3
Interface
type
port D1
port D3
1
2
3
4
RS-422
ON
OFF
ON
OFF
RS-485 4W
OFF
ON
OFF
ON
RS-485 2W
OFF OFF OFF OFF
Manchester
ON
ON
ON
ON
Table 2. Selecting D1 and D3 interface types using the
D1/D3 SEL dip-switch, S4
Note: The Factory Default Setting is RS-485 4W
Current loop output:
(Select RS-422 for current loop
operation). The output impedance of ports D1 and/or D3
can be made suitable for digital 20 mA current loop/TTY
applications by pulling a 2-pin jumper (pins 2-3) from
the board, thus inserting a resistor in the non-inverting
data output line (in order not to lose it, the jumper may
safely be parked on pins 1-2). See fig. 2 for the location
of the D1 and D3 Current Loop Output jumpers.
Current loop I/O should use only non-inverting lines and
signal ground (as in figure 3); input signalling on
A/GND needs >4V.
Figure 3. Current loop connections (port D1 and D3).
The “B” leads should not be connected.
RS-422,
RS-485,
and
Manchester
Line
Bias,
Configuration, and Termination:
In addition to
selecting the data interface type, the two Four position
dip-switches (see fig. 2) marked “DATA SELECT 1”
and “DATA SELECT 3”, must be set as prescribed for
proper operation. Refer to table 3.
Four Position Data Select Dip-Switches for D1 and D3
1
2
3
4
5
6
7
8
RS-485
4W
T
T
ON
ON
BR
BR
BD
BD
RS-485
2W
T
T
ON
ON
BR
BR
OF
F
OF
F
RS-422
O
N
O
N
ON
ON
BR
BR
OF
F
OF
F
Mancheste
r
O
N
O
N
OF
F
OF
F
OF
F
OF
F
OF
F
OF
F
RS485 Termination (T):
Switch positions 1 and 2
operate in tandem. When both switches are in the OFF
position, the RS-485 input is unterminated. When both
are set in the ON position, the differential input is
terminated with 120 ohms resistance.
For RS-485 installations, there might be anywhere from
2 to 32 RS-485 devices attached to the differential bus.
Normally, the devices at the two extremes of the bus are
terminated, while intermediate devices are not.
Input Configuration:
Switch positions 3 and 4 select
AC (OFF) or DC (ON) input coupling. RS-422 and
RS-485 inputs are always DC coupled and Manchester
inputs are always AC coupled.
RS-485 2W and 4W Line Receiver Input Bias (BR):
Switch positions 5 and 6, when ON, connect 390 ohm
bias resistors on the A and B inputs. The A input is
biased towards (+V) and the B input is biased towards
(-V). This bias ensures that the input line receiver
interprets the state of the differential bus as a logical
“zero” while all the drivers attached to the differential
bus are in a Hi-Z state. There must be at least one device
on the differential bus with the Line Receiver Input Bias
(BR) enabled. Typically, BR is enabled along with the
termination at one end (only) of the bus. However, if
other devices occupy the end positions on the bus, and
are terminated, it needs to be ensured that there is
adequate bias on the bus for proper operation.
Sometimes other devices provide bias. If not, enable BR
on one of the TETRA 43x0 units on the bus. To
determine if adequate bias is present, while there is no
data activity on the bus (disable the master unit if
required) attach the (+) lead of a VOM to the “A” lead of
the bus and connect the (-) lead to the “B” lead of the
bus and take a voltage measurement. For optimum
operation, the reading should be at least +200 mV. If it is
less, and there are operation problems, enable BR by
setting switches 5 and 6 in the ON position on at least
one TETRA 43x0 connected to the bus.
RS-485 4W Line Driver Output Bias (BD):
Switch
positions 7 and 8, when ON, connect 390 ohm bias
resistors on the A and B outputs. The A output is biased
towards (+V) and the B output is biased towards (-V).
This bias ensures that the input line receivers in other
RS-485 4W devices connected to the bus interpret
voltage level on the bus as a logical “zero” while the
TETRA 43x0 RS-485 4W output is in the Hi-Z state.
Sometimes other devices provide this bias. If not, enable
BD on one of the TETRA 43x0 units on the bus by
setting switches 7 and 8 in the ON position. To
determine if adequate bias is present, while there is no
data activity on the bus (disable the master unit if
required), attach the (+) lead of a VOM to the “A” lead
of the bus and connect the (-) lead to the “B” lead of the
bus and take a voltage measurement. For optimum
operation, the reading should be at least +200 mV. If it
is less, and there are operation problems, enable BD on
at least one TETRA 43x0 connected to the bus.
Table 3. Data I/O Configuration, Termination, and
Biasing for ports D1 and D3. See text for the definition
of “T”, “BR”, and “BD”.
Note: The Factory Default Setting is All switches ON
(RS485 4W, Terminated and Biased)
GND
B (IN-)
A (IN+)
GND
B (OUT-)
A (OUT+)
220 ohm,
internal
jumper removed
