>> Here is a circuit I devised recently for a simple RS232 based parallel
>> interface that had only three outputs available.
>THREE outputs with RS232???
>I only know of RTS and DTR. Or do you abuse the TxD line like the Erik-
>Hermann-ultracheap-RS232-programmer?

this is _not_ abuse!

TxD can be forced to any state by setting serial port controller to break.

>> Note for this circuit to work the MCLR line is tied to the system VDD via
>> a diode and 1k resistor on the target board
>That is another point to discuss. The data book says, MCLR must be at least
>0.85*Vdd, that is 3,06V at Vdd=3,6V, the difference is 0,54V.
>Microchip writes that the MCLR pullup resistor should be not greater than
>1k so that the voltage drop at MCLR input with respect to Vdd is smaller than

Microchip databook 1994 page 2-551 (DS30081C-page 17):
Pull-up R < 40KOhms is recommended (look carefully figure 4.2.4)

>a 1k : 10k voltage divider together with a diode? Just calculate yourself.

Microchip _recommends_ a use of diode in series to resistor at MCLR for ISP.
Databook 1994 page 2-567. That circuitry _works_

antti

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-- Antti Lukats                          Silicon Studio --
-- sis@rasi.lr.ttu.ee                    PO Box 3500    --
-- ftp://rasi.lr.ttu.ee/pub/sis          Tallinn EE0001 --
-- http://rasi.lr.ttu.ee/~sis            Estonia        --
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