Re: RS485 - MODBUS or PROFIBUS or ....

On 24 Jul 2009 14:28:01 -0500, bob <bob@xxxxxxx> wrote:

Looks like alll RS-232 transmitters transmit + and -
voltage levels, but hardly any of them xmit higher than +- 6 Volts
these days.

The RS-232 standard was designed in the 1960´s to connect
electrotechnical Teletypes (and early VDUs) to a modem and transfer it
over existing telephone network to a central computer along with local
devices.

The modem was usually in the adjacent rack from the computer or
Teletype, so the 15 m maximum distance was more than adequate and 20
kbit/s was more than even any baseband modem could carry over
telephone wires.

Typical current loop constant current sources had an open circuit
voltage of 24-60 V and telephone circuit voltages are in the order of
40-60 V range. No wonder that the allowed voltages were so high. The
minimum voltage levels are so high, that you could even drive directly
some reed relays :-).

The power dissipation was not an issue with the low speed, short low
capacitance cable connections and since discrete semiconductors and
resistors were used, it was easy to dissipate any power losses.

The situation is quite different today, quite long cables are often
used with a high cable capacitance. At each output transition, the
cable capacitance must be charged or discharged through the
transmitter to the corresponding supply voltage, in which some energy
is dissipated to heat in the transmitter.

With the current much higher line speeds used, these transitions occur
much more frequently charging and discharging the line capacitance
more frequently and hence dissipating a much larger average power in
the transmitter. With integrated circuits and even with multiple
transmitters on a same chip, the power dissipation would simply kill
the chip if driving high capacitive loads at high speed using a high
supply voltage.

Looking at the situation at the receiver, the important thing is that
the transition between -3 V to +3 V at the receiver is fast enough,
compared to the line speed used. The transmitter is required to
provide sufficient current in this voltage range to charge or
discharge the distributed line capacitance.

The problem with the cable distributed capacitance and distributed
resistance is that the initial transmitter output voltage swing must
be larger than +/-3V in order to achieve the +/-3 V voltage swing at
the receiver in a realistic (non-infinite) time.

Once the voltage at the receiver is outside +/-3 V, there is not much
point in feeding large amounts of current into the line capacitance,
so it is sufficient to feed the very small current consumed by the
high input impedance receiver in the steady state.

Clearly, the transmitter supply voltage must be larger than +/-3 V
even assuming zero voltage loss switching to charge the cable
distributed capacitance through the cable distributed resistance to
achieve the +/-3 V swing at the receiver in reasonable time (say t=RC
time constant).

The +/-6V transmitter voltage swing is a good compromise, allowing at
least 20 kbit/s into a low capacitance (<15 m) cable (as required by
the RS-232C standard) with a reasonable power dissipation.

Paul

.

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