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C-10 Field Control™ Genius® Bus Interface Unit User’s Manual
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October 1999 GFK-0825F
C
Bus Length
The maximum bus length for shielded, twisted-pair cable is 7500 feet. Some cable types are
restricted to shorter bus lengths. In turn, the bus length determines which baud rate may be
selected.
If the application requires greater bus length, fiber optics cable and modems can be used, as
explained later in this chapter.
Bus Length and Baud Rate for Busses with Phase A Devices
If a bus has any Phase A Genius products (catalog numbers IC660CBDnnn, IC660CBSnnn,
IC660CBAnnn, IC660HHM500, or IC660CBB900/901), the bus must use 153.6 Kbaud “standard” and
the maximum bus length is 2000 feet. Therefore, only the cable lengths listed under “153.6s” are
permitted (“153.6e” refers to 153.6 Kbaud extended, which is not compatible with 153.6 Kbaud
standard).
Baud Rate Selection
A Genius I/O or communications bus can operate at one of four baud rates:
153.6 Kbaud standard, 153.6 Kbaud extended, 76.8 Kbaud, or 38.4 Kbaud.
Follow these guidelines when selecting the baud rate for a bus:
1. All devices on a bus must operate at the same baud rate (other busses in the system may
operate at different baud rates).
2. If there are any older Genius products on the bus (catalog numbers IC660CBDnnn,
IC660CBSnnn, IC660CBAnnn, IC660HHM500, or IC660CBB900/901), the bus must be set
up to use 153.6 Kbaud standard.
3. If the cable length is between 4500 and 7500 feet, you must select 38.4 Kbaud. This data rate
only supports a maximum of 16 device on the bus.
4. If the cable length is between 3500 and 4500 feet, select 76.8 Kbaud.
5. If cable length is between 2000 and 3500 feet, select 153.6 Kbaud extended.
6. If the cable length is less than 2000 feet, either 153.6 Kbaud standard or 153.6 Kbaud extended
can be used. The products are set to operate at 153.6 Kbaud standard when shipped from the
factory. The use of 153.6 Kbaud extended is recommended, especially if the system will
include a dual bus with Bus Switching Modules.
In noisy environments, 153.6 Kbaud extended provides improved noise immunity with little effect
on bus scan time. If a system is experiencing excessive blinking of the bus controller's COMM OK
light, or if the I/O blocks' I/O Enabled LEDs go off frequently, 153.6 Kbaud extended should be
used.
The baud rate selected should be indicated on all blocks, especially if different busses in the facility use
different baud rates. Before connecting a Hand-held Monitor to a functioning bus, check that it has been
configured to the correct baud rate. If not, change the HHM baud rate selection, turn off the HHM,
connect it to the bus, then turn the HHM on.