CHAPTER 4– DATA COMMUNICATIONS INTERFACE
26 MultiLoad II/ RCU II Explosion Proof Lite (EXL) Installation Guide - Part # 6070
RS-422/485
The RS-422/485 communications protocol is designed for multi-point (i.e. computer to multiple
devices, also called multi-dropped) communications up to 4,000 feet (1,220 Meters).
RS-422 requires 4-wires (2 twisted pair) for full duplex communications and utilizes a transmit pair
of wires (TDA & TDB) and a receival pair of wires (RDA & RDB).
RS-485 requires 2-wires (1 twisted pair) for half duplex communications and utilizes a single pair
of wires (TDA/RDA & TDB/RDB) for transmittal and receival
This manual will typically refer to both RS-422 and RS-485 as simply RS485 2 Wire or RS485 4
Wire.
Although a ground wire is not required, the common mode voltage between the MultiLoad
II/ RCU II and the field device must be within -7Vdc to 12Vdc. To correct situations where
this is not the case, the DC common is available on COM 0 and COM 1 by using TB2, pin
five (revision 1.1 Power Supply/Comm boards only) and TB 3, pin three. Connect the
MultiLoad II/ RCU II DC common to the signal ground of the field device
Wire used must meet the following characteristics:
- 24 AWG stranded.
- 4-wire, two twisted pair with overall shield.
- 2-wire, one twisted pair with overall shield.
- 30 pF maximum between conductors.
- 1,000 ohm impedance.
- Maximum length: 4,000 feet (1,220 Meters).
- Maximum stub length: 15 feet.
All exposed shields must be properly insulated to prevent short circuits.
All shields must be continuous, soldered, and properly insulated.
RS-232
The RS-232 protocol is designed for point-to-point (i.e., computer to a single device)
communications limited to 50 feet requiring a minimum of 3 wires: transmit, receive and ground.
Additional wires are required for hardware handshaking when using printers and data loggers.
Wire used must meet the following characteristics:
- 24 AWG.
- 3 conductor with overall shield (data only) or 8 conductor with overall shield (full
handshaking).
- 30 pF maximum between conductors.
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