RS-232 signals left floating?

sunblock

I am currently cleaning up, and trying to figure out a small ExpressPCB project hat was left on our facilities work bench by a guy who decide he liked California over Virginia and moved.
The board has jumpers for several of the RS-232 signals between two uControllers, (i.e. RTS), where in one position the jumper sends RTS through to CTS on the corresponding uController.
The other position of the jumper goes to a position on the board where something was obviously not soldered yet.
A note say that that position was to "ground" the signal, instead of leaving it floating.
Does this make sense to anyone?
And if so, what might the item be that is missing?
I know the output of the RTS pin is 3V3.
I've always left the control signals for RS-232 floating... maybe incorrectly so?

VR/JW-Sunblock

Phil Pilgrim (PhiPi)

Most RS232 transceivers are "fault-proof," in that the RS232 inputs are biased to ground or to the negative supply (marking). So it should not be necessary to ground them separately. On the logic side of the transceiver, however, you will probably have to ground a handshaking signal to assert it on the RS232 side, if it's not otherwise being used for handshaking. This can be done through a resistor to Vss.

-Phil

kwinn

Generally not a good idea to leave input signals floating, but if the software does not use that signal it should not create a problem. A minimal serial connection between two systems would be gnd>gnd, tx>rx, rx>tx.

sunblock

Phil Pilgrim (PhiPi) wrote: »

Most RS232 transceivers are "fault-proof," in that the RS232 inputs are biased to ground or to the negative supply (marking). So it should not be necessary to ground them separately. On the logic side of the transceiver, however, you will probably have to ground a handshaking signal to assert it on the RS232 side, if it's not otherwise being used for handshaking. This can be done through a resistor to Vss.

-Phil

Thank you Phil! I've looked at thing again, with glasses this time, and I looks like the position of the jumper that would "Ground" the RTS output signal would go to a LED (little bag of red LEDs were taped to the project board). At further inspection I think the intention was that if the RTS signal was no passed through to CTS, it would be routed through an LED for visual inspection and then to ground? Does that make sense? I measure it at 2V5... There is no documentation on the LEDs, and there are no resistors. This is like doing SUDOKU on the last hour of a 13 hour flight from Korea.

Phil Pilgrim (PhiPi)

sunblock,

You will need to be much clearer about which side of the transceiver you're referring to as input or output, the logic side, or the RS232 side? A schematic would be even more helpful.

-Phil