Solution to MSTP frame desynchronization

Updated on 2021.7.13

We had discussed BACnet MSTP weakness to frame desynchronization in below:

BACnet MSTP frame lost synchronization

Attack BACnet MSTP by frame desynchronization

But what is BACRouter’s solution to this problem, let’s look for the clues from the standard.

9.5.2 Variables

SilenceTimer: A timer with nominal 5 millisecond resolution used to measure and generate silence on the medium between octets. It is incremented by a timer process and is cleared by the Receive State Machine when activity is detected and by the SendFrame procedure as each octet is transmitted.

9.5.3 Parameters:

Tframe_gap: The maximum idle time a sending node may allow to elapse between octets of a frame the node is transmitting: 20 bits times.

Tturnaround: The minimum time after the end of the stop bit of the final octet of a received frame before a node may enable its EIA-485 driver: 40 bits time.

Tpostdrive: The maximum time after the end of the stop bit of the final octet of a transmitted frame before a node must disable its EIA-485 driver: 15 bit times.

9.5.5 The SendFrame Procedure

If SilenceTimer is less than Tturnaround, wait (Tturnaround – SilenceTimer).

9.2.3 Timing

Transmitter disable: The node shall disable its EIA-485 driver within Tpostdrive after the beginning of the stop bit of the final octet of a frame in order that it not interfere with any subsequent frame transmitted by another node. This specification allows, but does not encourage, the use of a “padding” octet after the final octet of a frame in order to facilitate the use of common UART transmit interrupts for driver disable control. If a “padding” octet is used, its value shall be X’FF’. The “padding” octet is not considered part of the frame, that is, it shall be included within Tpostdrive.

(It’s unclear that whether the Tturnaround include “padding” octet, but in 135.1 testing standard, chapter “Verify T turnaround”: If the reference master employs a “padding” octet of X’FF’ as the last octet of every frame, then the time shall be measured starting from the trailing edge of the stop bit of the octet that precedes the X’FF’ “pad” octet in the frame transmitted by the reference master)

So in a valid frame,  the maximum bus idle is Tframe_gap plus tailing bit “1” in the previous octet. it’s 29 bits time (assuming previous octet is X’FF’)

Considering “padding” octet, the minimum bus idle between 2 frames is Tturnaround – Tpostdrive + 9 (tailing bit “1” in the “padding” octet), it’s 34 bits time.

BACRouter use a revised RSM to implement previous logic:

  1. When the time between receiving 2 sequential bytes is longer than 20 bits time, the receiving frame is aborted.
  2. Idle time on the bus greater than or equal to 33 bits time means there is a new frame.
  3. To be compatible with devices not respecting to Tturnaround, any data following valid frame will be regarded as new frame.

In 115200bps, one bit time is only 8.7us. To precisely measure duration of idle line, the timer granularity of BACRouter is set to only 5us. It help to resist to frame desynchronization, and reach 98.8% bandwidth utilization on 115.2kbps because BACRouter no more waste time when 40 bits Tturnaround is over.

Attack BACnet MSTP by frame desynchronization

As pointed out by previous article “BACnet MSTP frame lost synchronization” , BACnet MSTP has a design flaw on frame synchronization, but how to utilize it to perform attack and strictly obey the standard at the same time?

We make some assumptions here:

  1. There are at least 3 devices on the bus with MAC address 1, 8,10. The device 1 is carefully designed to perform attack. Device 8 and 10 are innocent.
  2. Device 1 supports extended frame, device 8 and 10 are not.
  3. The timers of 3 devices is precise enough.

The work flow of device 1 is:

  1. When get token, send out frame A
  2. Pass token to MAC address 2
  3. When get token again, send out frame B
  4. Pass token to MAC address 2
  5. goto step 1 again.

