I am wondering how many modbus modules can be connected to the CPU 1516-3 PN/DP.

I am wondering how many modbus modules can be connected to the CPU 1516-3 PN/DP.

Thanks in advance for the answer.

Hello,

from your question isn't clear what you really ask, sry. Modbus, which Modbus, which modules? What you want connect to CPU, via which interface in case of type of Modbus?

Hello jjjinn,

From point of view of protocol Modbus RTU/ASCII (same for ASCII and RTU) allow slave addressing from 1 to 247, so one master can reach 247 slaves.

But the main reason of slave number limitation is the sum electrical load of the group of RS485 receivers. Generally an RS485 transmitter can drive 32 slave + 2x 120Ohm  terminating resistors (at both ends of line) - this group is called "network segment". If you want to have more RS485 devices on the network you have to put a RS485 repeater per every 32 devices.

This is also mentioned in the PtP module datasheet

In particular cases you can check the master max load data in datasheet and input impedance data for every slave also in respective datasheets, sum all loads and check. And probably you can find that the particular master allow more then 32 of particular slaves. But I haven't found max load data for Siemens PtP module, only above mentioned max 32 slaves.

RS422 generally implies point-to-point connection (i.e. only 2 nodes on the network). This is also mentioned in the PtP module datasheet. I doubt you intend to use RS422.

I would also note the following. Designing Modbus network you have to pay attention to - what is the update time for the network (for each slave). The matter is that Modbus master queries slaves (reads/writes data from a slave) - slave-by-slave. And from practice, the biggest delay here is the time delay each slave requires to "answer" the master query.

E.g. if it takes 100 ms for each slave to generate answer, having just 10 slaves you have actual update time for every slave of 1 sec. You have to estimate if it's OK for your application. E.g. if you query temperature sensors - even many seconds may be OK. If you're trying to control something via Modbus you would want to have smaller update time.

If this is critical for you application - this can be the limiting factor for the amount of slaves per master. And if you need to have more slaves in this case you may need more masters (more Modbus networks) with less slaves per one master.

 Hello,

jjjinn

CPU: CPU 1516-3 PN/DP,6ES7516-3AN02-0AB0

RS422/485 module:

communication module,CM PtP, RS422 / 485, high function, 6ES7541-1AB00-0AB0

Protocol: modbus-rtu, modbus-ACSII

I am trying to connect an RS 422/485 module to a CPU. How many modbus-RTU can be connected to the CPU? Also, how many modbus-ACSII can be connected to the CPU?

 again pls, you ask how many modules you can connect to CPU or what, because from your question is clear only protocol not module/device. About protocols, Modbus ASCII isn't supported, only Modbus RTU.