Taps for industrial MODBUS network


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We are installing a MODBUS network that consist in three (3) tracts of shielded twisted pair cable 22AWG that starts from the MODBUS master device (DCS MODBUS module).

One tract go directly to a slave device at a distance of 240'. The others two tracts go each one to one tap to derivate the cable in order to end in two independent slaves device inside same cabinet. The distances of these tracts are 100' and 165'.

My questions are: what tap we can use to this application?? and if could we have some communication problems with these very close device??

We found in the web one tap SIEMENS model 500-5606 and one tap MODICOM 990 NAD 230 00, but possibly these taps works only in their own networks.

Please to advise ASAP in this.

Bob Peterson

Presumably this network is RS485. If so, there are no taps allowed. It has to be daisy chained from device to device.
Nevermind, I found it in Section 3.1 of the Modbus Over Serial Line Specification and Implementation Guide V1.02. There's also a figure included:

Now, keep in mind, the spec also states cable length limitations when using passive taps in section 3.4.3:
The derivations must be short, never more than 20m. If a multi-port tap is used with n derivations, each one must respect a maximum length of 40m divided by n.
This is consistent with RS-485 recommendations that stubs are allowed, but stub lengths must be kept as short as possible.