We recommend that before you proceed further you check
that there is sufficient power derived from the serial port to
drive all the connected controllers. The power you can
derive from PC serial ports varies greatly from PC to PC, and
the amount of power needed to run the RS485 bus varies
according to the length of the cable, the number of
instruments connected, and the biasing requirements.
To check whether there is sufficient power measure the
voltage present on pin 9 of the ‘D’ connector relative to
ground. This must be at least 5.5V at all times. It is important
to measure the voltage on pin 9 with all the controllers
connected. If the power derived from the serial port is
insufficient, an external power supply can be connected to
pin 9 of the connector.
The details above are general requirements, if you are
installing more than 20–30 controllers, or more than 300m of
cable, then additional biasing and power will be required.
To fit additional biasing you need to connect two 620 Ohm
resistors externally as shown in diagram 5. This then requires
an external power supply to be fitted, this provides the
extra power for the biasing resistors, and also the power to
drive all 128 controllers over 1200 meters.
EXTERNAL POWER SUPPLY
The recommended external power supply requirements are
+5.5V to +14V and min 100mA.
6
KK SYSTEMS K485ADE
If you have purchased the KK Systems RS232/485 converter
from CAL Controls then the specific requirements for this
converter are detailed here. Please note that the KK Systems
K485ADE are only designed to work at 9600 baud rate, with
8 data bits, no parity and 1 stop bit.
Termination Resistors
It is not generally possible to derive
sufficient power from a PC232 port to drive a 120
Ω
resistor
with RS485 voltage levels. A capacitor e.g. 1000pF should be
connected in series with any such terminator, to eliminate
the DC loading which a resistor alone would produce.
Note:
The controlled slew rate driver used in this converter
makes termination at either end of the bus unnecessary for
cables shorter than approximately 300m, this assumes the
use of Belden 9841 cable.
Bias Resistors
There are 2 100K
Ω
bias resistors fitted
internally to the converter, so generally this should be
sufficient.
1
5
6
9
KK485-ADE
Tx /Rx (+)
1
2
3
4
5
6
7
8
9
10
11 12
13
14
15
16
To
additional
units
3300/9300/9400
Tx /Rx (-)
Gnd
Diagram 6
