{"id":5086,"date":"2019-09-16T20:18:59","date_gmt":"2019-09-16T19:18:59","guid":{"rendered":"https:\/\/www.purplemeanie.co.uk\/?p=5086"},"modified":"2026-05-28T11:50:53","modified_gmt":"2026-05-28T10:50:53","slug":"ecu-diagnostics-part-12-osi-7-layers-for-caterham-diagnostics","status":"publish","type":"post","link":"https:\/\/purplemeanie.co.uk\/index.php\/2019\/09\/16\/ecu-diagnostics-part-12-osi-7-layers-for-caterham-diagnostics\/","title":{"rendered":"ECU Diagnostics – part 12 : OSI 7 Layers for Caterham Diagnostics"},"content":{"rendered":"\n

Communications protocols are often called protocol stacks… they’re layers, or stacks, of different protocols… one layered on top of the other from lowest level (hardware\/physical) to highest level (software application).<\/p>\n\n\n\n

This is where the OSI 7 Layer model comes in… it standardises how we think about the layers and allows us to talk about where each protocol and hardware\/software sits in the layer stack.<\/p>\n\n\n\n

More info on the OSI 7 Layer Model can found here<\/a>.<\/p>\n\n\n\n

If we now load the OSI model with the layers for the MBE ECU protocol stack then the table looks like this:<\/p>\n\n\n\n

\"Layered<\/figure>\n\n\n\n

From the table above…<\/p>\n\n\n\n

MBE ECU OSI Function<\/h3>\n\n\n\n

Application<\/h4>\n\n\n\n

At the highest OSI level we have the application layer. This is what the user sees, turning all the lower level layers into something pretty to see.<\/p>\n\n\n\n

mbe.py<\/h4>\n\n\n\n

This is my helper class. It takes the more complicated layers below and provides a simpler abstraction layer for the application to use.<\/p>\n\n\n\n

MBE ISOTP<\/h4>\n\n\n\n

I’ve called this layer the MBE ISOTP layer. It adds a communication layer on top of the ISOTP packetization layer. This MBE ISOTP layer provides “commands” for config and data extraction from the ECU. It does so “on top” of the ISOTP layer below, or another way to think of it is to say the MBE ISOTP packets sit “inside of” of the ISOTP packets.<\/p>\n\n\n\n

ISOTP<\/h4>\n\n\n\n

This is the standard used to extend CAN bus packets from 8 bytes up to a maximum of 4095 bytes per packet and sits on top of the CAN bus protocol.<\/p>\n\n\n\n

It also provides for an additional paradigm called flow control. In theory flow-control should allow CAN bus devices to negotiate the timing between CAN bus frames that each device can support.  <\/p>\n\n\n\n

HOWEVER – The MBE\/SBD Easimap and ECU… DO NOT USE FLOW CONTROL. This caused us some concern to start with and meant we had to adjust some of the ISOTP libraries we were using, see here<\/a>.<\/p>\n\n\n\n

CAN Bus<\/h4>\n\n\n\n

We’ve talked about this layer a lot, the most relevant post about it can be found here<\/a>.<\/p>\n","protected":false},"excerpt":{"rendered":"

Maps the Caterham diagnostics stack onto the OSI model, from CAN bus hardware through MBE ISO-TP layers up to Python tooling and applications.<\/p>\n","protected":false},"author":1,"featured_media":5099,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"advanced_seo_description":"","jetpack_seo_html_title":"","jetpack_seo_noindex":false,"_jetpack_newsletter_access":"","_jetpack_dont_email_post_to_subs":false,"_jetpack_newsletter_tier_id":0,"_jetpack_memberships_contains_paywalled_content":false,"_jetpack_memberships_contains_paid_content":false,"footnotes":"","jetpack_publicize_message":"","jetpack_publicize_feature_enabled":true,"jetpack_social_post_already_shared":true,"jetpack_social_options":{"image_generator_settings":{"template":"highway","default_image_id":0,"font":"","enabled":false},"version":2},"jetpack_post_was_ever_published":false},"categories":[20],"tags":[58,66,34,60,62],"class_list":["post-5086","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-ecu-diagnostics","tag-can-bus","tag-caterham","tag-ecu-diagnostics","tag-mbe","tag-python"],"jetpack_publicize_connections":[],"jetpack_featured_media_url":"https:\/\/purplemeanie.co.uk\/wp-content\/uploads\/2019\/09\/ECU-OSI-Layers.png","jetpack_sharing_enabled":true,"jetpack_shortlink":"https:\/\/wp.me\/p8yl38-1k2","jetpack-related-posts":[{"id":4726,"url":"https:\/\/purplemeanie.co.uk\/index.php\/2019\/08\/31\/ecu-diagnostics-part-1-introduction\/","url_meta":{"origin":5086,"position":0},"title":"ECU Diagnostics – part 1 : Introduction","author":"John Martin","date":"August 31, 2019","format":false,"excerpt":"Introduction and index for the Caterham ECU diagnostics series, covering MBE ECU traffic, CAN bus, OBD-II, ISO-TP, test setup, protocol decoding, and supporting Python and Wireshark tooling.","rel":"","context":"In "Caterham Blog"","block_context":{"text":"Caterham Blog","link":"https:\/\/purplemeanie.co.uk\/index.php\/category\/caterham-blog\/"},"img":{"alt_text":"","src":"","width":0,"height":0},"classes":[]},{"id":5112,"url":"https:\/\/purplemeanie.co.uk\/index.php\/2019\/09\/20\/ecu-diagnostics-part-13-three-diagnostic-protocols-in-the-mbe-9a4-ecu\/","url_meta":{"origin":5086,"position":1},"title":"ECU Diagnostics – part 13 : Three Diagnostic Protocols in the MBE 9A4 ECU","author":"John Martin","date":"September 20, 2019","format":false,"excerpt":"Introduces the three diagnostic protocols found on the Caterham MBE 9A4 ECU: MBE-Broadcast, OBD-II, and the MBE ISO-TP protocol used by Easimap.","rel":"","context":"In "Caterham Blog"","block_context":{"text":"Caterham Blog","link":"https:\/\/purplemeanie.co.uk\/index.php\/category\/caterham-blog\/"},"img":{"alt_text":"","src":"","width":0,"height":0},"classes":[]},{"id":5192,"url":"https:\/\/purplemeanie.co.uk\/index.php\/2019\/09\/20\/ecu-diagnostics-part-14-software-framework-mbe-py\/","url_meta":{"origin":5086,"position":2},"title":"ECU Diagnostics – part 14 : Software Framework mbe.py","author":"John Martin","date":"September 20, 2019","format":false,"excerpt":"Introduces the Python mbe.py framework for sending requests to the Caterham MBE 9A4 ECU and formatting diagnostic responses into readable values.","rel":"","context":"In "Caterham Blog"","block_context":{"text":"Caterham 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