Pin 5

Let me start with a lovely advert making the rounds here in the UK about a series of laptop. The punch line could never be closer to the truth.....

"This baby is bad, very very bad!"

Although I do honestly believe they were not intending it to be interpreted this way, I have never found a company actually admit that their laptops were not to be considered in earnest!

It has been found on a very high number of predecesors that Pin 5 on the Com port is open circuit. This leads to communications failures between the laptop and any device connected to the port, especially when the connected device features an optically isolated RS232 connection. The outer case of the Com port is, however, grounded. Connecting Pin 5 to the case within the lead would solve the problem on laptops but a problem then exists when using this same lead on desktop PCs.

Placing a short between Pin 5 and the case on a normal PC would cause high currents through the motherboard as the case is by no means the 0V found on the motherboard. Personal experience of having to repair an installation where a PC kept locking up turned out to be this very short between pin 5 and the case (put on by an overeager technician).

new ground connection in lead

However, no more than about 0.1V has ever been found between Pin 5 and the case. Using, as suggested, a minimum of 10 ohms per leg (thus 20 ohms total) would result in a maximum current of 5mA. It is extremely doubtful that this would upset anything. Many installations, including emergency control rooms, have been fitted with this modification without any problems caused. It does, however, also ensure that no matter what is connected on the port, be it only the case, Pin 5, or both, and be it PC or laptop, the signal ground is now available.

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© 07.01.01