[time-nuts] Mounting GPS Antenna on Steel Roof

Bruce Griffiths bruce.griffiths at xtra.co.nz
Thu Mar 6 23:51:23 UTC 2008


Chris Kuethe wrote:
> On Thu, Mar 6, 2008 at 3:08 PM, Matthew Smith <matt at smiffytech.com> wrote:
>   
>>  So my Trimble ACE II and Moto Oncore modules will tolerate RS232 levels
>>  without conversion?  Are we absolutely sure on this - I don't want to
>>  fry anything!
>>     
>
> Well, I plugged my Oncore UT+ into my PC and it didn't work until I
> put a level shifter in... no damage but no data either. So far I've
> been lucky in that every device I tried that needed a level shifter
> wasn't damaged by the lack of one.
>
> CK
>
>   
The powered down input resistance of a classic 1489 RS232 receiver is 3k 
minimum.

The classic 1488 RS232 driver has a 10mA short circuit current limit.
To check if a 10mA current flowing in the CMOS output stage protection 
diodes compromises the long term reliability of the GPS receive can only 
be determined by consulting the datasheet for the CMOS output device. In 
any case, such currents flowing in the logic substrate may alter the 
chip timing and timing noise characteristics. For example when the 
inputs of a 74HC04 are overdriven the propagation delay and the 
associated timing jitter change significantly.

Thus although the device may not fail anytime soon due to 10mA currents 
flowing in the input protection diodes, there is usually no guarantee 
that the chip will still operate correctly when significant protection 
network current is flowing.

Bruce




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