Why do internal modems for PCs always have a huge black or white plastic casing covering half of them?

Question

Hi Jeff - thanks for the answer, but that doesn't explain why it's only on modems and anything else connected to the PSTN. Even the most expensive, fragile, complex PCB add-in cards for PCs don't have them on. ..

Answer 1

It contains the isolation circuit that separates the telephone line system from the mains driven PC. It is a requirement that any device connected to the telephone system that is not powered from the telephone line has an approved isolation system to protect the line. You'll find the same parts in external modems, cordless phones, answer machines &c.

Answer 2

They have them for two reasons:

1: As Colin C said telephone line voltages( at +48 and -48V) could cause severe damage to sensitive cards, components in your PC.

2: It helps prevent internal computer cables coming into contact with the telephone line which can induce noise on to the transmission line (when electrically insulated not connected as in example 1 above); the more noise on the line the slower the modem speed.

Answer 3

The plastic covers are to insulate against the dangerous voltages from the line. Network cards see 2 - 3 v, speakers etc do not bring in voltage. Telephone lines carry 50 and 90v This can do serious damage to other components, even indirectly through the internal cables.

Answer 4

This plastic casing provides protection from heat and delicate components that might get damaged when being handled. While each manufacturer has their own uniqe design, it's almost certain those components inside the plastic casing may have been manufactured by a different vendor and designed that way for easy assembly.