# Master and slave confusion over Modbus and number of maximum devices

**URL:** <https://forum.inductiveautomation.com/t/master-and-slave-confusion-over-modbus-and-number-of-maximum-devices/27659>\
**Category:** General Discussion\
**Created:** [August 22, 2019, 12:43pm UTC](https://forum.inductiveautomation.com/t/master-and-slave-confusion-over-modbus-and-number-of-maximum-devices/27659 "2019-08-22T12:43:46Z")\
**Posts on this page:** 4\
**Page:** 1

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**Author:** ![miculacristianradu](https://avatars.discourse-cdn.com/v4/letter/m/b782af/32.png) [@miculacristianradu](https://forum.inductiveautomation.com/u/miculacristianradu)\
**Post date:** [August 22, 2019, 12:43pm UTC](https://forum.inductiveautomation.com/t/master-and-slave-confusion-over-modbus-and-number-of-maximum-devices/27659/1 "2019-08-22T12:43:46Z")

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Hello guys,

I have a beginner qestion.  
According to Modbus terminology protocol there’s need to be one master and multiple slaves. (or one slave)

**Let’s assume I have 10 place’s where I have 1 PLC and 4 I/O’s devices attached to it for each place** , and my PLC’s communicate with a SCADA server over TCP/IP network.

**The Master for the PLC’s is the SCADA server and the master for the I/O devices is are the PLC’s?**

**Another question is how many devices cand Ignition support over Modbus protocol?**

The theory that Modbus protocol can have a maximum of 247 devices is available only on devices connected to the PLC’s?

Thank you in advance guys!

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**Author:** ![pturmel](https://sea1.discourse-cdn.com/inductiveautomation/user_avatar/forum.inductiveautomation.com/pturmel/32/3295_2.png) [@pturmel](https://forum.inductiveautomation.com/u/pturmel)\
**Post date:** [August 22, 2019, 12:50pm UTC](https://forum.inductiveautomation.com/t/master-and-slave-confusion-over-modbus-and-number-of-maximum-devices/27659/2 "2019-08-22T12:50:06Z")

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> [@miculacristianradu](#):
>
> how many devices cand Ignition support

There's no specific limit, and it doesn't matter what protocol. The actual limit is how much memory and CPU cycles you need in the Gateway to service all of those connections. I'm aware of users with thousands of device connections in a single Ignition gateway.

As for the PLC's, the limits are entirely dependent on brand and model. In general, Ignition is only going to "see" the PLC's memory, not directly into the I/O connected to the PLC.

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**Author:** ![paul-griffith](https://sea1.discourse-cdn.com/inductiveautomation/user_avatar/forum.inductiveautomation.com/paul-griffith/32/37249_2.png) [@paul-griffith](https://forum.inductiveautomation.com/u/paul-griffith)\
**Post date:** [August 22, 2019, 3:25pm UTC](https://forum.inductiveautomation.com/t/master-and-slave-confusion-over-modbus-and-number-of-maximum-devices/27659/3 "2019-08-22T15:25:57Z")

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To add a bit to @pturmel - _each instance_ of a modbus driver in Ignition can itself connect to 255 devices, if the device it’s connecting to correctly follows the protocol. There’s no limit anywhere in Ignition on the number of drivers you can have.

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**Author:** ![miculacristianradu](https://avatars.discourse-cdn.com/v4/letter/m/b782af/32.png) [@miculacristianradu](https://forum.inductiveautomation.com/u/miculacristianradu)\
**Post date:** [August 23, 2019, 6:54am UTC](https://forum.inductiveautomation.com/t/master-and-slave-confusion-over-modbus-and-number-of-maximum-devices/27659/4 "2019-08-23T06:54:41Z")

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Thank you guys for the explanations!