wbc@moose.dartmouth.edu (Wayne B. Cripps) (01/30/91)
What should the voltage be on thinnet? - I get readings of -1.8 to -2.0 volts and .2 to .3 volts - is this in the range? is 1.2 volts ok? Wayne
cliffb@isavax.isa.com (cliff bedore*) (02/05/91)
In article <1991Jan30.155606.21529@dartvax.dartmouth.edu> wbc@moose.dartmouth.edu (Wayne B. Cripps) writes: > > >What should the voltage be on thinnet? - I get readings of >-1.8 to -2.0 volts and .2 to .3 volts - is this in the >range? is 1.2 volts ok? > > Wayne At the risk of stepping into something soft and gooey, I thought I'd put on my engineers hat for a while and comment on this. First. It will be very traffic dependent (assuming you're using a voltmeter that does some averaging). Having stated that and not knowing the details of an ethernet board, but knowing something about transmission lines, we can get a wag of ranges for the voltages. The lines are 50 ohms and are terminated in 1/4 or 1/2 watt resistors. (Mine is cause I did it myself and haven't had problems). Power (watts) = voltage ^2 / resistance or voltage = sqrt( power * resistance) voltage = sqrt ( 1/4) * 50 ) or 3.5 volts. for 1/4 watt power voltage = sqrt ( 1/2 * 50 ) or 5 volts for 1/2 watt power These are for steady state DC levels. Ethernet is 10mbit/sec square waves +- distortion from the coax. This means the the voltage measures via some sort of averaging/rms/other meter would be something less (depending upon the actual waveform, activity on the cable etc. (This is true for thick or thinnet. the impedance on the cable is the same) This the voltages you measure will probably be in the range for correct operation, but without looking at the waveform, you could have a short with some DC voltage giving you the same result. Cliff
jb@falstaff.mae.cwru.edu (Jim Berilla) (02/06/91)
In article <1991Feb5.131333.2047@isavax.isa.com> cliffb@isavax.isa.com (cliff bedore*) writes: >In article <1991Jan30.155606.21529@dartvax.dartmouth.edu> wbc@moose.dartmouth.edu (Wayne B. Cripps) writes: >> >> >>What should the voltage be on thinnet? - I get readings of >>-1.8 to -2.0 volts and .2 to .3 volts - is this in the >>range? is 1.2 volts ok? >> >> Wayne > >At the risk of stepping into something soft and gooey, I thought I'd put on >my engineers hat for a while and comment on this. > >First. It will be very traffic dependent (assuming you're using a voltmeter >that does some averaging). True. Don't use a voltmeter, use an oscilloscope. You'll see many interesting things. >Having stated that and not knowing the details of an ethernet board, but >knowing something about transmission lines, we can get a wag of ranges for >the voltages. Not true. The voltages on the ethernet are clearly defined. >The lines are 50 ohms and are terminated in 1/4 or 1/2 watt resistors. (Mine >is cause I did it myself and haven't had problems). > >Power (watts) = voltage ^2 / resistance or > >voltage = sqrt( power * resistance) > >voltage = sqrt ( 1/4) * 50 ) or 3.5 volts. for 1/4 watt power > >voltage = sqrt ( 1/2 * 50 ) or 5 volts for 1/2 watt power What *are* you talking about? (And take off that silly hat.) In the case of ethernet, the voltage depends on the amount of current pulled out of the tap. It's independant of the power rating of the terminating resistors. Remember that the tap appears as a 25 ohm load, i.e. it's connected to two 50 ohm transmission lines. For the AM7996 transceiver (common in a lot of Sun's), the voltages are specified as follows: High level is between 0 and -.1 volts, low level is between -1.625 and -2.2 volts. As stated above, this voltage is generated by a current sink (to -9V) by the chip. An ideal current sink has infinite impedance, and doesn't load the transmission line. If two or more stations transmit at the same time, the voltage on the line goes below -2.2 volts. The chip detects collisions on this basis. >These are for steady state DC levels. Ethernet is 10mbit/sec square waves >+- distortion from the coax. This means the the voltage measures via some >sort of averaging/rms/other meter would be something less (depending upon >the actual waveform, activity on the cable etc. (This is true for thick or >thinnet. the impedance on the cable is the same) > >This the voltages you measure will probably be in the range for correct >operation, but without looking at the waveform, you could have a short with >some DC voltage giving you the same result. You can use a meter for a few tests on the ethernet, but it's very limited. On an inactive network, an ohmmeter accross the tap will read 25 ohms. If there are any stations transmitting, the reading will be jittery. If the network is shorted, it will read close to zero ohms. If a terminator is bad, or there is a break in the line, it will read 50 ohms. A voltmeter might tell you something, but I wouldn't use it if there's a scope around. -- Jim Berilla / jb@falstaff.cwru.edu / 216-368-6776 "My opinions are my own, except on Wednesday mornings at 9 AM, when my opinions are those of my boss."
