Regarding limitation of the daisy chain connection of HDPM6000

[T.Okada]

Hello,

I would like to confirm the “Maximum number of concurrent Modbus TCP connections” listed as “25” in the HDPM6000 specifications. Does this refer to the maximum number when HDPM6000 units are connected in a daisy-chain? If they are connected in a star topology, is there effectively no limitation?

Also, I understand that the typical length limit for communication over LAN cables is about 100 m. Could you explain how this applies when connecting HDPM units in a daisy-chain? For example, if every segment between devices in the daisy-chain is under 100 m, would continuous, stable communication be possible—similar to using hubs or repeaters—even if the total length across 25 daisy-chained units is 1,000 m?

Regards,

Okada

[Mehran_Mehrnia]

Hi @T.Okada 

The specification "Maximum number of concurrent Modbus TCP connections: 25" refers to the maximum number of simultaneous Modbus TCP client connections that a single HDPM6000 can support. This limitation is independent of the network topology and applies whether the meters are connected in a star or daisy-chain configuration.

Regarding Ethernet daisy-chaining, the dual Ethernet ports on the HDPM6000 allow the device to forward Ethernet traffic similarly to an embedded switch. For 10/100BASE-TX Ethernet networks, each cable segment between adjacent devices should comply with the Ethernet standard maximum length of 100 m (328 ft).

Therefore, a total network distance greater than 100 m may be achieved in a daisy-chain arrangement, provided that each individual segment remains within the Ethernet specification. However, overall network performance will also depend on factors such as traffic loading, bandwidth utilization, and overall network design.

Additionally, in a daisy-chain topology, each HDPM6000 acts as a communication pass-through device. If an intermediate unit loses power, experiences a fault, or is physically disconnected, communication with all downstream devices in that chain may be interrupted until the connection is restored.

Best Regards,

-Mehran

L3 Expert Advanced metering

[T.Okada]

Hi @Mehran_Mehrnia 

Regarding “the maximum number of simultaneous Modbus TCP client connections that a single HDPM6000 can support,” you’re not saying that only 25 HDPM units can physically exist on the same network. Rather, you mean that a single HDPM cannot establish Modbus connections with, for example, 26 master devices at the same time, correct?

So it should be perfectly fine to have, for instance, 600 HDPM units on the same network, with a single master communicating individually with each HDPM over Modbus. That configuration is entirely acceptable, right?

Also, just to confirm: for HDPM daisy-chain connections, as long as the length between each device is kept under 100 m, there’s effectively no overall length limit. Is it correct that there is no specific limit on the number of devices that can be connected in a single daisy chain?

Regards,

Okada

[Charles_Murison]

Hello @T.Okada 

"600 HDPM units on the same network, with a single master communicating individually with each HDPM over Modbus." - Correct, this can be fine from perspective of the HDPM6000.

"for HDPM daisy-chain connections" - Each HDPM6000 with be a switch in the network, the network can handle having multiple switches. Note that all traffic for the daisy chain will flow through the first meter, all traffic - 1 will flow through the second meter. Eventually there will be a bottle neck and you will notice a drop in communication performance.

Before you ask, no there is no specific number that I can give, factors outside the meter will play a much bigger role, network speeds, quality of network cables, Electromagnetic noise in environment etc. Also the definition of performance can differ between each system. Asking for data once a minute would be very different than asking for data every 10 seconds. Also the amount of data and number of requests to collect all the data  would also be a factor to consider. 

This is not to say you can't have HDPM6000 is a daisy chain, just don't have 600 meters all 100m apart on a single loop and expect to get data from every meter once a second.

Regards,

Charles

[Mehran_Mehrnia · Accepted solution]

Hi @T.Okada 

1. Regarding the 25 Concurrent Connections and 600 Units on a Network

Yes, your understanding is correct. The specification of 25 concurrent Modbus TCP connections refers to the maximum number of simultaneous client connections that a single HDPM6000 head unit can support. It does not limit the number of HDPM6000 units that can exist on the same Ethernet network.

Therefore, having a large number of HDPM6000 units on the same network, including hundreds of devices, is possible provided the overall network infrastructure, IP addressing scheme, and supervisory system are designed to support that number of devices. From the HDPM6000 perspective, the 25-connection specification does not impose a limit on the total number of meters deployed on the network.

2. Regarding Daisy-Chain Length and Device Count

For Ethernet daisy-chain installations, each individual cable segment between adjacent HDPM6000 units should comply with the Ethernet limitation of 100 m (328 ft).

Based on the published HDPM6000 documentation, and as Charles mentioned, we are not aware of a specified maximum number of HDPM6000 head units that may be connected in a single Ethernet daisy chain. However, this should not be interpreted as an unlimited topology. Practical limits will depend on the overall network design and communications performance requirements.

Additionally, because a daisy-chain topology relies on intermediate devices to maintain communication to downstream devices, the loss of power, failure, or disconnection of a device within the chain may interrupt communications to the many devices located farther downstream.

Best Regards,

-Mehran

L3 Expert Advanced metering

[T.Okada]

Hello @Mehran_Mehrnia , @Charles_Murison 

Thank you for your reply.

I understand that in a daisy-chain configuration, there should be no issue as long as the distance between each HDPM6000 unit is less than 100 meters; however, since performance depends on the environment, you cannot state the maximum number of units that can be connected.

Thank you.