Frame A is a valid proprietary frame (hexadecimal);

55 ff 80 ff 01 00 1d a3 02 2b 72 fe 55 ff 03 08 01 00 11 a0 ff 55 ff 21 01 08 00 09 ce d4 f3 55 ff 00 01 08 00 00 bf

Frame B is also a valid proprietary frame as:

55 ff 80 ff 01 00 1d a3 02 2b fe dc 55 ff 03 0a 01 00 11 b1 ff 55 ff 21 01 0a 00 09 fd 8a 51 55 ff 00 01 0a 00 00 8c

Every thing will go well if there is no frame desynchronization, but after hours running, if device 8 losses synchronization with frame A header (It has same effect if device 10 losses synchronization when device 1 sends frame B) , device 8 find another frame when scan Frame A’s data portion:

55 ff 03 08 01 00 11 a0 ff 55 ff 21 01 08 00 09 ce d4 f3 55 ff 00 01 08 00 00 bf

It’s a Test-Request frame send to device 8,  so device 8 try to reply it after Tturnaround with a Test-Response frame:

55 ff 04 01 08 00 11 ae ff 55 ff 21 01 08 00 09 ce d4 f3 55 ff 00 01 08 00 00 bf

but at the same time, device 1 passes token by sending:

55 ff 00 02 01 00 00 73

So the first 8 bytes of two frames are collided, so device 10 drop invalid header, find data as below:

55 ff 21 01 08 00 09 ce d4 f3 55 ff 00 01 08 00 00 bf

When device 1 finishs sending, it starts receiving data and get the same as:

55 ff 21 01 08 00 09 ce d4 f3 55 ff 00 01 08 00 00 bf

For device 10, it get a valid Not-For-Us frame header, so it enter SKIP-DATA state, there is not enough data to skip, so device 10 will wait until Tframe_abort.

For device 1, it’s a BACnet-Extended-Not-Expecting-Reply frame header, because it support extended frame, so it validate header by procedure described in Addendum 135-2012an. Because the data length is too short, so it abort the frame enter IDLE state again, then find another frame:

55 ff 00 01 08 00 00 bf

It’s a token frame passing token to device 1, so device 1 get token then sending Frame B just after Tturnaround:

55 ff 80 ff 01 00 1d a3 02 2b fe dc 55 ff 03 0a 01 00 11 b1 ff 55 ff 21 01 0a 00 09 fd 8a 51 55 ff 00 01 0a 00 00 8c

As mentioned above, Device 10 still wait 1 byte to skip previous frame (because Tturnaround < Tframe_abort), so it miss this frame header. get wrong frame as:

55 ff 03 0a 01 00 11 b1 ff 55 ff 21 01 0a 00 09 fd 8a 51 55 ff 00 01 0a 00 00 8c

It’s another Test-Request frame send to device 10, thing repeats.

From above, every devices strictly obey standard, but once frame desynchronization occurs, the whole MSTP bus is stalled forever.

Read more on Solution to mstp frame desynchronization

BACnet MSTP frame lost synchronization

There are two concepts of frame:

  1. BACnet MSTP datalink layer frame, it has at least 8 octet bytes, including: 2 preamble bytes of 0x55 and 0xff, frame_type, destination_mac, source_mac, 2 data_len bytes, crc, and omissible data portion.  We call this type of frame as “MSTP frame”.
  2. EIA-485 frame, it is consisted of bits, including start bit, data bit, parity bit, stop bit. BACnet MSTP using non-return to zero (NRZ) encoding with one start bit, eight data bits, no parity, and one stop bit. The start bit shall have a value of zero, while the stop bit shall have a value of one. The data bits shall be transmitted with the
    least significant bit first. We call this type of frame as “byte frame”.

BACnet MSTP Receive Frame Finite State Machine (hereafter refer to RSM) distinguish starting of frame by preamble bytes. If there is no DataAvailable or ReceiveError within a frame for Tframe_abort (60 bits time, but Implementations may use larger values for this timeout, not to exceed 100 milliseconds) , the frame is aborted, RSM search for next frame again.

