UD78 User Guide
Issue code: 78nu2
13
4.4 PL1
Serial communications connector
Functions of the terminals
Terminal Name Function
1 0VSC 0V
2 TX\ Transmit output (inverted)
3 RX\ Receive input (inverted)
4
TX
TERM
TX termination-resistor
connection
5
RX
TERM
RX termination-resistor
connection
6 TX Transmit output
7 RX Receive input
8
TX\
TERM
TX\ termination-resistor
connection
9
RX\
TERM
RX\ termination-resistor
connection
Shell 0VSC 0V
Specification
RX (input) 2 unit-loads (EIA RS485)
TX (output) 2 unit-loads (EIA RS485)
2-wire (Transceiver mode) 4 unit-loads (EIA RS485)
Termination resistance 120
Ω
±
5% (no series capacitor)
In accordance with the EIA RS485 specification, the total load on a line must
not exceed 32 unit-loads. Each transmitter and receiver of the UD78 loads
the line by two unit-loads (in two-wire mode, each UD78 loads the line by
four unit-loads). This allows the following to be operated:
•
15 units in 4-wire mode
•
7 units in 2-wire mode
When line repeaters are used, up to 81 Control Techniques devices can be
operated. In this case the devices must be arranged in up to nine groups of
nine. A particular group or groups can be given commands without
affecting other devices or groups of devices.
