The Model RS485MS-2W Communications Module serves two very important functions.
From a communication network view, its primary purpose is to electrically isolate the
communications network from the high voltages present in SymCom’s units and to convert
the communications signals from the microprocessor’s 5-volt levels into RS-485 levels. The
second function of the RS485MS-2W is to provide power to the RM-1000 remote monitor.
SymCom’s units are connected to high AC voltages with a oating ground circuit. As long
as there are three balanced line voltages present, the resulting ground level will often be near
case ground. However, if one phase is lost or if the line voltages become unbalanced, the
oating ground may be as much as 480 volts above the case ground. The RS485MS-2W has
two high speed optical isolation chips on the receive and transmit pins and a low speed opto-
isolator on the transmit/receive pin to isolate the communication network from the AC line
voltages.
The signals from the microprocessor go through resistors to provide current limiting before
connecting to the 9-pin external connector. Therefore, the 9-pin SUB D connector on the
SymCom unit provides un-isolated CMOS signals and is NOT RS232 or TTL compatible.
The RS485MS-2W does not have support for four wire communications systems.
Installation Instructions for SymCom’s
Model RS485MS-2W Communications Module
Note: Do not plug a modem or any other PC compatible device directly
into the 9-pin SUB D connector on SymCom’s units.
To utilize SymCom’s RS-485 network, three key components are required:
1.
Modbus Slave Device –
SymCom model series 601, 77C, or 777 with the Model
RS485MS-2W
communications module
.
2.
18 to 24 AWG shielded twisted-pair cable –
The twisting of the wires helps
minimize the effect of noise on the data signal.
3.
Modbus Master Device –
RM-1000, RM-2000, PLC, or computer. On a PC,
PC compatible device, and a PLC without an RS-485 port an RS-232 to RS-485
converter is required.
2880 North Plaza Drive, Rapid City, SD 57702
(800) 843-8848•(605) 348-5580•www.symcominc.com
MODULE FUNCTIONS
RS-485 NETWORKING
WIRING
Slave Side:
The terminals of the
RS485MS-2W
are marked as follows:
Master Side:
RS-232 to RS-485 converter:
The converter is a port-powered device that changes TD and RD RS-232
lines to balanced half-duplex, two wire RS-485 signals. Send Data Control
automatically enables the RS-485 driver when data is present on the RS-232
side. The converter has a DB-9 female connector on the RS-232 side and on the
RS-485 side terminal blocks for signal and DC power supply connections. An
optional power supply may be required for laptops or other computers with low
power serial ports.
The converter will work with only a few RS-485 devices attached within a few
hundred feet. This con guration normally will not require terminating resistors
on the RS-485 network. If you have very many RS-485 devices or the network
must span several hundred feet or more, you may need to connect a 12-volt
DC external power supply to the converter. Terminating resistors may also be
required under these conditions. If terminating resistors are required, one set
of 120-ohm resistors should be placed at the two extreme ends of the network
across the A and B terminals. Only two terminating resistors are permitted
on each twisted pair of wires in the network. If the converter is being used
along with 120-ohm terminating resistors, external power will be required (see
Appendix, Figure 3).
When connecting more than one SymCom unit on an RS-485 network, only
one unit is connected to the RS-232 to RS-485 converter with A to RD(B) and
B to RD(A). All remaining units are wired with terminals labeled A connected
together and terminals labeled B connected together.
Model RS485MS-2W Communications Module Revision 05/23/03
Terminal
A
V+
V-
adapters. If the connections are OK, check the network wires. If
possible, use an Ohmmeter to verify the continuity of each wire.
If your installation does not use the RS-232 to RS-485 converter and
the wiring diagram in the Appendix, Figure 2 does not apply, try
switching the polarity of the wires. In other words, switch the
connections at the A and B terminals of the RS485MS-2W module. If
possible, send a START or STOP command from the Modbus Master
device to the Model 777, since the START and STOP commands will
be executed whether or not the Model 777 returns an acknowledgment.
You will need a voltage meter to measure voltages on the connectors of
the converter and the RS485MS-2W module. The voltages are
measured in the DC 10 volt range between case ground on the computer
or the metal sheath of the DB-9 converter to the RD(B) connection of
the converter module or to the (A) or (B) connections on the RS485MS-
2W module. Close the COMM port by shutting down the Modbus
Master program. Leave the converter module plugged into the COMM
port on your computer, but unplug the RS485MS-2W module from
the Model 777. The COMM port should not be open so there should be
no voltage at connections A and B on the RS485MS-2W module.
Measure the voltage with the voltage meter’s positive probe touching
the (A) terminal of the RS485MS-2W module and the voltage meter’s
negative probe touching the PC’s case. If the case is not metal, such as a
laptop computer, you can use the metal shell of the DB-9 connector of
the RS-232 to RS-485 converter for a ground. If there is voltage resent,
there may be another program using the RS-232 port on the PC. Next,
open the COMM port by starting the master program. When the RS-232
port is opened, you should be able to measure around 4 volts at the
RD(B) connection of the RS-232 to RS-485 converter. You should also
be able to measure 4 volts on the (A) labeled connection of the
RS485MS-2W module that plugs into the Model 777. You should
measure 0 volts on the (B) labeled connection. If the voltages are
switched at (A) and (B), the polarity is not correct and the two wires
should be switched. If you do not measure 4 volts on the (A)
connections of the RS485MS-2W module, the RS-232 port may not be
opened correctly and the Modbus Master program will not be able to
talk to the Model 777.
Note: In this section, “Model 777” refers to any Symcom RS-485 compatable
unit.
may have been programmed using a proprietary protocol from
Cimetrics. All Model 777’s that are programmed for Modbus have a
small sticker near the 9 pin interface connector that says “Modbus”.
These Model 777’s have the option “11” code in the part number. If you
have a Model 777 that is not Modbus, it can be re-chipped for a nominal
fee. Please contact SymCom, Inc. at the number at the end of this
section for more information.
If you continue to have problems, please contact SymCom, Inc. at
(800)-843-8848 or (605) 348-5580
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