1. modpoll

modpoll -b 9600 -p none -a 1 -r 98 -c 3 -1 /dev/ttyUSB0
 modpoll 3.7 - FieldTalk(tm) Modbus(R) Master Simulator
Copyright (c) 2002-2019 proconX Pty Ltd
Visit https://www.modbusdriver.com for Modbus libraries and tools.

Protocol configuration: Modbus RTU
Slave configuration...: address = 1, start reference = 98, count = 3
Communication.........: /dev/ttyUSB0, 9600, 8, 1, none, t/o 1.00 s, poll rate 1000 ms
Data type.............: 16-bit register, output (holding) register table

-- Polling slave...
[98]: 6
[99]: 8
[100]: 2290

2. jpnevulator

https://jpnevulator.snarl.nl/

jpnevulator --ascii --read --timing-print --tty /dev/ttyUSB0
 2020-06-28 16:31:46.136148:
01 03 00 61 00 03 54 15 01 03 06 00 06 00 08 08 ...a..T.........
F4 2E F0                                        ...

3. ioninja

https://ioninja.com/

https://tibbo.com/downloads/archive/ioninja/ioninja-3.15.0/ioninja-3.15.0-linux-amd64.rpm

sudo yum-config-manager --add-repo https://tibbo.com/downloads/archive/ioninja-rpm/ioninja.repo
sudo yum -y install ioninja

4. ser2net & socat

on srv030

vi /etc/ser2net.conf
///
3000:raw:600:/dev/ttyUSB0:9600 8DATABITS NONE 1STOPBIT

on a workstation

socat -x pty,link=/dev/ttyCOM1,nonblock,raw,echo=0,ignoreof,waitslave tcp:10.1.1.30:3000
modpoll -b 9600 -p none -a 1 -r 98 -c 3 -1 /dev/ttyCOM1
> 2020/06/29 19:40:36.879846  length=8 from=0 to=7
 01 03 00 61 00 03 54 15

5. references

http://jheyman.github.io/blog/pages/RS485Sniffer/

https://www.acmesystems.it/socat