Adding Siemens S7 PLC to existing RS485 bus.

[sommers9]

I currently have a RS485 full-duplex bus system with slaves running at 230k4 baud.

I'm looking to connect my Siemens S7-315 2PN/DP with this bus system.

I think I should use the CP-341, but I figure there is no way to connect the 230k4 bus, so I should lower the complete bus's speed to 115k2.

Apart from that, the datasheet tells that RS422 uses 4 wires and RS485 uses 2 wires.
Which confuses me, since RS485 can be full duplex.

Also, some sources state that you'll need a ground connection between devices and some state you shouldn't?
Since it's differential, you wouldn't need a ground (reference) since the counter-signal acts like a reference?

 

[JesperMP]

What protocol ?

 

[AustralIan]

I currently have a RS485 full-duplex bus system with slaves running at 230k4 baud.

I'm looking to connect my Siemens S7-315 2PN/DP with this bus system.

I think I should use the CP-341, but I figure there is no way to connect the 230k4 bus, so I should lower the complete bus's speed to 115k2.

> I concur.

Apart from that, the datasheet tells that RS422 uses 4 wires and RS485 uses 2 wires.
Which confuses me, since RS485 can be full duplex.

> I haven't seen the actual spec, but an RS422/RS485 compatible driver has electrical characteristics such that you can use RS422 on RS485. I suspect you will need to place the CP-341 in RS422 mode, which should actually be "RS422 and RS485 compatible full duplex".

Also, some sources state that you'll need a ground connection between devices and some state you shouldn't?
Since it's differential, you wouldn't need a ground (reference) since the counter-signal acts like a reference?

> Ground wire. RS485 spec requires the voltages to be within about +7V of each other. In practice, devices can cope with a little bit more than this. Unless you can be sure that the ground of one system is the same as the ground of the other system, you need to connect the ground wire.

> Not stated, what higher level protocol do they talk? will ASCII or one of the other protocols listed in the manual be acceptable? Model numbers of your slaves will help us determine any other incompatibilities.

 

[JesperMP]

I currently have a RS485 full-duplex bus system with slaves running at 230k4 baud.

I'm looking to connect my Siemens S7-315 2PN/DP with this bus system.

I think I should use the CP-341, but I figure there is no way to connect the 230k4 bus, so I should lower the complete bus's speed to 115k2.

> I concur.

Until you know which protocol is used, you cannot say if the CP341 is the solution or not.

 

[AustralIan]

JesperMP is right, and I should have been clearer that you need to find out what higher level protocol your slaves talk and make sure you can implement it in the CP-341.

 

[sommers9]

JesperMP is right, and I should have been clearer that you need to find out what higher level protocol your slaves talk and make sure you can implement it in the CP-341.

It uses default ASCII serial. (8 databits, 1 stop bit)

So it will only send an ASCII string, whatever happens with it is up to the slaves. The messages will be sent from/to a server and the PLC will only "bridge" them to/from the RS485/RS422 bus.

 

[JesperMP]

For plain ASCII, a CP340 is enough,

 

[mk42]

For plain ASCII, a CP340 is enough,

Wouldn't he need the cp341 for the higher bus speed? I thought the cp340 only went up to 18kbps or something like that.

 

[sommers9]

Wouldn't he need the cp341 for the higher bus speed? I thought the cp340 only went up to 18kbps or something like that.

That'll depend on how low/high I want the bus-speed to be.

But yeah, I figure that 1152kbps is about the max for any of the Siemens RS485/RS422 bus systems.

I'll just get one and check if it'll work in my setup.

 

[JesperMP]

mk42 is right.
CP340 goes to max 19.2 kbps
CP341 goes to max 115.2 kbps