Convert BASgatewayLX CSV Configuration To BACRouter JSON

BASgatewayLX (Hereinafter referred to as LX) supports CSV configuration file. There are decent documents about the format and dozens of configuration samples for vary devices on their website. The convert tool we provided could convert the CSV file to JSON file which could be imported in BACRounter’s WebUI to create a Modbus slave station.

Convert on our website

Download and extract it to disk, open index.html by your web browser

Matters need attention:

Though LX claims the object name should be unique, but the accessory “Profile Checker” does not check duplicated object name. We found a lots duplicated object names in the samples. So we decided to disable object with duplicated name in converting.

Some CSV files have characters out of UTF8 code page, we guess they are ISO-8859-1 encoding. So if we fail to decode CSV by UTF8, we fall back to ISO-8859-1.

LX supports maximum 64 bytes of object name and description (With ISO-8859-1 encoding?). BACRouter supports 64 bytes too, but with UTF8 encoding. There has possibility that 64 bytes in ISO-8859-1 is larger than 64 bytes when encoding as UTF8. We will tail-cut too long name and description and promote to user.

BACRouter doesn’t allow 2 output objects mapped to overlapped Modbus address, but LX seems to allow it. We will disable the conflicted object in converting.

LX defines register order for every 32 bits data, but BACRouter has to set unitary byte order for all objects. If the consistent byte order definitions are found when converting, the previous definition will be accepted, the conflict will be promoted to user.

BACRouter has object instance range of 0~999 for every slave, but LX seems no limitation. If instance larger than 999 is found, the object will be auto assigned a new instance.

Offline configuration and import/export

From firmware v4.x,Offline configuration had been introduced. Now user could configure a BACRouter without device on hand, then export configuration to a file. When commissioning on the field, user just need to import previously exported file, “Save&Reboot” to take effect the configuration.

From v4.04,The object definitions for Modbus gateway could be import/export as CSV format. User could effectively setup object to Modbus address mapping In batches with Excel/WPS/Libreoffice, then import it to WebUI for detailed modify.

Offline configure on our website

Download and extract it to disk, open index.html by your web browser (Because IE/Edge doesn’t support Local Storage for file:// URL, the user library for binary text and most used engineering unit will not be kept after leaving the web page)

There are 3 types of configuration file:

BACRouter Configuration File

This type of configuration file keeps all settings in BACRouter. If the firmware version of exporting one is same to importing one, WebUI should not complain of the configuration file.

Modbus Master Configuration File

This type of configuration file keeps setting under the scope of Modbus master.  When importing it, collision detecting logic based on current mapping mode will check every enabled slave in this master, if collision is found, the slave will be disabled.

If the master is RTU/ASCII type, it may also be disabled if there are contention for RS485 port.

With these problem, WebUI will prompt user to re-check configuration.

Modbus Slave Configuration File

This type of configuration file keeps setting under the scope of Modbus slave.  When importing it, if collision detecting logic based on current mapping mode found problem, the slave will be disabled.

It is allowed to import TCP salve configuration file in RTU/ASCII master, and vice versa, but the slave may be disabled if Modbus parameter setting do not work.

With these problem, WebUI will prompt user to re-check configuration.

Modbus Gateway in BACRouter

Update on 2021-06-07  for version 4.05

From firmware version 4.x,  BACRouter has builtin Modbus gateway functionality. This article will try to explain the underlying mechanism.

Modbus Read

BACRouter will periodically read point’s value and cache it.  “Update interval” defines how often BACRouter update the value.

To improve performance, BACRouter will read as more as possible data in one transcation. In Modbus standard, at most 2000 bits (coil or discrete) or 125 registers (input or holding register) could be read in one request, but specific Modbus slave device may only support a lower value.  In BACRouter’s WebUI, there is a Modbus parameter edit dialog in “Slave Settings”, “Multiply read bits” and “Multiply read registers” parameters define the ability of the slave device.

BACRouter will not segment reading of single point even the “Multiply read registers” is less than registers used by the point.

Usually the data we care for has discontinuous Modbus address, for example, We only care for coils on address 1 and 2000, though we could read coil data from address 1 to 2000 in one request and skip returned data from address 2 to 1999,  sending two requests independently for coil on address 1 and coil on address 2000 maybe more efficient. “Bits skipped” and “Registers skipped” parameter define how many unused data units between 2 cared data units are allowed to be skipped. As previous example, if “Bits skipped” is set to 1998, the BACRouter will try to read coil 1 and 2000 in single request. If “Bits skipped” is set to 1997, then BACRouter will read Coil 1 first, then Coil  2000, because bits skipped is 1998 and is greater than the setting of 1997.

On the WebUI page for slave, the “Read Group” button will show how the BACRouter will group the reading and test it. User should pay special attention on server exception of “Illegal data address”.

Modbus Write

BACRouter will not combine multiple writing demand into one writing request. It always queue write request immediately after receiving command.

For writing single data unit, there are two Modbus function codes available. For example, function code 5 is writing single coil,  function code 15 is writing multiple coils but the quantity of coil to write could be assigned to 1. Some devices only support one of them.  “Coil write singly” parameter will specify which function code is used.

