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GM7HUD > TECH 20.12.05 11:45l 41 Lines 2108 Bytes #999 (0) @ WW
BID : 71545_GB7ESX
Read: GUEST DL1LCA OE7FMI
Subj: re: A puzzling fault
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Sent: 051220/0823z 71545@GB7ESX.#31.GBR.EU $:71545_GB7ESX [Witham, Esx]NNA V3.1
Regard RS232 signalling levels derived from charge pump circuits....
Dont for get that RS232 specs demand that the signal is less than -3V ot
greater than +3V. So you dont really need much voltage swing.
In theory that is. In practice you may need a bit more "oomph" if you try
slewing the lines at high speeds (115kbps etc.)
Talking of charge pumps, devices like MAXIM's MAX232 series normally
use 10uF caps in the charge pump, but you can nearly always get away with
0.1uF caps. The voltage available is lower but it means you can build the
charge pump circuit on a smaller board.
Of course if your junk box is like mine then when you go looking for 4x10uF
caps the only ones you can find are 10uF 450V monsters and using 0.1uf
decoupling caps is the only option :-)
As a final point, companies like MAXIM, Linear Technology and Dallas are
a delight to deal with. They have comprehensive websites that dont restrict
access to any of their data and best of all you can place small orders
directly with the company rather than having to find a distributor who
#will still process small orders. Chips like the MAX232 are easy to find at
hobby suppliers but the more esoteric devices like the MAX3232 (3.3v not 5v)
is harder to get. Likewise if you need one in SMD to fix something.
This is when companies like the 3 above shine, they'll ship you 1 or 2 often
with no carriage charges in many cases. Of course you can get free samples
but dont be a cheapskate and abuse such services!
It's quite amusing that now in 2005 we are still mucking about with RS232.
Think about it... a standard from the middle 60s when +/=15V supplies were
common. Nowadays we have a CPU often running at 1.5V which needs gubbins to
shift its IO levels to 3.3v or 5V. Then we shift those signals to something
around +/- 10V for RS232 signalling. At the remote end another chip drops
them down to 5V or 3.3V followed finally by shifting to core levels.
On battery powered gizmos... that's a lot of expense to achieve a slow
error prone signalling scheme. That's why USB is just sooooooooo much better!
73 de Andy GM7HUD
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