2-13
2
two-wire RS-485 network. Use the “SYST:COMM:SER:RS485 ON” com-
mand to configure the units. The ON setting causes the unit to tristate its serial
transmitter when not transmitting which free's the network so the Modbus can
respond to the message. Also set the 8099’s internal W4-W6 jumpers to 422
for RS-422 and RS-485 signals as shown in section 2.7.
8099
DB-25P
2
3
14
16
18
12
SD(A)
RD(A)
SD(B)
RD(B)
Term+
Term-
Tx/Rx-
Tx/Rx+
(a) Basic 8099 RS-485 Connection
(Requires an external termination network)
8099
DB-25P
2
3
14
16
18
12
1 K
Vcc
SD(A)
RD(A)
SD(B)
RD(B)
Term+
Term-
Tx/Rx-
Tx/Rx+
1 K
220
(b) Using the 8099’s Internal Termination Network
Figure 2-11 8099 RS-485 Connections
Two-wire RS-485 networks need a termination network to bias the lines in the
'mark' state when neither unit is transmitting. This prevents each receiver from
inputting noise when nothing is being transmitted. Use one termination network
for short cables of 200 feet or less. For longer cables, use a termination network