In the same way, “Register write singly” is applied to register writing. If any BACnet output object maps to multiple holding registers, it implies the slave device support function code 16 for write multiple registers, function code 16 will be used for writing any register whatever “Register write singly” is set.

Offline

If any read/write request failed, BACRouter will retry it, when the continuous fail count reachs 3, BACRouter will regard the slave device as offline. In offline state, BACRouter will retry the request every “Offline update interval”.

The BACnet side reaction for offline event will be described later.

Modbus Serial Bus

BACRouter supports serial slave devices with different baudrate, parity, RTU/ASCII mode on same RS485 bus. Before sending request, BACRouter will change it serial port parameter based on slave device’s settings.

It should be safe to have 2 devices with same slave address but different baudrate or RTU/ASCII  mode, but preventing collision based on different parity might not be enough, because some devices do not examine parity error.

Single Device Mapping Mode

If VBUS network port is not enabled, BACRouter will work in single device mapping mode, every Modbus slave device will be mapped into exclusive object instance space of 1000 in the only BACnet device defined in “Application Settings”.

The actual BACnet object name will be organized as “master_name|slave_name|object_name”,  The delimiter “|” is select-able by user.

As recommended by standard, BACRouter will create one StructuredView object for each slave device.

When the slave device turn to offline in this mode, all points’ reliability will be set to “COMMUNICATION_FAILURE”, if slave device is back to online, upon updating of point value, the point’s reliability will be set back to “NO_FAULT_DETECTED”.

Virtual Device Mapping Mode

If VBUS network port is enabled. Each slave device will be mapped to a virtual BACnet device on VBUS network. The MAC address will start from 1.

In this mode, each slave shall has a unique “Device instance”. The actual device name will be “master_name|slave_name”, The delimiter “|” is select-able.

When the slave device turn to offline in this mode, as recommended by standard, the virtual BACnet device will stop sending/receiving any packet to simulate offline state. if slave device is back to online, after updating all point’s value, the virtual BACnet device will quite offline state.

Slave Device’s Status

For every slave device, BACRouter will create 3 objects in gateway device, one object is Binary_Input with name of “online”, which represents online/offline status of slave device. another object is Analog_Input with name of “update_delay”, which is recently average update time delayed over “”Update interval”, the last one is Analog_Input with name of “fail_rate”,  which count a relative long time fail rate on Modbus side.

Those objects’ name have prefix of “master_name|slave_name|comm|”, where “|” is select-able delimiter.

Object Instance Assignment

Object Instance is defined for every object. When the editor dialog for a object is submitted and the object is enabled, the object instance claimed by the object will be assigned, if that assigning fail, a automatically selected instance will be used.

Value Object

AV/BV/MV is behaving same as AI/BI/MI, except that when AV/BV/MV is mapped to writable Modbus address (0X or 4X), the present value of the object is writable, the writing will be forwarded to Modbus side.

Output Object

The value of output object is still being polled from Modbus side and set to object’s Relinguish_Default property. If the value read back does not match last value written, BACRouter will set the reliability of that object to “UNRELIABLE_OTHER”,  and BACRouter will queue another write after a certain time.

Because of above verifying mechanism,  the BACRouter will prevent defining two output objects with overlaping Modbus writable address.

NaN For Float And Double

NaN is a special value defined by IEEE-754, BACRouter does not accept NaN (Infinite is still valid for BACRouter).

When a point is defined as Float and Double, but NaN is read from slave, BACRouter will set the reliability to “UNRELIABLE_OTHER”.

Interoperability between extended MS/TP frame capable devices and legacy devices

Original BACnet MS/TP data link specification only supports NPDU length up to 501 bytes, which is much shorter than 1497 bytes of Ethernet and IP data link. It limits transmission performance, increases complexity on application layer, especial when two IP/Ethernet networks are conjoined by a MS/TP network.

Extended frame was designed to solve this problem. The detail could be found here. Briefly, this addendum added two new frame types as:

  • 32: BACnet Extended Data Expecting Reply
  • 33: BACnet Extended Data Not Expecting Reply

Frame type 32 is extended from frame type 5 (BACnet Data Expecting Reply),  the special of it is that it is encoded by COBS and the NPDU length it carried is in range of 502 to 1497 bytes.

In the same way, frame type 33 is extended from frame type 6 (BACnet Data Not Expecting Reply)

Extended frame support was added into BACnet standard since revision 16. There are still lot of devices installed or on the market that do not support it. The interoperability between extended frame capable devices and legacy device is discussed below.

  • Non-router legacy device and extended frame capable device: Because all messages sent to legacy device are application layer message, the “Max APDU Length Accepted” from Device object property or confirmed service request primitive should be respected, the NPDU length will not exceed 501 bytes. So there are no problem with this configuration.
  • Legacy router and extended frame capable device: NPDU that should be relayed to other network through legacy router with length over 501 bytes will be discarded, no reject-message-to network with reason “Message Too Long” will be responded. Even more, the “Max APDU Length Accepted” of legacy router may be determined by other port that has a NPDU length larger than 501 bytes (It is allowed by standard), so NPDU for local application layer sent to legacy router will still possibly be carried by extended frame and discarded. So this configuration may cause problem in field.

