On 1/8/2012 6:48 PM, David Woolley wrote: > Mike S wrote: >> On 1/8/2012 5:19 PM, unruh wrote: >> ... >>> >>> Make sure you properly terminate the cable with 100 ohms. Otherwise you >>> will get reflections back and forth along the cable, and the cable will >>> slowly fill with charge, broadening the pulse by a huge amount. It may >>> also be that the signal is decreasing due to resistance in the line. >> >> The proper termination for EIA-232 is 3000-7000 ohms. > > EIA-232 deliberately mis-terminates in order to make the line > capacitive, and slow down the rise times.
Yes, and no. It doesn't treat cabling as a formal transmission line. They do not deliberately "mis-terminate to make the line capacitive," and they limit load capacitance to 2500 pF. The standard says "The maximum instantaneous rate of voltage change shall not exceed 30 volts per microsecond," so that's handled by the transmitter, not by screwing with transmission line characteristics. It also doesn't require the drive current needed to achieve the expected signal levels into 100 ohms. Do so, and all bets are off. Is it better to have some jitter, or no detected signal at all?
> -232 is actually mis-terminated at both ends, but with different impedances.
No, again. Per the standard "The source impedance of the generator side of an interchange circuit including any cable to the interface point is not specified," although it places an upper limit of 100 mA on the driver.
EIA-232 signals aren't intended for transfer of precision timing, so you have to live with what it can do, if you want it to work.
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