This article presents a simple yet highly practical circuit that allows two RS232 terminals to operate in parallel on a single serial host interface – without requiring any additional power supply.

The circuit has proven to be reliable and universally applicable through many years of practical use in the laboratory. Besides its use within the O-M-S-U system, it is equally suitable for a wide range of other applications involving classic RS232 interfaces, particularly for service, diagnostic and debugging purposes.

The entire circuit consists of only four diodes and a single resistor, resulting in an extremely simple, robust and easily understandable solution.

Illustration: J. Lange (AI-generated)
 

 

Circuit Description

The complete circuit is shown in Figure 1.

At the heart of the circuit is a passive isolation network consisting of four diodes (D1 to D4) and a high-value resistor (R1). The purpose of this arrangement is to electrically isolate the transmit lines of two RS232 terminals while allowing both terminals to remain connected simultaneously to the host's receive input.

The terminals are connected via connectors JP2 and JP3, while the host is connected through JP1. The ground lines (GND) of all participants are directly interconnected, providing a common reference potential.

Function of the Diode Matrix

Diodes D1 to D4 are arranged to provide bidirectional isolation between the two terminal outputs. When one of the terminals transmits data, the signal is routed to the host through the corresponding diode path. At the same time, the remaining diodes prevent the signal from feeding back into the transmit line of the second terminal. This effectively prevents direct contention between the RS232 output drivers.

Only a single diode is present in the active signal path. The resulting voltage drop of typically 0.6 V to 0.7 V is negligible compared with the signal levels commonly used by RS232 interfaces.

The Role of Resistor R1

Resistor R1, with a value of 100 kΩ, performs an important protective function within the circuit.

It provides a defined high-impedance balancing path between the two sides of the diode matrix. This becomes particularly important when both connected terminals simultaneously drive different voltage levels, for example +12 V and −12 V.

Without suitable isolation, a significant equalisation current would flow between the output stages under these conditions. Resistor R1 limits this current to a harmless level, thereby preventing excessive loading of the RS232 drivers involved.

Due to its high resistance value, the influence on the actual signal levels can be considered negligible.

A Thought Experiment: Opposing Output Levels

To illustrate the function of R1, let us consider a critical operating condition.

Assume that Terminal 1 drives an output level of +12 V, while Terminal 2 simultaneously outputs −12 V. This results in a potential difference of 24 V between the two output stages.

Under these conditions, the diode matrix would create a current path connecting the two RS232 drivers. Without additional protection, a substantial equalisation current would flow, approaching a short-circuit condition between the two output stages.

This is where resistor R1 comes into play.

Located in the current path between the two sides of the diode matrix, it limits the current to a harmless level. Instead of an almost short-circuit-like current flow, only a small and well-defined equalisation current remains.

This current can easily be estimated:

This effectively protects the output drivers without impairing the operation of the circuit.

The O-M-S-U CPU is equipped with a female DE-9 connector and internally implements crossed signal routing. This allows standard USB-to-RS232 adapters to be connected directly without requiring an additional null-modem cable.

The adapter presented here has been designed accordingly.

The host interface (JP1) is implemented as a 9-pin DE-9 male connector and plugs directly into the CPU. The two terminal interfaces (JP2 and JP3) are implemented as 9-pin DE-9 female connectors, allowing standard USB-to-RS232 adapters to be connected directly.

Signal routing within the adapter has intentionally been kept simple.

JP1-TXD (Host DE-9 Pin 2, male) is connected directly to JP2-RXD (Terminal 1, DE-9 Pin 2, female) and JP3-RXD (Terminal 2, DE-9 Pin 2, female). This allows the host output to be distributed simultaneously to both terminals.

The transmit lines of the two terminals, JP2-TXD and JP3-TXD (DE-9 Pin 3, female), are connected to opposite sides of the diode matrix and combined at JP1-RXD (Host DE-9 Pin 3, male).

This arrangement allows both terminals to send data to the host without interfering with each other.

The ground connections (DE-9 Pin 5) of all connectors are directly interconnected, providing a common reference potential.

Application of the Circuit

The solution presented here is not intended as a standards-compliant extension of an RS232 interface, but rather as a deliberately simple passive coupling arrangement for practical applications.

Its main advantages are the minimal component count, the complete absence of an additional power supply and its robustness against wiring errors. This makes the circuit particularly suitable for service, testing and diagnostic environments, such as those frequently encountered within the O-M-S-U system.

The basic concept behind this circuit is not new and has been described in similar forms in various applications. A comparable implementation was documented on the website of Thomas Krueger, which, unfortunately, was no longer available at the time this article was written (May 2026).

A compact 3D-printable enclosure is available for this project and can be freely downloaded from the accompanying project files.

The enclosure data is provided in STL format, allowing it to be adapted for use with virtually any common 3D printer.