BACnet, Modbus and the integration layer where the project money actually goes. The protocols are the easy part: what neither of them standardises is what a point means, and that gap is where the integration budget goes.
Four ideas to carry into the next specification conversation you have.
BACnet is ANSI/ASHRAE 135, and ISO 16484-5 republishes the same text about an edition behind. What a device can do is set by its Protocol Revision: 135-2020 was PR 22, and 135-2024 plus its published addendum is PR 31. Addenda approved together share one PR, and addendum 135-2020ce kept the MS/TP letters but replaced the master-slave wording with multidrop serial bus/token passing.
Section 4.4, verbatim: "The pre-mapping between the MODBUS data model and the device application is totally vendor device specific." The specification fixes big-endian for one 16-bit item and says nothing about a 32-bit float, so word order is a question to ask, not an assumption. The 40001 everyone types appears in no current Modbus specification: it comes from Modicon PI-MBUS-300, June 1996.
Since addendum 135-2016bm (June 2018) the limit is 32 unit loads per segment, and the PICS carries the transceiver's unit loading. At an eighth of a unit load the electrical budget allows up to 256 devices. Only 9600 and 38400 baud are required, and 115200 is the only rate whose recommended segment drops, to 1000 m.
Every commandable point applies the first non-NULL value of its 16 priority slots. The standard's own example puts the scheduler at 9 and a manual override at 8, so one forgotten override outranks the schedule until someone writes NULL back. Annex H makes gateways implement all 16 entries, and still lets a conforming gateway answer Result(+) before the write has reached the device.
Per ANSI/ASHRAE 135 and its addenda, the Modbus specifications, NCA NCS-1:2020 and the Mostadam manuals. Every source is linked at the foot of the page.
| Item | Value | What it means |
|---|---|---|
| BACnet identity | ASHRAE 135 / ISO 16484-5 | ISO republishes the 135 text about an edition behind. Cite the Protocol Revision |
| Protocol Revision | PR 22 → PR 31 | 135-2020 was PR 22; 135-2024 plus addendum cu is PR 31. Addenda approved together share a PR |
| MS/TP segment limit | 32 unit loads | Not 32 devices (addendum bm, 2018). An eighth-unit-load transceiver allows up to 256 on the electrical budget |
| MS/TP required baud | 9600 and 38400 | The rest are optional. Only 115200 shortens the recommended segment, to 1000 m |
| Priority array | 16 slots | First non-NULL wins. The standard's example: scheduler at 9, manual override at 8 |
| Gateway, Annex H | all 16 entries | Its one data mandate. A gateway may still answer Result(+) before the write lands |
| Modbus data model | vendor specific | Section 4.4, verbatim. Ask for the address base, data type and word order |
| Modbus security | zero mentions | MODBUS/TCP Security: port 802, TLS 1.2 or better, mutual X.509v3 certificates |
| BACnet/SC transport | TLS 1.3 | A WebSocket to a primary or failover hub; certificates signed by the site's CA |
| Saudi certificate limits | 20/10/5/3 | Maximum years, NCA NCS-1:2020 s. 5.2 via ECC 2-8-3: root, intermediate, end-user MODERATE, ADVANCED |
| Saudi protocol mandate | none | Mostadam names no protocol. Commercial D+C: E-02 metering and PMM-01 commissioning are Keystone |
Five corrections worth carrying into the next specification.
An Object_Name only has to be unique inside the device that holds it; the one exception is the Device object's own name, which must be unique across the internetwork. There is no naming convention and no controlled vocabulary, so three vendors can name one zone temperature three ways and all three conform. That is the gap Project Haystack and the Brick Schema close. ASHRAE 223P, now titled "Semantic Data Model for Analytics and Automation Applications in Buildings", is still not published: its first formal public review ran in mid-2025 and a second from April to June 2026, so do not print a publication year for it.
The words "security", "authentication" and "encryption" appear exactly zero times in the 50-page Modbus Application Protocol Specification. A secure version exists and almost nobody specifies it: MODBUS/TCP Security on port 802, TLS 1.2 or better with mutual X.509v3 certificates, first published in July 2018 and now at v36 (2021). Modbus TCP is profile CP 15/1 in IEC 61784, while the serial line has no IEC profile, and the Modbus Organization's tested-devices page lists two companies, so a "Modbus certified" claim should name what was tested.
ASHRAE deleted Clause 24, Network Security, with addendum 135-2016by in November 2019, because it had "very limited industry acceptance". BACnet/SC was approved the same day: each node holds one live WebSocket connection over TLS 1.3 to a primary or failover hub, and every certificate is signed by a CA the site controls, in the standard's words "The signing CA shall be controlled by the site." The standard sets no certificate lifetime and no renewal interval. In Saudi Arabia the NCA's National Cryptographic Standards (NCS-1:2020), binding through ECC control 2-8-3, cap certificate validity instead: root CA at most 20 years, intermediate at most 10, end-user at most 5 at the MODERATE level and 3 at ADVANCED.
Annex H is normative, and its one mandate about data is that "Gateways are required to implement Priority_Array properties correctly with all 16 entries." Its other hard rules are about timing and silence, such as aborting a request that overruns APDU_Timeout. The rest is a local matter: a conforming gateway may return Result(+) to a write it has not yet performed, and may read back the value you wrote instead of the value in the device. Since addendum 135-2012al, a gateway presenting virtual BACnet devices must also supply a second PICS for them.
The four Mostadam manuals run to 878 pages and never name BACnet, Modbus, KNX, DALI, LonWorks or MQTT, although they do name building management systems. The commercial Design + Construction manual makes E-02 Energy Metering and PMM-01 Systems Commissioning Keystone credits, required whatever rating is targeted, with sub-meters covering at least 90% of energy demand; its guidance says meters should feed a system "capable of being remotely connected to Sustainable Building". The residential manual makes metering a keystone only from Silver, and the code's metering rules sit in SBC 1001 section 603. The named person is the Commissioning Agent under PMM-01: at least five years of commissioning experience, reporting directly to the owner, not the contractor's employee, and appointed before the design-phase commissioning review starts.
Seven lines, each naming a document, a quantity or a date. None of them can be answered with a brochure, which is the only test a specification clause has to pass.
a. Each BACnet device: a PICS stating its Protocol Revision as a number and its transceiver unit loading.
b. Each gateway presenting virtual devices: a second PICS for them.
c. Each MS/TP segment: unit loads summing to 32 or fewer.
d. Each Modbus device: a register map giving address base, data type and word order.
e. At handover and at the end of the defects period: every non-NULL priority slot, with its reason.
f. If BACnet/SC: the site CA and its renewal procedure, within NCA NCS-1:2020 limits, in the O&M manual.
g. A Commissioning Agent with at least five years' experience, reporting to the owner, appointed before the design-phase commissioning review (Mostadam PMM-01).
Line e is the one nobody writes, and it is the only one that finds an override nobody cleared.
The deliverables that make an integration verifiable are specific, and the clause above lists them: a PICS with the Protocol Revision and the transceiver unit loading for every device, a second PICS for any gateway's virtual devices, a register map with address base, data type and word order for every Modbus device, and a report of every non-NULL priority slot at handover and again at the end of the defects period. That last one is how an override nobody cleared gets found.
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