FI20215620A1

FI20215620A1 - Serial bus termination

Info

Publication number: FI20215620A1
Application number: FI20215620A
Authority: FI
Inventors: Matti Kalalahti; Tommi Lavikko
Original assignee: Valmet Automation Oy
Priority date: 2021-05-26
Filing date: 2021-05-26
Publication date: 2022-06-15
Also published as: CN115408319B; CN115408319A; FI129635B

Abstract

Esillä olevan keksinnön mukaisen esimerkkinäkökohdan mukaisesti tarjotaan laitteisto elektronisen viestintämoduulin (10) yhdistämiseksi sarjadataväylään, käsittäen: väylän päätepiirin (11), joka käsittää ensimmäisen kanavatransistorin, FET (engl. field effect transistor) (20), joka käsittää ensimmäisen diodin (21), toisen FET:n (30), joka käsittää toisen diodin (31), ja päätevastuksen (40). Päätevastus on yhdistetty ensimmäisen ja toisen FET:n vastaaviin kollektoriliittimiin. FET:t on yhdistetty ohjausjohtoon (50) laitteiston aktivoimiseksi väylän päättämiseksi ja sovitettu yhdistämään ensimmäisen johdon ja toisen johdon päätevastukseen. Väylän päätepiiri on sovitettu estämään ensimmäisen johdon ja toisen johdon yhdistäminen päätevastukseen vasteena sille, että tulojohto deaktivoi väylän päättämisen.

Description

SERIAL BUS TERMINATION FIELD

[0001] The invention relates to terminating an expandable serial bus.

BACKGROUND

[0002] Industrial automation systems control an industrial process by means of various field devices, e.g. regulating devices, control devices, sensors, transmitters and the like, which are connected to the system. A typical field device is a control valve provided with a valve controller. A field device is typically controlled by a process controller using an appropriate control algorithm on the basis of the measurement results obtained from the process and the set values. Field devices may be connected to a field bus, which may be a serial bus.

[0003] Serial bus arrangements are widely used in various technical fields, including industrial automation control, for networking control units. RS-485, also known as TIA or EIA-485, is an example of a serial communication standard. PROFIBUS and controller area network (CAN) based systems are some examples in which serial buses are applied.

[0004] In many environments, data buses need to be very stable on account of the interference immunity, in particular, and are often in the form of a linear bus. The bus is thus terminated at both physical ends of its geometrical extent with a respective terminating resistor (bus termination) matched to the characteristic impedance of the bus lines. As a = result, reflections produced on the bus lines in the case of higher-freguency data transmission & rates can be minimized and the bus can be operated in an optimum manner. Since the rates 3 of current bus systems have been increasing on account of their real-time capability and N increasing bus load, correct bus termination is very important for reliable communication. x = 25 [0005] Modularity and flexible expanding of a serial bus by new I/O modules is

O S important for automation control systems. When a new I/O module is added to a bus, a

LO N specific bus terminal component may need to be manually inserted to terminate the bus.

O N

[0006] DE102009056563 discloses a bus system, having bus subscribers with impedance, manageable as terminating resistor when mass potential is not supplied to input of galvanically separated controllable circuit.

SUMMARY OF THE INVENTION

[0007] The invention is defined by the features of the independent claims. Some specific embodiments are defined in the dependent claims.

[0008] According to an aspect, there is provided an apparatus for connecting an electronic communication module to a serial data bus, comprising a bus termination circuit comprising a first field effect transistor comprising or coupled with a first diode, a second field effect transistor comprising or coupled with a second diode, and a termination resistor. A source terminal of the first field effect transistor is connected to a first line of the bus and a source terminal of the second field effect transistor is connected to a second line of the bus. The termination resistor is connected to a drain terminal of the first field effect transistor and — to a drain terminal of the second field effect transistor. The first field effect transistor and the second field effect transistor are connected to a control line for activating the apparatus for terminating the bus and adapted thus to connect the first line and the second line to the termination resistor. The first diode is adapted to connect the first line to the termination resistor when gate-to-source voltage of the first field effect transistor is below a threshold — voltage value and while source voltage of the first field effect transistor is higher than drain voltage of the first field effect transistor. The second diode is adapted to connect the second line to the termination resistor when gate-to-source voltage of the second field effect N transistor is below a threshold voltage value and while source voltage of the second field > effect transistor is higher than drain voltage of the second field effect transistor. The bus 2 25 termination circuit is adapted to prevent connecting the first line and the second line to the 2 termination resistor in response to deactivating termination of the bus by the input line. a