Because preamble bytes is allowed on other portions of MSTP frame (Extended frame introduced by Addendum 135-2012an is an exception, it use COBS encoding to avoid 0x55 existing in data portion), so RSM may parse MSTP frame beginning from data portion of previous MSTP frame.

The minimum time gap between MSTP frames is Tturnaround (40 bits time), it is less than Tframe_abort, so if RSM lost sychronization of MSTP frame, it may parse the wrong MSTP frame across actual MSTP frames.

There are several cause of losing synchronization:

  1. Program defect on sending or receiving device. It could be eleminated by code review and test.
  2. Timer precision. BACnet MSTP standard only requires a 1% precise timer with a resolution of 5 ms or less. So it is hard to check out Tframe_abort. Because there is only 5 ms of error space between delay and timeout (Tusage_delay to Tusage_timeout, Treply_delay to Treply_timeout), it is very likely causing collisions for slow responding devices.
  3. Noise on bus line. Noise causes byte frame error and crc error, RSM will abort previous MSTP frame in both situations.

Some may argue that MSTP frame integrity is protected by header crc8 and data crc16. Even without considering malicious devices(Here is a attack example) and random data collision, there are still applications may send valid frame in data portion.

(Updated 25 June 2021) For example, There are some products that have implemented a packet capture function on MSTP bus, then the captured data may be transferred by BACnet service (PrivateTransfer or AtomicReadFile?). What would happen if there is electronic noise on the bus when the data is passing through a MSTP network?

It is not only possible to cause MSTP bus being blocked, but also device
malfunction if the wrong frame is a APDU or break whole BACnet
inter-network if the wrong frame is a I-Am-Router-To-Network.

Byte Frame Desynchronization

For the byte frame, losing synchronization usually is caused by noise, no termination or absence of biasing. The symptoms include regarding data bit of 1 as idle line, regarding data bit of 0 as start bit of new byte. In addition to data error, the losing synchronization of last byte in a MSTP frame will introduce measure error of idle time between MSTP frames.

Read more on Solution to mstp frame desynchronization

How fast device feature of BACRouter improves MSTP token pass rate?

The test criterion is: 115.2kbps, 80m cable. 1 router and 10 mstp master devices, max_master=127, no NPDU traffic on bus, define non used mac as fast device.

Router mac

Fast device timeout config

Other device’s mac

Token pass rate(round/min)

Percentage Base no fast












































































BACRouter Changelog

Old Firmware

4.19 2022.05.20 Download

Modbus Gateway CSV file importing:

1. There is a bug ignoring Modbus data unit type from 2th point.

2. Mulltistate point without states defined need not to be forcedly disabled.

4.18 2022.02.07 Download

From v3.00,  we added a integrity check: A relayed NPDU with 255 hop count is regards as malformed and will be dropped. The log will show:    [WARN]npdu_decode_pci: hop count should not be 255 for relayed npdu

Dropping such NPDU may causes interoperability problem with other routers. Though the hop count is incorrect here, BTL test does not completely cover it, So there are still BTL passed routers have such bug.

The more detail could be found here:, Thank MaxBurn very much for reporting and helping us to locate it.

The solution is changing the hop count to 254 and accepting it. The log will show: [WARN]npdu_decode_pci: hop count should not be 255 for relayed npdu, change to 254

4.17 2022.01.10 Download

More informative log

ReadPropertyMultiple: potential buffer overflow when generating large reply apdu.

Race condition when device instance is modified by BACnet service.

When processing write-bdt bvll request, if BDT is not changed, configuration saving will not be fired to save flash’s life.

Race condition when network number of VBUS is changed by initialize-routing-table request while processing confirmed request for virtual device.

MS/TP bug introduced by 4.10 may cause incorrect timing when receiving bad frame head.

4.16 2021.12.13 Download

BBMD: BDT push misses myself.

Modbus: WebUI bug on Master copy.

4.15 2021.9.15 Download

Modbus: more convenient Master/Slave copy.

