PXC001 - maximum number of MODBUS slaves on line

Hello,

I am interested what is the maximum number of slaves (devices) which can be connected (without repeaters) to the one PXC001 MODBUS line.

In document CA2Y9772en_06 (Desigo™ PX Open PX Modbus Engineering guide) on page 5 is stated:

PX Open Modbus master
The PX Open Modbus master allows the integration of 1…247 Modbus slaves.

I am a little doubtful about this because I know that Simatic MODBUS modules have limitation of 32 slaves (without repeaters), so I am unsure about this. Is this really true?

I am aware about other aspect of MODBUS line (length, speed, etc.).

Hello,

32 devices without repeater is the RS485 limit

So it's the limitation for the PXC001 too.

For the baudrate, stopbit, etc. there is with PXC001 just one configuration possible

In TX-Open you can have several configurations for the settings of slaves.

Hi,

In document from  modbus.org

(Modbus_over_serial_line_V1_02.pdf), on page 27 stands:

Hi,

An interesting topic.

I have some slides from a Siemens Modbus course that I did several years ago. (PX and TX Open). In addition to the "The PX Open Modbus master allows the integration of 1…247 Modbus slaves." text, it also mentions "However the actual number that can be connected to the PXO is dependent upon the Modbus device".

This marries up with the post above which makes reference to "In this case these MODBUS devices must be documented to say how many of such devices are authorized without repeater". 

Cable length is an important part to bear in mind.

Number of points per 3rd party device is also important. i.e. Compare a Modbus master that tries to read 1 point from a 3rd party device compared to a Modbus master trying to read 25 points from a 3rd party device. Then apply the same thinking for 32 devices.

For years the rule-of-thumb has been to limit the 485 bus to maximum of 32 devices before the requirement for a repeater. It's worth noting that BACnet MSTP (it's of course different, but it does use 485 as it's medium) has a much higher limit of 128 devices. However the baud rate needs to be high, a minimum of 76.8 from memory.

To answer your original question, yes - depending on the 3rd party manufacturer, it may be possible to connect more than 32 devices to a PXC001 Master without a repeater.

Anybody who is engineering a Modbus network with more than 32 devices should, by default, and where possible, be providing repeaters as well. It's good practice to do it this way.

Also - the 2nd post mentions "For the baudrate, stopbit, etc. there is with PXC001 just one configuration possible". This is incorrect. The PX software requires a Communication Block (COMBLK) and also a Message Block (MSGBLK) to be present in the software for the PXC001 to start communicating over Modbus. The COMBLK contains the settings for Baud rate, parity, stop bits, timeouts, delays etc. Much the same as the TX Open settings. The MSGBLK contains info on the License points used and available, and more diagnostic info from the PX-Open application.

Regards,

Hi _CJ,

I have all these parameters in mind which influence eacho other (speed vs. length and number of DPs and devices). In practice, if there is a problem with electrical parametrs of the line or with throughput / bandwidth, it can be solved with speed decrease (if allowed by application requirements).

I was concerned about hard limit of the devices, i.e. if there is electrical drivers limitation.

In summary, number of 32 devices is a minimum which MODBUS standard conformed device shoul fulfill.

It is not a maximum, but it depends from manufacturers.

We had implemented 35 Siemens RDF302 on the MODBUS/KNX protocol convertor and it worked without problems. But I don't know is it an exception or a rule.

The maximum amount of devices on a 485 line is 32 unit loads. 

485 is a current loop so adding move devices = more current in the loop. 

The max count of devices depends on the 485 transciever chips in the device, they can be 1/2 unit loads, 1/4 unit loads or even 1/8 unit loads. If all devices were 1/4 unit loads you theoretically could have 128 devices.

Repeaters are "invisible" to a PXC001 and operate as a "current booster".

I don't actually know myself but I would guess the hard coded limit of Modbus devices on a PXC001 would be about 255 minus a perhaps a few reserved addresses. 

In practice I've found that cable selection, length, terminations and noise are limiting factors before approaching the theoretical maximum limits.

Bill

Sydney