BACRouter supports extended frame from very early version. From firmware version 3.18, we introduced “Extended frame” option on BACRouter’s MS/TP configuration, if there are legacy router that does not support extended frame on the bus, this option should be disabled to avoid Interoperability issues.

It’s worth noting that even “Extended frame” option is disabled, unlike legacy router, BACRouter will still be interoperable with extended frame capable devices.

(Screenshot has been updated on Aug 5, 2021. Because extended frame is mandatory from standard revision 16, from firmware 4.13, we move this option to extended configure mode.)

 

 

Fixed/Auto/Forced baudrate for MSTP

Update on 2020-11-20 (Appended info. about JCI module)

MSTP baudrate is always painful for field technician. If the baudrate is wrong, device can’t join a MSTP bus.

Most devices have fixed baudrate. To modify the baudrate setting, technician have to physically access the device and change dip switches. Some devices support changing baudrate by BACnet service. but before that they should already have correctly baudrate setting for BACnet service to access it.

Some vendors implement auto baudrate, but introduce more problem than it solves. There are two types of auto baudrate mechanism:

  • Starting detection: The device detects and adopts baudrate on the bus when it starts. then never changes baudrate.
  • Dynamic detection: The device does same as starting detection type when it starts, but if it find there are error on bus for a predefined time, it considers that the baudrate is changed, it detects baudrate again.

For both types, it is difficult to change baudrate when devices is working. Simply changing baudrate on all fixed baudrate devices can not work, because auto baudrate devices are still working on old baudrate. The solution is to power off all auto baudrate devices, then power on all auto baudrate devices(Don’t power off/on auto baudrate devices one by one)

Our new firmware(>=2.0) introduces new baudrate management mechanism(Patent pending). There are 3 types of baudrate mode for BACRouter : Fixed/Auto/Force:

  • Fixed baudrate mode works as most traditional devices.
  • Auto baudrate mode is same as above-mentioned dynamic detection. The predefined time to re-initiate detection is 10 error frames, it usually take several seconds.
  • Forced baudrate mode is same as auto mode except that when the device get token, it changes baudrate to predefined value.

When there is a device with forced baudrate mode, the baudrate on the bus will be forced to predefined value. Devices with auto baudrate mode will automatically synchronize to predefined baudrate. Devices with fixed baudrate mode but baudrate setting different with predefined value will not be seen on bus (It’s easy to check out in “Recent active devices” field from BACRouter’s runtime info). Devices with starting detection type may run on wrong baudrate, they will not be seen on bus too, but powering off/on them one by one will synchronize them to forced value.

More than one device with forced baudrate mode could coexist on a bus, but the baudrate values on them should be same.

JCI FEC/IOM modules implements baudrate dynamic detection mechanism, the re-detect interval is about 150 sec on the test.

On a test bus , BACRouter cooperate perfectly with FEC2611, IOM3731,  the baudrate is dynamically controlled by BACRouter from 9.6k ~ 76.8k, FEC2611 and IOM3731 will catch up after 2.5 minutes.

BACnet MSTP auto addressing

Updated on 2020.3.25 for firmware version 3.x

Because the same time online for all devices could not be guaranteed, there is no auto addressing solution could avoid MAC conflict. We remove this feature on firmware 3.x. To help determine max_master and unused MAC on bus, “Sniffer mode” could be enabled, then “Current max master” could be obtained from run time info. Unused MAC also could be chosen referred to “Recently active devices”.

Every device on a MSTP bus should have a unique MAC address.  For master device, the available address range is 0~127,  and 128~254 for slave device.

Usually MAC address is set by DIP switch, jumper, LCD screen, firmware downloaded by configuration tools. Some devices support MAC address modification through BACnet object/property, but before doing that, it should have a valid MAC address to join BACnet network.

If the unique MAC address could be automatic obtained like we get IP address just by plugging notebook into home/office network, it would save a lot of time in commission.

There are several solutions had been discussed.    Now seems committee prefer  “Zero-Config” (addendum 135-2012bb)

BACnet stack has implemented “Zero-Config”.

“Zero-Config” only works on fixed configuration that Max-master is 127 and automatic assigned address range is 64~127. If not, it may cause mess.

To avoid above limitation, BACRouter implements proprietary auto addressing solution and keep compatible with “Zero-Config”.  It has some attractive features:

  1. Learning Max-master from bus traffic.
  2. Assigning MAC address from highest unused one.

So users have more freedom on MAC address schema,  For example, leave address 0~30 for fixed address devices, set Max-master as 40, so automatic addressing devices would use 31~40.

Both Zero-Config and BACRouter’s solution have trouble when a automatic addressing device is pulled out bus then plugged in again without reboot, because a new attached automatic addressing device would occupy the same address.(BACRouter is more weak in such situation because of it’s predictable address assigning), So

ALWAYS power on automatic addressing device after attaching to bus.

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 12.1.3.4 “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