Modbus: supports batching modify slave’s Modbus parameter.

Modbus: MultiState objects could be mapped from bit range in a register.

4.14 2021.8.25 Download

There is a bug on rescue upgrade that old configuration is not deleted which may prevent WebUI from loading.  If old version WebUI could not be loaded because of invalid configuration, please rescue upgrade to this version twice, first time it will upgrade firmware, the second time it will delete old configuration.

A bug on Modbus virtual device mode is fixed which cause malfunction when virtual devices are more than 5.

A WebUI bug when Modbus slave number overflowing limitation is fixed.

The limiations of Modbus master and slave are changed from 100 to 128.

A WebUI bug when disabling “Packet capture mapping” is fixed.

More name delimiter characters are added. Internal object names are modified to avoid collision with new name delimiter characters.

4.13 2021.8.5  Download

Modbus slave settings add “Batch Address” functionality.

Move MS/TP “Extended frame” option to extended configure mode.

4.12 2021.7.23  Download

A long time bug from 3.x on ms/tp may cause ms/tp port stop working.

4.11 2021.7.16

There is a bug in 4.10 introduced by MS/TP timing revision,  which is caused by higher compiling optimization.

4.10 2021.7.13

BACnet protocol revision is changed to 16

Polarity in binary value object is proprietary property.

MS/TP timing revision for padding byte.

Modbus master: Batch Point, Batch Instance, Batch Import

Modbus slave: Import Point

Modbus master/slave: UI for copy and  move up/down.

4.09  2021.7.02  Download

Compatible to new hardware revision.

BTL compatibility: max_info_frames/max_master are writable from BACnet side.

4.08  2021.6.22

BTL requires routers sending who-is-router-to-network to looking downstream router before reject messages.

Bugs which will be triggered when VBUS is enabled have been fixed.

4.07  2021.6.13

UI bug introduced in 4.05 may prevent user to access WebUI in certain situation. (Please note that http://IP/app/upgrade is a rescue entry to upgrade firmware)

Some minor UI bugfixs.

4.05  2021.6.7

Modbus object name remove top level “modbus_master”.

Object name delimiter “|” become configurable.

Value object is allowed to overlap with output object.

Some UI bugfixs.

4.04  2021.6.1

Lots of update on Modbus Gateway module: Multistate object, Value object, Binary text, Read group, Batch conversion, Import/Export CSV object definition.

Convert BASgatewayLX CSV Configuration To BACRouter JSON

4.03  2021.4.23

Modbus Gateway

Offline Configuration

3.23  2021.3.19

Bugfix for BACnet mapping module for capture control feature. The bug may cause daemon restart if capture is restarted from BACnet side.

Bugfix for max_apdu_length_accepted for outgoing request on ms/tp if extended frame feature not enabled.

More infos including “total/free memory”/”Booting time” are added into “System Settings” page on WebUI.

3.21  2021.2.22

Bugfixs for mstp slave proxy: daemon segfault&restart when binding removing. Incorrect hop count when send I-Am reply on other ports.

Reply APDU and network reject messages should respect busy status of destination net too.

3.20  2021.1.26

Limitation of creating network port is removed, only total network port rule applies now. It’s more easy for user to adjust and switch configuration of port.

If root password has not been set yet, the button to set root password on “System Settings” page will has red color to remind user.

3.19  2021.1.20

 Soc bug that prevent MS/TP from startup will occasionally be triggered on 3.18

3.18  2021.1.18

MS/TP add “Extended frame” option.

Reject-Message-To-Network should be unicasted to compliant to BTL.

TTL of registered foreign device should plus 30 seconds to compliant to BTL.

Confirmed service sent to broadcast address should be dropped on application layer to compliant to BTL

BBMD should drop unicasted forwarded npdu from local subnet with same udp port.

Bug on multiple BBMD detection when checking existence of entry in BDT with different udp port is fixed.

3.17  2021.1.13

WebUI adds network settings page, which will show routing table.

