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2.5 Limitations There are certain limitations and several options which must be considered in planning your installation. 2.5.1 Physical Limitations In a redundant EPLCG application, the primary and secondary EPLCG modules generally mount in the same rack, but cannot be located in the same dual node module. They are normally installed close to each other because of interlink or relay panel cable length restrictions. If your system uses an interlink cable, its length is fixed at 3 meters. Alternate cable lengths are not available. If your system uses a relay panel, the standard cable length to secondary EPLCG is 2m, but alternate cable lengths are available. However, if a longer relay panel cable is used, the amount added to the relay panel cable must be subtracted from each of the Port 1 and Port 2 cables. Obviously, the length of a substitute relay panel cable must be less than 15 meters (50 feet). 2.5.2 Single vs. Multidrop Cabling There must be only a single cable from a port to the PLC, modem, or communications controller that port is to service. If you want to use a Modbus protocol multidrop arrangement, you must place a local modem at the EPLCG with remote modems connected to each of the PLCs in the network. Allen-Bradley (A-B) protocol multidrop arrangements always connect through an AllenBradley communications controller (a CIM, for Communications Interface Module). Since this communications controller supplies the multidrop connections, only a single cable is needed from the EPLCG port to the A-B controller. 2.5.3 Cable Lengths The cables from the EPLCG ports cannot be longer than 15 cable-meters (50 cable-feet). If the distance to a PLC or communications controller exceeds this limit, you must use short-haul modems. See subsection 2.6 for modem considerations. EPLCG Planning, Installation, and Service2-10 5/01 2.5.4 2.5.4 Direct Connection If you are connecting a single PLC (or an A-B communications controller) to one of the ports and the cable-length from the EPLCG to the PLC is less than 15 cable-meters, you can use an EIA-232 direct-connection (no modems). In this arrangement, the EIA-232 cable supplied by Honeywell must be specifically wired to a connector which mates your PLC. Subsections 3.2.7 and 3.2.8 show cable wiring schemes for several types of PLCs and interface devices. 2.6 EPLCG TO PLC CONNECTIONS 2.6.1 Modem Usage and Selection Direct-connection, short-haul modems (sometimes called line-driver), or signal converter devices can be used with the EPLCG. As mentioned earlier, direct-connection is limited to a maximum of 15 cable-meters between EPLCI I/O or Relay card. Signal converters are devices that convert signals between EIA-232 an EIA-422 or -485, and are commonly used to provide extended distance or multidrop configurations. A short-haul modem presents an EIA-232 hardware interface to the EPLCG or PLC similar to that presented by conventional telephone modems. The short-haul modem, however, uses dedicated lines (not telephone lines) and may take liberties with interface protocol that would not be acceptable in conventional telephone modem communications. Conventional telephone modems are not typically used with the EPLCG because they severely limit bandwidth, and their necessary low speeds (baud rate) might degrade EPLCG performance. The EPLCG also does not support the handshake signals commonly required for modems, including Request-To-Send (RTS), Clear-To-Send (CTS), Carrier Detect (CD), Data Set Ready (DSR), and Data Terminal Ready (DTR). Various device and cable configurations are feasible. Consult with a communication link specialist or vendors for devices and cabling appropriate to your application.