Campbell Scientific MD485 RS-485 Multidrop Interface - Manual (Page 10)

MD485 RS-485 Multidrop Interface
The baud rate for each port is set independently, so they can be
different. However, in some instances, mismatched baud rates
can cause an MD485 buffer overflow and create communication
failures. Regardless of the baud rates set on other interface ports,
the RS-485 ports of all MD485s in a system must be set to the
same baud rate. NOTE
3.2 Power Supplies
The typical base station MD485 connected directly to a PC uses a wall
transformer to supply 12 VDC power. You can order the optional wall
transformer (CSI Item # 15966). In a phone to MD485 base station
configuration (without datalogger) the MD485 can obtain power from a PS100
Power Supply with an A100 Null Modem Adapter. (The PS100 with an A100
attached replaces the PS512M. Either may be used whenever the PS100/A100
is referenced in this manual.)
The typical remote MD485 will be connected to a datalogger CS I/O port and
get its 12 V power from that. If your datalogger is an earlier unit without 12 V
on CS I/O pin 8 (see Table 2) or if the remote MD485 is connecting to a
datalogger RS-232 port, there is an optional Field Power Cable available (CSI
Item # 14291) with tinned leads to connect to power at the datalogger 12 V
output terminals and barrel connector to plug into the MD485’s “DC Pwr”
jack. If 120 VAC is available at the site, the wall transformer (CSI Item #
15966) is an option.
A 12 V supply may connect to either the MD485’s “DC Pwr” jack or CS I/O
pin 8 (or both, since there is diode isolation between supply inputs). The 12 V
supply inputs are diode protected against the application of reverse polarity
power.
3.3 RS-485 Cable
The connection between MD485s is made with the CABLE2TP-L two-twisted-
pair cable with shield and Santoprene jacket. Insulation colors of the twisted
pair are red/black and green/white. One pair is used for the differential data
(“A” connects to “A”; “B” connects to “B”), and one line of the other twisted
pair is used for the signal ground (third connection on the MD485 terminal
block). This is shown in Figure 3. The cable shield should be connected to a
chassis or earth ground (NOT the signal ground) at one end as shown in that
figure.
When connecting to equipment that uses “-” and “+” terminal markings, the
MD485 “A” terminal is connected to the “-” terminal and the MD485 “B”
terminal is connected to the “+” terminal.
If an application requires both the maximum distance (4000 ft) and the highest
data rate (115k) of the MD485, a better quality cable, such as a polyethylene
data cable could be used.
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