Modbus Plus Throughput - ABB TPU2000 Technical Manual

communication status registers reside at 40712 through 40179. Section 5 of this document lists the method to
access and use these registers.

Modbus Plus Throughput

The Manual Titled Modicon Modbus Plus Network Planning and Installation Guide Copyright 1995, AEG
Schneider Automation, Inc., lists the methods to calculate Modbus Plus network throughput. It is recommended
that the aforementioned text be consulted to perform a specific network throughput analysis.
The same principles for any protocol analysis apply to Modbus Plus Protocol analysis. Modbus Plus is a very
efficient protocol since it's bandwidth is effectively utilized by using the hybrid features of an HDLC protocol with
token passing. The ability of the network to carry out 32 individual conversations and 2 Global Data broadcast
conversations is a very useful capability of the network. Combined with a high baud rate of over 1 megabaud, fast
throughput is assured.
A typical Modbus Plus network is depicted in Figure 5-78. A Programmable Logic Controller is connected to a
TPU2000R protective relay accessing data along one of its 8 data slave paths. A Personal Computer Host is not
used as a device in this example since it is difficult to predict the latency of the host device. As seen from the
example calculated with Modbus, host latency (in this case the PLC), network latency, and IED latency
(TPU2000R) all must be evaluated in their contributions to overall network throughput. The PLC is using a Master
instruction to access data on the network. The amount of logic in the PLC is 1K of ladder instructions operating
with a combined scan rate of 4 mS per K of logic. A PLC physical input is assumed to be using in triggering the
data for this example. The latency of the I/O module is assumed to be 1 mS (125 VDC Input Module).
For a network with two nodes on a network as illustrated in Figure 5-78, the first step is to calculate the token
rotation time using a master instruction.
Using the Token Rotation Time on Page 74 of the aformentioned Modbus Plus Manual, 890 USE 100 00 Version
2.0 where
TR = Token Rotation
DMW = Average number of words per Data Master Path used in the network (Maximum = 100)
DMP = the number of Data Master Paths used continuously in the network
GDW = the average number of global data words per message used in the network (Maximum = 32)
N = the number of nodes on the network.
Thus the token rotation is calculated according to the formula:
TR = (2.08 + 0.016 * DMW) * DMP + (0.19 + 0.016 * GDW) * GDN + 0.53 * N
In this example the PLC is continuously requesting 16 words of data. Only 1 path in this example (Path 1 is being
utilized). For the sake of simplicity, no Global Data is being used on the network. The calculation for the token
rotation time for the network in Figure 5-78 is:
TPU2000/2000R Modbus/Modbus Plus Automation Guide
Modicon Compact PLC
MSTR
Address 1
40110 - kWatts A High 16 bits
40110 - kWatts A Low 16 bits
40111 - kWatts B High 16 bits
40112 - kWatts B Low 16 bits
40113 - kWatts C High 16 bits
40114 - kWatts C Low 16 bits
40115 - kWatts Three Phase High
40116 - kWatts Three Phase Low
40117 - kVars A High 16 Bits
40118 - kVars A Low 16 Bits
40119 - kVars B High 16 Bits
40120 - kVars B Low 16 Bits
40121 - kVars C High 16 Bits
40122 - kVars C Low 16 Bits
40123 - kVars Three Phase High
40124 - kVars Three Phase Low
40125 - SPARE REGISTER
:
40139 - SPARE REGISTER
Figure 5-78. Modbus Plus Network Throughput Example
ABB TPU 2000R
Modbus Slave Addr =1
Read Power Information
C
E
Response From Relay
Read from
Data Path 1
4X Mapping
40283 - kWatts A High 16 bits
40284 - kWatts A Low 16 bits
40285 - kWatts B High 16 bits
40286 - kWatts B Low 16 bits
40287 - kWatts C High 16 bits
40288 - kWatts C Low 16 bits
40289 - kWatts Three Phase High
40290 - kWatts Three Phase Low
40291 - kVars A High 16 Bits
40292 - kVars A Low 16 Bits
40293 - kVars B High 16 Bits
40294 - kVars B Low 16 Bits
40295 - kVars C High 16 Bits
40296 - kVars C Low 16 Bits
40297 - kVars Three Phase High
40298 - kVars Three Phase Low
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