·
Any good·tester would need to test for continuity between pins, show pin configuration ie is the cable 1 to 1,·identify any shorts between pins and finally show resistance of each core to show up any bad joints or if cable length is a problem.
My initial thought is·of using two I/O's per core say P0 as an output and P1 as an input with the core under test between them (a BS2's 16 I/O's giving the capability of testing up to an 8 core cable), obviously as this is a wire the resistance would hopefully be very low and therefore i'd make sure of a 220 ohm resistor connected to each output pin or would it be better to have a 110 ohm resistor( is there such a thing?)·on each output and input pin being connected by the core under test in series to give 220 ohms?
This setup I believe would work ok for straightforward continuity and short testing and also cable pin configuration ie 1 to 1 or crossover.
Programming would make sure only one core is "live" at any one time in order to isolate cores for the testing of any shorts or even intentional connections·ie an output would show up on multiple inputs if more than one of the cores are connected and a confirmation on a debug screen or LCD would ask if the cable is wired that way or not.
The problem i forsee is when you come to reading resistance, the circuits shown in the Basic Stamp reference·Manual under the RCTime function page (pdf page 368 fig 5.33) show the RC circuit connected between one I/O pin and ground this would mean several cores would have to be grounded at one end and not good for short testing purposes so would it be ok to imitate the ground connection per each core using the ODD numbered pins ie in the above P0 was an output and P1 an input could P1 under a subroutine call called "resistance" change from an input to an output and made LOW and P0 made high for the Capacitor charging and then an input to read the change from 1 to 0 as discharge takes place.
Any reading would have to subtract the 220 ohms in circuit to gain the actual value of resistance, I presume·I'd do this by finding the RCTime for the 220 ohm resistor first and subtract that from the total RCTime. I'm not quite understanding how to convert RCTime back into Ohms though at the moment in order to display it and would be grateful for help on this.
Sorry for the long post but I would graciously accept any thoughts as to whether firstly its viable using the BS2 and debug screen , or if using an LCD display i may have to go for the BS2pe for the extra I/O's and to whether i have overlooked anything that may potentially cause problems with the concept.
·
Comments
I leave to others the job of figuring out how to measure the resistance, for the expected and desired values of resistance (a small fraction of an ohm) are not going to be in a range easily measured with a Stamp.
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· -- Carl, nn5i@arrl.net