ModSim - A Simple Modbus TCP Device Simulator

modsim is a simple Modbus TCP device simulator deployed at modsim.topmaker.net:502. It works as a remote Modbus-TCP (virtual) device, which has already been populated with all 4 types of registers, i.e.

• Coil Type
• Discrete input Type
• Input register Type
• Holding register Type

It is recommended to use modpoll tool to communicate with it, the example configuration is as following,

device,modsim01,1,,
poll,coil,0,16,BE_BE
ref,coil01-08,0,bool8,rw
ref,coil09-16,1,bool8,rw
poll,discrete_input,10000,16,BE_BE
ref,di01-08,10000,bool8,rw
ref,di09-16,10001,bool8,rw
poll,input_register,30000,20,BE_BE
ref,input_reg01,30000,uint16,rw
ref,input_reg02,30001,uint16,rw
ref,input_reg03,30002,uint16,rw
ref,input_reg04,30003,uint16,rw
ref,input_reg05,30004,int16,rw
ref,input_reg06,30005,int16,rw
ref,input_reg07,30006,int16,rw
ref,input_reg08,30007,int16,rw
ref,input_reg09,30008,uint32,rw
ref,input_reg10,30010,uint32,rw
ref,input_reg11,30012,int32,rw
ref,input_reg12,30014,int32,rw
ref,input_reg13,30016,float32,rw
ref,input_reg14,30018,float32,rw
poll,holding_register,40000,44,BE_BE
ref,holding_reg01,40000,uint16,rw
ref,holding_reg02,40001,uint16,rw
ref,holding_reg03,40002,uint16,rw
ref,holding_reg04,40003,uint16,rw
ref,holding_reg05,40004,int16,rw
ref,holding_reg06,40005,int16,rw
ref,holding_reg07,40006,int16,rw
ref,holding_reg08,40007,int16,rw
ref,holding_reg09,40008,uint32,rw
ref,holding_reg10,40010,uint32,rw
ref,holding_reg11,40012,int32,rw
ref,holding_reg12,40014,int32,rw
ref,holding_reg13,40016,float32,rw
ref,holding_reg14,40018,float32,rw
ref,holding_reg15,40020,uint64,rw
ref,holding_reg16,40024,int64,rw
ref,holding_reg17,40028,float64,rw
ref,holding_reg18,40032,float64,rw
ref,holding_reg19,40036,string16,rw

Quick Start

A docker image has been provided for user to directly run the program,

docker run -p 5020:5020 topmaker/modsim:latest

The above one-line command will create a virtual Modbus TCP device running at localhost:5020.

To verify it, you can test by modpoll tool with the following command,

modpoll \
  --tcp localhost \
  --tcp-port 5020 \
  --config https://raw.githubusercontent.com/gavinying/modpoll/main/examples/modsim.csv

Note: add sudo before docker run command if you want to use the Modbus TCP standard port 502.

Local server behavior

Run the simulator without Docker with python -m modsim --comm tcp --port 5020. The default server exposes the four Modbus tables with the sample data shown above. Coil and holding-register writes are supported; discrete inputs and input registers are read-only. Requests outside an initialized address range return a Modbus exception. The server handles SIGINT and SIGTERM by stopping its listener so the port can be reused after shutdown.