Environmental Effects; Shielded Cables - HP A4500A - VMEbus Single Board Computer 744 Motherboard Manual

8 A note on standards

Environmental effects

Shielded cables

74
Environmental effects, such as device-to-device ground shifts and conducted
emissions can affect the reliable operation of RS-422 and RS-423 interfaces. The
best measure of these effects is the common-mode voltage. Common mode voltage
is defined differently for RS-422 and RS-423.
According to the RS-422 standard, the common mode voltage at the receiver must
be less than 7 volts to ensure reliable operation. The common mode voltage is
defined by the sum of ground potential difference between the driver and receiver
ground points, common mode noise, and the common mode offset (the drivers
common mode voltage).
The RS-423 standard specifies a common mode voltage of 4 volts. In this case, the
common mode voltage is defined by the sum of the ground potential difference
between the driver and receiver ground points and common mode noise.
Shielded cables increase the immunity of data communications cables to
electromagnetic energy and reduce common mode noise seen by the receiver. They
also decrease emissions from the data communications cable.
When using shielded cables, it is advisable to connect the shield to the chassis on the
system side only.
Shielded cables are not required by the RS-422 or RS-423 standards.
Limiting the difference between the ground points at the system and the peripheral
is important to ensure that the common mode range of the receiver is not exceeded.
This is more difficult as distance sbecome greater between the system and the
peripheral, and is highly dependent on the environment. It is often necessary to
connect the system and peripheral to the same ground point. Otherwise, a large
difference in ground potential may develop.
A large ground potential difference may cause permanent damage to a system or to
peripherals. Whilst Hewlett-Packard equipment is protected against this kind of
damage, no such protection can be one hundred percent effective.