A bug on slave proxy is fixed: when there is a unicasted who-is send to slave device, the device instance range carried by the service should be respected, same as broadcasted one.

3.15  2020.10.26

MS/TP duplicate detection algorithm which is learned from duplicating of JCI FEC2611 and IOM3731 is added.

3.14  2020.10.23

MS/TP duplicated MAC detection is implemented. Duplicated MAC will be shown by Red color in “Recently active other devices”. Because of complexity of behavior on duplicating situation, the detection is inaccurate and for reference only.

A potential bug introduced from 3.0 in the low level MS/TP state machine is fixed.

3.13  2020.10.5

Based testing and clients’ feedback, many(maybe most) devices does not obey the MS/TP receive frame finite state machine described in BACnet standard when they encounter error.  Our proprietary “Timeout Interrupt” feature will cause trouble or degrade performance in this situation.  So “Timeout Interrupt” feature is removed.

More statistics is added to MS/TP runtime information page.

WebUI bug on BIP’s Foreign Device Mode introduced from 3.08 is fixed.

3.11  2020.7.14

Fix mstp calculation bug on time used for invalid packet.

Fix bug on mstp slave proxy introduced by 3.08

Some minor WebUI improvements.

3.10  2020.7.1

The config file format is changed much between 2.x and 3.x,  so from 3.00 to 3.09, when upgrade firmware, the upgrading script will install new config file.  This version add a “Keep config” option when upgrade firmware. If config file have something wrong, the WebUI will popup a warning when it is opened.

3.09  2020.6.30

Fixs bug introduced in 3.08 when fetch mstp runtime info.

Some minor WebUi improvements.

3.08  2020.6.28

Bugfix: 1 hop broadcast distribution is optional for BBMD, From version 3.00, BACRouter force other BBMDs to use 1 hop distribution if cross-network broadcast support is enabled. It’s incorrect behavior and fixed in this version.

BDT editing UI is improved. When submitting, BDT entries will be checked to detect possible misconfigure. The checking is also taken when receiving Write_BDT BVLL message.

If multiple BBMD support is not enabled, when receiving forwarded npdu from BBMD not present in local BDT, the npdu will be dropped and logged. It will help distinguishing configure problem of BBMD on most situation. “Report multiple BBMD” function in last verion is covered by this function, so it’s removed.

The filename of downloaded config is set as IP_Version.json, for example:

Config uploaded from file will be edited in WebUI before saving to eliminate any potential incompatibility.

3.06  2020.5.22

Ethernet is disabled by default. If BIP and Ethernet are enabled at the same time, a warning of possible circular route will be promoted.

The default capture size increases to 4M.

3.05  2020.5.15

Capture control function is mapped to BACnet.  Each datalink port has its “Capture buf_size” and “Capture command” Multi_State_Value. “Capture buf_size” could be set between “64K” to “16M”. “Capture command” could be set between “Stop&Clean”, “Start”, “Stop”.

3.04  2020.4.28

Password for root could be modified from WebUI now.

3.03  2020.4.23

BACRouter verifies other router’s live by query its routing table with Initialize-Routing-Table message. But addendum 135-2012AL removed the enforced supporting of Initialize-Routing-Table from router. So in this version BACRouter also monitors Reject-Message-To-Network message that could only be sent by router.

BIP BVLL header of forwarded npdu is 6 bytes larger than other npdu, so there was potential bug on packet length verification that is fixed on this release.

3.02  2020.3.26

Minor bugfixs for UI

3.01  2020.3.17

“Download config” and “Upload config” are added to “System setting”

3.00  2020.3.10

Add Packet capture feature:

Supported on all ports(Ethernet, BIP, MSTP).

Because Wireshark could not decode MSTP extended frames, MSTP extended frames are converted to traditional data frames. Fortunately, Wireshark doesn’t complain about overflowed data length.