cliffb@isavax.isa.com (cliff bedore*) (02/08/91)
In article <1991Feb6.033424.21632@usenet.ins.cwru.edu> jb@falstaff.mae.cwru.edu (Jim Berilla) writes: >In article <1991Feb5.131333.2047@isavax.isa.com> cliffb@isavax.isa.com (cliff bedore*) writes: >>In article <1991Jan30.155606.21529@dartvax.dartmouth.edu> wbc@moose.dartmouth.edu (Wayne B. Cripps) writes: >>> >>> >>>What should the voltage be on thinnet? - I get readings of >>>-1.8 to -2.0 volts and .2 to .3 volts - is this in the >>>range? is 1.2 volts ok? >>> >>> Wayne >> >>At the risk of stepping into something soft and gooey, I thought I'd put on >>my engineers hat for a while and comment on this. >> >>First. It will be very traffic dependent (assuming you're using a voltmeter >>that does some averaging). > >True. Don't use a voltmeter, use an oscilloscope. You'll see many interesting >things. > >>Having stated that and not knowing the details of an ethernet board, but >>knowing something about transmission lines, we can get a wag of ranges for >>the voltages. > >Not true. The voltages on the ethernet are clearly defined. I didn't say they weren't clearly defined. I said we could get a range of what was reasonable. We engineers call this a worst case analysis. > >>The lines are 50 ohms and are terminated in 1/4 or 1/2 watt resistors. (Mine >>is cause I did it myself and haven't had problems). >> >>Power (watts) = voltage ^2 / resistance or >> >>voltage = sqrt( power * resistance) >> >>voltage = sqrt ( 1/4) * 50 ) or 3.5 volts. for 1/4 watt power >> >>voltage = sqrt ( 1/2 * 50 ) or 5 volts for 1/2 watt power > >What *are* you talking about? (And take off that silly hat.) >In the case of ethernet, the voltage depends on the amount of current >pulled out of the tap. It's independant of the power rating of the >terminating resistors. Remember that the tap appears as a 25 ohm load, >i.e. it's connected to two 50 ohm transmission lines. > What I am talking about is how much voltage you can put through a 1/4 watt 50 ohm resistor without exceeding its power limits. Not circuit impedance! The above analysis is true. If you put 2 50 ohm 1/4 watt resistors in parallel,you get a 25 ohm 1/2 watt resistor. This reduces the resistance but keeps the power dissipation the same so the maximum voltage will be the same. (Ohms Law; He was a hero to us engineers) The 2nd calculation was assuming the specs call for 1/2 watt terminators. The idea of this was to get an idea of what would be the max voltage you should see on an ethernet line using a voltmeter. Given that the above calculations do that, I'll keep my hat on; thank you very much. The point is sometimes circuits don't pull current, they get pushed voltage. If the voltage-current combination (power) exceeds the rating of the resistor, it will burn up. If you think this isn't important get a 50 ohm resistor (47) from Radio Shack and plug it into your 110 outlet (but wear eye protection and gloves). Or run 15-20 volts down your thinnet and be prepared to look for a new job >For the AM7996 transceiver (common in a lot of Sun's), the voltages are >specified as follows: High level is between 0 and -.1 volts, low level >is between -1.625 and -2.2 volts. As stated above, this voltage is >generated by a current sink (to -9V) by the chip. An ideal current sink >has infinite impedance, and doesn't load the transmission line. If two >or more stations transmit at the same time, the voltage on the line goes >below -2.2 volts. The chip detects collisions on this basis. Had you been as quick to answer the original question as you were to critcize my answer, we could have saved a lot of bandwidth on the net. I waited 4 days to see if someone would post an answer. Since no one did, I gave the above estimate of what would be reasonable. . . > Jim Berilla / jb@falstaff.cwru.edu / 216-368-6776 >"My opinions are my own, except on Wednesday mornings at 9 AM, > when my opinions are those of my boss." In spite of my above comments, I do appreciate knowing what the true voltages are for ethernet. I've filed them away for the next time the question comes up. Cliff