EVM 1
EVM 2
Disabled
Disabled
R9 and R10 Removed
T
EVM 1
EVM 2
Disabled
Disabled
T
T
Overview
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1.3.1
Point-to-Point
The point-to-point configuration is shown in
Figure 5
. This configuration requires two EVMs, and although
this is not the intended mode of operation for PROFIBUS/RS-485 devices, it is used for high noise or
longer (higher loss) transmission lines. Due to the increased drive current, a single 100-
Ω
termination
resistor on the EVM results in a differential bus voltage (Vod) twice as large as the normally doubled
terminated line. This practice is acceptable as long as the combination of input voltage and common-mode
voltage does not exceed absolute maximum ratings of the line circuits.
This configuration is shown in
Figure 5
. It consists of a single driver on the left, and a single receiver on
the right. Note that the termination resistors (R9 and R10) have been removed from the driver EVM.
This configuration shows the receiver disabled on the driver (JUMP2 connected to Vcc) EVM and the
driver disabled on the receiver EVM (JMP4 tied to GND1). Note that with only a single termination, the
differential bus voltage is higher than normal.
Figure 5. Point-to-Point Simplex Circuit
This configuration also can have a termination at the source and load (parallel terminated as shown
Figure 6
) thereby, keeping nominal PROFIBUS/RS-485 signal levels.
Figure 6. Parallel Terminated Simplex Circuit
1.3.2
Multipoint
The multipoint configuration is the primary application of PROFIBUS/RS-485. The RS-485 standard allows
for any combination of drivers, receivers, or transceivers up to a total of 32 unit loads on the line. The
ISO1176T device allows the total number of nodes to be increased because ISO1176T represents just 1/5
of a unit load. This allows up to 160 nodes to be connected onto a single bus.
Figure 7
shows a
representation of a five-node multipoint configuration using five ISO1176T transceivers. Increased drive
current in addition to the wide common-mode input, allows this ISO1176T PROFIBUS transceiver to drive
multiple receivers over longer line lengths. Notice that the termination resistors (R9 and R10) are only
installed on the EVMs at each end of the bus.
6
ISO1176T/35T/3086T Evaluation Module
SLLU122A – April 2010 – Revised July 2010
Copyright © 2010, Texas Instruments Incorporated