MSTP packet could be downloaded as “Interval format”. it could be used to show exact idle time before packet by selecting “Seconds Since Previous Captured Pakcet” as “Time Display Format” in Wireshark. It is very helpful to analyze timing and performance problem.

Continuous download is supported. Automatic traffic logging is possible with the API.

MSTP auto addressing feature is removed.  Because the same time online for all devices could not be guaranteed, there is no perfect auto addressing solution. To help determine max_master and unused MAC on bus, “Sniffer mode” may be enabled, then “Current max master” could be obtained from run time info. Unused MAC also could be chosen referred to “Recently active devices”.

MSTP support real time analysis for max master and show it on run time info, if the value is not matched to the setting, “unmatched” is showed after it.

MSTP support sniffer mode. it could analyze and record data without interfering the bus.

The WebUI of MSTP port add options for basic mode and extend mode.

The timeout settings for MSTP are changed from Integer to Float.

MSTP port’s slave proxy is only support broadcasted Who-Is in previous version. Now it support unicast too.

MSTP port’s slave proxy send unicasted I-AM instead of broadcasted one in previous version.

There is a bug in previous version that Who-Is packet readed by MSTP slave proxy module bypass source address routing verify. it’s fixed this time.

BBMD module add option to log error when multiple BBMDs are found in local lan.

Modify feature introduced by version 2.17. “Accept unmatched destination IP” option is provided, if it is selected, only warning message will be logged, else error message will be be logged and the packet will be dropped.

Many WebUI improvements.

2.18  2019.4.16

Improved IP&Ethernet processing performance.

Process 0 in unconfirmed COV notification service is reserved for unsubscribed notificationCOV. Not longer reports error when can not find matched process for 0.

2.17  2019.4.2

In previous version, when BACRouter receive Original-broadcast BVLL from unicast address or Original-unicast BVLL from broadcast address, it would print error message then drop it.  A customer reported that Johnson Controls’ CCT sends Original-broadcast BVLL to unicast address, so now we only print warning message.

2.16  2019.2.25

From version 2.0.8,  only proprietary network layer messages would be relayed. Now all unknown network layer messages will be relayed.

2.15  2017.10.19

Redirect URL from “/” to “/?” to support Edge web browser (avoid keep asking authorization)

MSTP: Fix bug in standard state machine which would cause passing token to self when duplicated token is found.

MSTP: With standard state machine device in sole master mode would send max_info_frames * Npoll frames after polling one master. A more reasonable behavior is implemented that device send max_info_frames * Npoll frames after finish polling all masters.

2.14  2017.10.13

Fixed bug introduced from 2.0, it should not wait reply when it send expecting reply request to mac address 255 on MSTP.

2.13  2017.9.19

Potential bug (barely) may be triggered if MSTP auto mac and baudrate forcing are enabled together.

2.12  2017.7.10

Optimizing route entry protection: Deletion from route hop will bypass protection.

Support devices utilizing 6.5.3 methods 1 and 4 for establishing the address of a BACnet router for a particular DNET.

2.11  2017.6.16

This update help building up routing table quickly in large BACnet inter-network.

When BACRouter startups, it broadcasts out Who_Is_Router_To_Network without network number specified to learn routing info from other routers.

When Who_Is_Router_To_Network/Router_Busy_To_Network/Router_Available_To_Network without network number specified is received, BACRouter reports network number up to 1120 instead of 112 in previous version.

2.10  2017.6.13

Bugfix: ReadRange on property_list should not return object_identifier; object_name; object_type.

Delay 1 second for sending Network_Number_Is when router startup, because if router is a BIP foreign device, the foreign device registering have not completed yet.

2.09  2017.6.10

Protect route entry 60 seconds from last activity. Avoid broadcast storm in circular routing path.

2.08  2017.6.7

Bugfix: Vendor proprietary network message should be correctly relayed.

When relay Reject-Message-To-Network with reason 4(Too long), use unicast address instead of broadcast address.

Relax daemon’s max_apdu_retries to 10, in accord with WebUI.

