Pings Over MODBUS But There Is No Reply


Thread Starter


Before stating my problem and asking the question, I will detail my system:

Terminal OTB1E0DM9LP: receives sensor state signal (digital input), connected through Ethernet port (MODBUS TCP/IP Protocol) to Radio Antenna 1.

Radio Antenna 1: sends signal of sensor state to Radio Antenna 2.

Radio Antenna 2: receives signal of sensor state from Radio Antenna 1. Connected to Switch Ethernet.

Switch Ethernet: PACM340 (Modbus TCP/IP Protocol) and Radio Antenna 2 are connected to it.

The problem:
From the Switch Ethernet, if I ping the Terminal OTB1E0DM9LP, all 4 packets are received (ping successful). However, the PACM340 does not see the Terminal OTB1E0DM9LP (it is well configured, I tested that by connecting it directly to an Ethernet port of an expansion card of the PACM340). Besides, I used CAS MODBUS SCANNER (from my laptop connected to the Switch Ethernet) to read the sensor state register of the Terminal OTB1E0DM9LP, and no response was received (I did the same test by connecting my laptop directly to the Terminal, and it was successful). It seems that there is a problem of data integrity in the section of the wireless network (I am not a networking expert, but is my guess), but I would like to receive help from someone with more experience in this area.

Additional information:
Each terminal has its own IP Address.

There are several other Terminals OTB1E0DM9LP connected to the network in the same way with their own Radio Antenna, and everything works fine.

The connection of all the Terminals were tested in my office and everything worked fine.

Can the ping be successful and at the same time the data (over MODBUS TCP/IP protocol) cannot be read?

Thanks for your help

> Can the ping be successful and at the same time the data
> (over MODBUS TCP/IP protocol) cannot be read?

Yes, if the port being used by MODBUS (normally 502) is not open or blocked at any point, it will fail while ping uses ICMP.

Good luck,


Lynn August Linse

I think you need to read the 'Ethernet radio' docs carefully!

Many do severe packet filtering, so might not even allow ICMP to pass unless it has been enabled; Broadcasts might also be limited. It is the typical "How to fit a Quart of data through Pint-sized pipe?"