Re: RS232 treshold revisited


"René" <rjz~nospam~@xs4all.nl> wrote in message
news:nmda12hcg0nlom2dr3gu2qaldplkrv4ukt@xxxxxxxxxx
I need to design a PC compatible RS232 I/F in a very small, battery
operated, cheap device. (isolated housing) 1200bd speed. RX / TX only.

The "royal way" would be obvously to use some MAX232 derivative, but
the battery drain budget (1 year out of a CR2032), BOM budget and
space budget make that highly unattractive.
Question. Is the RX line always going to be available, when the TX is
required?. RS232, is never going to be a 'low power' interface. You need
to be able to drive at least a 3K load, with +/-3v (at the receiver -
supposedly +/-5v at the transmitter), and given that the
idle condition is 'driven', you are talking a minimum of over 3mA...

As I see it I am left with 2 alternatives:

1. Hook the device ground to the minus voltage of a RS232 gate ,
and have a transistor + collector resistor switch between + and -
RS232 voltages at the collector. (or use a fet)
(use 2 any non-rx/tx RS232 pin to get steady + and - voltage out of
the PC, set dedicated RS232 PC driver to provide these voltages in a
steady mode)

Advantage: Full RS232 swing available
Disadvantage: device grounded to neg RS232 voltage, unsolid ground.

2. Use TTL method, switching between pos value (derived from
steady state RS232 pin), and true RS232 ground. Here it is assumed
most (all?) PC RS232 I/F's actually switch at TTL level tresholds.

Advantage: solid ground
Disadvantage; will not work if treshold is close to, or below 0V.

Knowing full well that both methods are sub-optimal and non-compliant,
I lean towards method 2.

Question: On which PC's will I run into trouble? Any other low cost /
low energy solutions? What's with Apple PC's?
Look at the DS275. Close to compliant, and the solution used in most low
power devices. It steals the -ve voltage, from the incoming RX line, and
uses this to drive the TX. However even with this (or your solutions), you
should consider using a protocol at the PC end, which accepts 'break'
between packets, and resynchronises when the packet starts, and then have
your driver switch off when not actually sending. Otherwise, even with a
low power solution, a CR2032, will run down too fast.

Best Wishes



.

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