2.07  2017.6.5

Compliant to BACnet Router and BBMD device profile.

Try to purge invalid route entry if no reply is received for Who_Is_Router_To_Network or Initializing_Routing_Table. This feature helps to rescue from mess network configuration.

Avoid sending too many I_Am_Router_To_Network with circular routing path.

Reduce memory consumption in complicated inter-network.

2.06  2017.6.1

Avoid BBMD broadcast storm. If Forwarded-NPDU is received from broadcast address, do not re-broadcast it.

When relay Reject-Message-To-Network with reason 1/2/4, use broadcast address.

Discard Reject-Message-To-Network with incorrect route info.

Check source net of incoming NPDU.(Trickily handle Initializing-Routing-Table)

2.05  2017.5.23

Bug: global broadcasted initializing-routing-table should be executed.

Bug: WebUI BIP mode select should not be applied without submiting.

Limit number of route entry in runtime info page to avoid request failure in very large internetwork.

2.04  2017.5.11

Fix WebUI Bug for verifying BDT.

Save configuration and restart when receive reinitialize-device service request.

2.03  2017.5.7

BIP BBMD NAT mode support udp port modifying.

BIP BBMD NAT mode do not listen to local net broadcast address and not send broadcast to local net.

BIP foreign device mode do not listen to local net broadcast address.

2.02  2017.5.5

Regards device sent Who-Is-Router-To-Network has no route to that network.(Delete that route entry from route table)

When Initializing-Routing-Table request modify network number of route port, I-Am-Route-To-Network is sent to report new network number.

Fix bug introduced by ver2.0 for saving new BDT modified by BVLL Write-Broadcast-Distribution-Table message.

2.01  2017.5.3

Add upgrade entry “http://ip/app/upgrade for rescue.

More intuitive log.

Improve network reject behavior.

Fix overflow bug when send I-Am-Router-To-Network(if network numbers reported > 738).

Sending I-Am-Router-To-Network at dynamic time interval.

2.0  2017.4.19

Option to enable Device object inside router.

Allow setting modification from BACnet side.

Support What_is_Network_Number and Network_Number_Is message

Option to support MSTP max_info_frames by occupy time.

Introduce MSTP new concept of auto/forced baudrate.

Guarantee of MSTP message delay(10 seconds).

WebUI is more intelligent.

1.22  2016.10.26  Download

Increase max route table entry from 1024 to 65534(unlimited).

1.21  2016.10.18

Slave Proxy: Increase ReadProperty(Multiple) sending rate if remote device is fast device.

BVLL response NAK for BBMD-related BVLL request but BBMD is not enabled. (Addendum 135-2012ax-5)

BVLL response NAK Upon receipt of a Distribute-Broadcast-To-Network message from a un-registered foreign device. (Addendum 135-2010ad-10)

1.20  2016.10.8

BIP provide option to accept buggy broadcast to

Reduce minimum value of register interval for BIP Foreign Device Mode from 30 seconds to 15 seconds.

1.19  2016.9.23

Change webui reboot to restart, if IP/Netmask/Gateway/DHCP is not changed, fastly restart self

MSTP add fast device timeout interrupt feature

MSTP fast device minimums timeout is set to 0ms (actually 1.5bits plus 50us after turnaround)

MSTP fast device minimums token pass timeout is limit to 20ms

MSTP runtime info shows devices is active in past 10 seconds

1.18  2016.9.13

BIP BBMD support NAT, add two config parameter: “Accept BDT push from other” and “Accept foreign device register”.

Show Ethernet MAC address on “Setting” page.

1.17  2016.9.8

Requested by some OEM customer, now login authorization domain could be customized when place order.

1.16  2016.9.7

Bugfix: slave proxy should not response to who-is from port that device is not proxied on.

Bugfix: write to remote BBMD BDT fail

Lots UI improving.

1.15  2016.9.1

New slave proxy feature.

MSTP add no_turn_around frame count and padding frame count diagnosis information.

