Siemens MPI cable

Hello,

We have become accustomed to using a standard serial cable to connect from an MPI port (on a PG laptop) to a Siemens S7 MPI port.

We ran into a problem where Siemens claimed that using such a cable can damage the S7 CPU.

So, we purchased a 901-0BF00-0AA0 MPI cable. After reading the pinout, I found that it has a reduced pin count, but each wire runs straight from pin-pin as it does in a standard cable.

Now, I know that mfgs will commonly sell you cables you don't need or cook up special cables just to increase revenue, but this seems like we've been snowed.

Can anyone comment on:
- why we would need a reduced pin count or this cable in general
- what cables you use for MPI and why
?

Thanks!

In fact, since MPI is RS485, it really only needs two pins to work correctly. But because it is used on high baudrates, you better use cable with the correct internal impedance. If not, you can get standing waves which can damage not only the PLC, but also the MPI converter. At 187.5 kbaud this is relatively unlikely, but if you use the Profibus possibilities then you're talking 1.5 MBaud up to 12 MBaud. And at those speeds you almost certainly get standing waves if the cable isn't the correct impedance.

Kind regards,

2 x Profibus connectors + 2-4m of profibus cable = very nice MPI-cable substitute.

We were tired of Bubba using the MPI-cable as a Null-modem, or attempting to at least, leaving it all over the place and never where it's supposed to when we need it. The nice pink/purple cable is quite obviously PLC related, so they don't touch it.