[time-nuts] Mounting GPS Antenna on Steel Roof

[Bruce Griffiths]

Matthew Smith wrote:
> Quoth Hal Murray at 2008-03-06 20:07...
>   
>>> I will be doing (4) as soon as I've found the time to make up a TTL to
>>> RS232 board.
>>>       
>> I suggest trying it without anything fancy.
>>
>> All the GPS toys I've tried work fine without any conversion gear.
>>
>> The normal TTL to RS-232 chips include an inverter.  That will break things 
>> unless you add another inverter.  (or your gear expects an inverter)
>>
>> The usual switching point for RS232 receiver chips is 1.4 volts.  That's the 
>> switching point for TTL inputs, 2 diode drops above ground.  That works fine 
>> for 3V CMOS drivers.
>>
>>
>>     
>
> 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!
>
> Cheers
>
> M
>
>   
Matthew

All that has been stated is that the receiver modules wont be damaged 
when driving an RS232 input.

However when an RS232 input drives a 3V CMOS input there are no guarantees.
In the latter case a series resistor at the CMOS input can be used to 
guarantee the COS gate isnt destroyed.

In general it would be better to use a CMOS - RS232 converter chip, it 
can be socketed and used as a fuse (considerable cheaper than using the 
GPS receiver outputs as a fuse) should anything untoward occur.

Also CMOS devices arent rated to continuously drive a short circuit a 
series resistor to limit potential fault currents to the datasheet 
maximum is advisable.

Bruce