Some UI improved.

Fix decode bug when no turnaround frame following a padding frame.

1.14  2016.8.24

It’s a big release, many new features and bugfix:

MSTP add setting for Treply_timeout and Tusage_timeout.

MSTP add fast device feature, allow user define some nodes as fast device, apply as short as 1 milliseconds to Treply_timeout and Tusage_timeout

MSTP add more diagnosis information as: mac conflict frame count, send conflict count, token lost count, pass token fail count, reply timeout count, frame error count, token duplicate count.

MSTP enhance compatible to some devices no respecting 40bits turnaround requirement.

MSTP fix bug decode extended frame.

MSTP fix bug calculate test response header crc on specified situation.

1.13  2016.8.16

Low level api change, not affected.

1.12  2016.8.12

Add serial no.

1.11  2016.8.10

Firmware filename has version and crc.

Show version on left tree menu.

Auto refresh page after reboot or firmware upgrade .

Other UI bugfixs.

CCN Gateway changelog

2.9 2017.5.30

Add Compatibility to a strange 30RQ chiller.

Remove feature introduced by 2.2 (Release forced point when initialization).

2.8 2016.10.26

Buffer overflow while convert Unicode string to utf8.

2.7 2016.10.3

WebUI use basic authorization method.

WebUI table index overflow (Thanks to Nitin)

2.6 2016.9.13

WebUi show Ethernet address.

2.5 2016.9.13

Discard character out of 0-9, a-z, A-Z in string.

2.4 2016.8.26

Selectable 0.001,0.01,0.1,10,100,1000 multiply mapping

BACnet version merge bugfixs from BACnet Router firmware.

2.3 2016.8.16

Low level api changed, prepare for future product

Bug introduced in 2.1 is fixed: continuous error after communication fail.

2.2 2016.8.12

When initialize device, if point is forced by high priority, release it.

2.1  2016.8.10

Firmware filename has version and crc.

Show version on left tree menu.

Auto refresh page after reboot or firmware upgrade .

Other ui bugfixs.

Bandwidth of BACnet MSTP

A common problem of BACnet MSTP is low bandwidth utilization. Some researches disclose that more than 50% traffic on MSTP bus is token frame.

Unfortunately, many poor implement of MSTP make it worse. For example, in standard, the Tturnaroud is 40 bits, that is said, on 115200bps, node only need to wait 0.35 millisecond to send frame when it receive token. But in practice, there are a lot of devices wait at lease several milliseconds.

Our implement of MSTP try best to improve bandwidth. In previous laboratory test, we achieve 98.8% bandwidth utilization on our BACnet router. The test criterion is:

115200bps, 4 node with mac address from 0 to 3, max_master set to 3, no NPDU. in theory, the max token relay rate is 960 per seconds. we get 948 per second.

In another test, we send a read property request (23 bytes frame) each second, then get a Reply_Postponed frame, then a  read property response (29 bytes frame). we still get token relay rate of 943 per second.

Update Feb, 2017, We make a stress test for ReadProperty service on above configuration, set max_info_frames to 10 in all devices, 2 devices request, the other 2 devices reply. The result is 153~154 ReadProperty requests are served each second.

Update March,2017. Thank to our detailed runtime info, user could get token pass rate easily. 9 months after we release first firmware, We perform above test again, then get 956.6 per second, 99.6% bandwidth utilization.

Demo Gateway

For user to get familar with CCN Gateway, we build up simulated DEMO Gateway.

This demo Gateway link with 6 CCN chillers, each has same bus number of 0, element addreses are from 1 to 6.

The username to acces is:  admin, no passowrd is needed.

For modbus gateway, the acces point is

For BACnet gateway, the access point is

User could use modscan or VTS to test the modbus/BACnet function. The port number is defined in the Gateway’s general setting section.

Because the accsss is open, if there are users operate it concurrently. it would behave strangely. If you want exclusively operate it, please contact us, we would give you other private access point.



Posted in CCN