Electronic Design

Add A Dual-Port COM Switch To Your PC

Did you ever wish you had another COM port on your PC without needing an additional COM card? This article describes a simple RS-232 electronic switch that splits one COM port into two. By toggling the RTS hardware line, the Dual Port COM Switch circuit will electronically switch between two devices connected to a single COM port.

RS-232 transmit, receive, and RTS from the PC connect to the Dual Port COM Switch circuit via connection to IC-1 (see the figure). IC-1 is a MAX232A RS-232-to-TTL converter chip with an onboard ±10-V charge-pump power supply. The transmit line and RTS line from the PC are converted to TTL logic levels via IC-1. If the RTS line is set to logic HIGH, it will pass the converted TTL transmit signal to IC-2 via the NAND gate IC-4. IC-2, another MAX232A chip, then conditions the TTL transmit signal back to the appropriate RS-232 driver voltages for the device connected to Port B. The receive signals from Port B, in turn, also are NAND-gated, via IC-4, back to IC-1 and the PC.

A logic level LOW on the RTS line routes the transmit and receive signals in a similar fashion to and from the PC and Port A. Indicator lights LED1 and LED2 provide a simple visual indication of which port is active.

Switching between Port A and Port B, using Microsoft Visual Basic, is a simple matter of setting the COM control’s RTS property to True or False:

COM.RTS = True ;connect PC to Port B
COM.RTS = False ;connect PC to Port A

The switch is activated using Microsoft QuickBasic (see the listing).

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