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Maxim > Design Support > Technical Documents > Application Notes > Interface Circuits > APP 3580
Keywords: redundant-transceiver, RS-232, overvoltage protection, max211
APPLICATION NOTE 3580
Redundant-Transceiver RS-232 Link Has ±40V
Overvoltage Protection
Jul 22, 2005
Abstract: Parallel RS-232 transceivers can offer extra reliability in safety-critical applications. But such
redundant transceivers put extra load on the data bus. This application note shows how the problem can
be overcome by buffering the transceivers with a line protector. Overvoltage protection is increased in
the process.
The RS-232 standard is intended primarily for point-to-point communications between one transmitter
and one receiver, but in some circumstances it may be necessary to connect more than one transceiver
to the link. For example, redundant transceivers provide extra reliability in safety-critical applications.
Multiple transceivers load the data bus, however. For instance, if two transmitters—one from each of two
separate MAX211 transceivers—are connected in parallel (to the same link), the unused device (whether
shut down or with power removed) will adversely affect the active device by loading the data bus. Figure
1's circuit avoids this problem while increasing the overvoltage protection from that of the transceivers to
that of the line protectors (±40V).
Each of the parallel RS-232 transceivers (IC1 and IC2) is buffered by a 2-terminal, multichannel line
protector (IC3 and IC4). The line protectors normally exhibit about 60Ω im between each input-output
pair, but that resistance goes to a high impedance if power is removed or if either terminal rises to within
1.5V of a supply rail. The line protectors are powered by charge pumps internal to the transceivers, so
either will lose power if its associated transceiver loses power or is shut down. Thus, an inactive
transceiver is automatically disconnected from the line.
To ensure that the line protectors have sufficient power-supply headroom to accommodate the
transceivers'
±5V transmitter-output specification, external diode-capacitor charge pumps boost the
transceivers' V- outputs to a more negative level. The 100kΩ resistors discharge this negative rail when
the transceiver is shut down or turned off. The system's various states are summarized in Table 1.
Table 1. System States
Tx CONDITION Tx (V+) Tx (V-) LINEPROTECTOR (V-)
Active ~2VCC ~-2VCC ~-3VCC
Shutdown VCC Ground Ground
Power Off Ground Ground Ground
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