Siemens MP377 Panel PC – DIP Switch Configuration - Page 1
Forum section: electronics / projects

author: Z. Khalid
author_rank: Contributor
timestamp: September 18, 2025, 11:06:00 pm
content: Hi there!Issue Summary:The user manual for the Siemens MP377 panel PC specifies that when all DIP switches (S1) are set to the OFF position, the communication protocol is configured for RS-485 communication. When DIP switches 1 and 4 are set to OFF and DIP switches 2 and 3 are set to ON, the communication protocol is configured for RS-422 communication.However, the back cover of the PC, indicates that when the DIP switches are in the OFF position, the communication protocol is set to PPI, MPI, or DP (Profibus, Multi-Point Interface, Point-to-Point Interface). When DIP switches 1 and 4 are OFF and 2 and 3 are ON, the communication protocol is set to RS-422 or RS-485.I find these conflicting descriptions confusing and would like to understand if I am mistaken or missing some critical detail. Could someone please clarify whether there is additional information or context that explains this discrepancy? I am unable to reconcile the differences based on the current documentation.Please refer to the attached photograph.Thank you!

author: jwet
author_rank: Frequent Contributor
timestamp: September 19, 2025, 04:04:11 am
content: This reminds me of my days at Maxim listening to customer inputs and FAE inputs on their interface requirements so we could design parts around them.No offense but in my experience, when Germans get involved in things, they have a tendency to take over. This is true with Bosch in Automotive and Siemens in fieldbusses. They are both confusing.RS-485 and RS-422 are differential signalling physical layer standards. RS-485 is usually half duplex using 2 wires and RS-422 is full duplex usuually implemented as 4 wires.PPI/MPI/DP are "protocols" that run on these physical layers. Point to point is the oldest and slowest, MPI is semi modern and supports up 12M and Profibus is sort of the current standard. There are good Wiki's on all this stuff, its ambiguous and messy- best source I've found. It reached peak insanity around 2000 when all this stuff was common in the field.There are similar issues with CAN. CAN is an established automotive physical interface standard that exists in several variants. There is also a protocol called CAN that in many industrial environments use an RS-485 interface and occasionally an automotive CAN phy standard.Its messy, you are rightfully confused.

author: Z. Khalid
author_rank: Contributor
timestamp: September 19, 2025, 07:02:16 am
content: Thank you for your detailed explanation. That being said, can we draw a conclusion regarding the DIP switch configuration:The DIP switches primarily define the physical communication interface. For example, the setting determines whether the port operates electrically as RS-422 (typically 4-wire differential signaling) or RS-485 (typically 2-wire differential, half- or full-duplex). These settings govern the electrical characteristics of the communication channel, ensuring proper signaling, termination, and compatibility with connected devices.The actual communication protocol (PPI, MPI, or DP/Profibus) operates at a higher layer and is not determined by the DIP switches. Instead, the protocol is configured separately within the HMI’s firmware or software settings, which control message framing, timing, and network addressing. In other words, the switches set the “physical highway,” while the firmware/software selects the “language” that travels over it.This interpretation reconciles the manual and the back cover diagram in the first scenario, when the DIP switches is set to OFF: one specifies the physical interface, the other indicates the supported protocols compatible with that interface; however, the inconsistency remains in the second scenario, where the switches is set to different positions (S1,S4 are set to OFF and S2,S3 are set to ON).

author: jwet
author_rank: Frequent Contributor
timestamp: September 19, 2025, 07:09:23 am
content: I think you've got it. Good luck. I made an edit to my response to make it less ambiguous going forward- it didn't change anything. Use the thanks button for thanks, Its how I measure my impact here.