User manual

Connect Tech Xtreme/104-Plus, PCI-104 and PCI/104 Express Family User Manual
Revision 0.12
30
……power considerations continued
A typical current draw under RS485 is about 900mA (4.5 Watts) which occurs under the following
conditions:
 All ports RS485 4-wire mode.
 Bias and Termination (B&T) circuit installed all ports (provides bias and termination on
the RX+/- signals).
 Remote end or device end with 120 ohm termination resistor installed on RX+/-.
 5-10 remote devices per port.
The lowest power RS485 operations would occur when no termination resistors are installed. If the desired
baud rate is less than 250kbps, then the slow slew rate option can be used to ensure reliable operation when
no termination resistors are installed.
The typical power with all ports under RS232 is about 800mA (4 Watts) on the 5V rail.
19 Thermal Considerations
Under worst case conditions the self-heating effects caused by the XIG, XFG or XOG due to power
dissipation and high enclosure ambient temperatures could cause the maximum operating temperature of
the product to be exceeded. It is up to the PC/104 integrator to perform a thermal analysis and thermal
experiments to ensure that the ambient temperature around the product PCB does not exceed +85C under
the desired operating conditions.
Considerations for adequate air flow or cooling around the product and distance from other hot PC/104
cards or power supplies should be made to ensure safe and reliable operation.
These thermal considerations are not unique to optically isolated boards, however optically isolated
electronics like the XIG, XOG and the XFG have DC-DC converters which dissipate a typical 0.15 Watts
each. This means that an 8 port opto board will dissipate around 1.2 Watts from the DC-DC converters
alone.
In conclusion isolated boards do consume more power and dissipate more heat than non-isolated boards
and this must be considered when doing a thermal analysis.