Calibre UK UCA93 - Manual (Page 5)

CALIBRE
Issue 1.0 Page 2
01/07/03
IO 1 is a 5V TTL input and can only be used on 5V tolerant systems.
Pin Function
1 NC – DO NOT CONNECT
2 IO 0
3 IO 1
4 0V
1.6. Bus Capacitance Limitations/Cable Choice
The maximum allowable capacitance on the I 2 C bus in normal mode depends on the value of the SCL
and SDA master pull-ups, but never exceeds 400pF. Refer to Phillips Technical Handbook Book 4 Parts
12a and 12b for further details. Care should be taken in choosing a length and type of interconnecting
cable, which will not exceed this limit.
For most systems with a distance of a few metres between the I 2 C Communications Adapter and the
target system, screened cable is NOT recommended, as it is likely to introduce too much capacitance.
However, the EMC performance of an unscreened cable is always potentially poorer than a screened
one. The Adapter's EMC performance even with an unscreened cable is good - but this may not be true
of the target system! If you are in any doubt at to the best way to connect up your system with EMC in
mind please contact your supplier or Calibre for advice.
1.7. Variable Voltage Bus Power Supply
Pin 7 on the “D” connector is connected to the variable bus voltage power rail. Power for external
circuitry can be drawn from here, but care should be taken never to short it to 0V or to exceed 250mA
loading. It is short circuit and overload protected by a self-resetting thermal fuse but prolonged shorting
could cause the UCA93 to generate an excessive amount of heat.
The bus voltage is set via software, in the range 2V to 5V.
NOTE that if a bus voltage greater 4.2V is selected the output will be 5V.