MCT-MC
4.3
FDC MCT-MC
- 144 -
Appendix
A.1.3 Interface Standards
An interface is a means for electronic systems to interact. It’s a specific kind of electrical wiring configuration.
It has nothing to do with how data is sent over that connection. The two most common interfaces used today
are RS-232, which provides a simple 1 to 1 connection and RS-485, which provides a multi-drop connection
where more than one device can be placed on the same line. The MC communications interface is RS-232,
but can be changed to RS-485 through the use of an external RS232/485 adapter.
EIA-232 (Full Duplex)
An EIA-232 (formerly RS-232C) interface uses three wires: a single transmit wire; a single receive wire; and a
common line. Only two devices can use an EIA-232 interface. A -3 to -24 volt signal indicates a 1 and a +3 to
+24 volt signal indicates a 0. The EIA-232 signal is referenced to the common line rather than to a separate
wire, as in EIA-485. Thus, an EIA-232 cable is limited to a maximum of 50 feet, due to noise susceptibility.
EIA-485 (Half Duplex)
An EIA-485 interface uses two wires: a T/R+, a T/R- line. A -5-volt signal is interpreted as a 1, a +5-volt signal
as a 0. As many as 31 slave devices can be connected to a master on a multi-drop network up to 4000 feet
long.
Wiring
Few PCs today provide the standard EIA-232 port (usually referred to as RS-232). In these instances, you
must use a USB to serial adapter. There are numerous adapters available on the market, many of which can
provide either an RS-232 connection, RS-485connection or both. Due to the possible variations between
computer hardware, operating systems and software, the selection of a USB to serial adapter is outside the
scope of this manual. Nearly all adapters provide the same functionality so the selection of an adapter typically
falls to making sure it is compatible with the computer’s operating system.
To connect to an EIA-485 multi-drop system. The standards do not specify the wire size and type. Use of 24
AWG twisted pair provides excellent results. If shielded cable is used, terminate the shield at one end only.
Always follow the manufacturer’s instructions supplied with the interface converter. See Biasing of Buses next.
Biasing of Buses
The EIA-485 standard requires the bus to be biased for reliable communication. This requires termination
resistors to be placed across the T/R+ and T/R- wires. One resistor is placed at the PC where it connects to
the EIA-485 bus. The second resistor is placed at the last controller on the network. Do not place resistors at
each controller. The impedance of the wires used for the bus determines the resistor value. For twisted pair,
the value is typically 120 ohms. In addition, it may be necessary to have a pull-up and pull-down resistor
between the power supply and ground of the interface adapter.
Check the documentation that came with your interface adapter. Biasing the bus reduces reflection of signals
sent down the bus. These reflections are sometimes referred to as a standing wave. This condition is most
notable when communicating at high baud rates over longer distances.
A.1.4 Interface Converters
The purpose of an interface converter is to allow two different buses to be connected together. Interface
converters are required when connecting an EIA-232 port to an EIA-485 bus. The EIA-485 bus is a half duplex
bus. This means that it can only send or receive data at any given time. Some interface converters on the
market provide the ability to have full duplex with the EIA-485 bus. This is accomplished by using two receivers
and transmitters tied in tandem. This type of converter will not work with the MC controller. Be sure that the
model you purchase is designed for half duplex.
Summary of Contents for MCT-MC 4.3
Page 1: ...MCT MC 4 3 User Manual MCT MC 4 3 User Manual Rev A February 2018...
Page 176: ...MCT MC4 3 FDC MCT MC 176 Appendix...
Page 177: ...MCT MC4 3 Appendix 177 FDC MCT MC Open Frame Power Supply Specifications...
Page 178: ...MCT MC4 3 FDC MCT MC 178 Appendix...
Page 179: ...MCT MC4 3 Appendix 179 FDC MCT MC A 6 Power On Delay Relay Specifications GE1A C10HA110...
Page 180: ...MCT MC4 3 FDC MCT MC 180 Appendix...