[0009] According to a second aspect, an electronic communication module is or D comprises the apparatus of the first aspect or an embodiment thereof. According to a third S aspect, a communications module installation or connecting assembly unit, such as a slave I/O mounting base unit, for connecting a communications module to a serial bus, is or comprises the apparatus of the first aspect or an embodiment thereof.

[0010] According to a fourth aspect, there is provided an expandable serial bus arrangement, comprising a sequence of apparatuses according to the first aspect. The sequence may comprise at least a first electronic communication module or a first mounting base unit thereof comprising the bus termination circuit and a second electronic communication module or a second mounting base unit thereof comprising the bus termination circuit. The bus termination circuit of the second electronic communication module or the second mounting base unit is adapted to, in response to coupling the second electronic communication module as last module in the sequence, connect the termination resistor to the bus and cause disabling termination of the bus by the first electronic communication module. The arrangement may comprise an installation platform adapted to supply operating voltage in the control line for activating the second electronic communication module or the second mounting base unit to terminate the bus.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] FIGURE 1 illustrates an example of a serial bus arrangement,

[0012] FIGURE 2 illustrates an example of a serial bus arrangement according to at least some embodiments,

[0013] FIGURE 3 illustrates a bus termination circuit according to at least some embodiments,

[0014] FIGURE 4 illustrates a bus termination circuit according to at least some embodiments, and

N N [0015] FIGURES 5a and 5b illustrate examples of serial bus waveforms. 3

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E EMBODIMENTS S 25 — [0016] An industrial automation system may be used for controlling industrial 5 processes such as manufacturing, production, power generation, fabrication, and refining N processes. A bus system is typically applied for connecting field devices. I/O modules may be mounted on arail or rack and included as part of the bus system.

[0017] Figure 1 illustrates an example of a serial bus arrangement comprising slave I/O modules 1a, 1b, 1c. The data bus comprises a first line 2 and a second line 3, connected to a master (module) 4. The lines are terminated by termination elements or resistors 5 and 6, to minimize reflections produced on the bus lines. When a new module is added to the bus, the lines need to be terminated by a termination resistor 6. The I/O modules may be fixed to a DIN rail or other type of installation platform, and a separate termination resistor module comprising the termination resistor 6 may need to be manually coupled to the platform to terminate the bus each time the I/O module configuration is modified.

[0018] There is now provided an apparatus for improving expandable serial bus — termination, comprising a specifically adapted bus termination circuit as further illustrated below. The bus termination circuit may be included in each communication module, such as the modules 1a-1c, or in a mounting base unit. The mounting base unit may be connected to or may comprise the bus, and comprises a connector or connector assembly for connecting at least one communication module to connect the module to the bus.

[0019] Figure 2 illustrates a simplified example of a serial bus system, in which separate termination resistor may be avoided. Each electronic communication module, or in the following examples slave I/O mounting base unit, 10, 12, comprises internal serial bus termination circuit portion (BTCP) 11, 13. The mounting base units may be connected to the bus in sequence. The bus termination circuit portions are electronically adapted to switch — bus termination resistor at the bus by the last I/O mounting base unit in the sequence automatically. In the example of Figure 2, the I/O mounting base unit 10 is coupled into the bus as the last unit. Upon connecting the unit 10 to the bus lines 2 and 3, the BTCP 11 disables the previous BTCP 13 of the I/O mounting base unit 12 (which was earlier the last S I/O mounting base unit in the seguence) and switches its termination load to the bus lines 2, LO 25 3.

O N [0020] A control line, or an input thereof, to the BTCP 11, 13 may selectively activate E or deactivate the bus termination resistor in the respective module. In some embodiments, 2 the bus termination resistor may be activated in the last I/O mounting base unit 10 of the D sequence by an appropriate triggering control input 14, in some embodiments by continuous S 30 — supply voltage as further illustrated below.

[0021] The BTCP 11, 13 may be adapted to prevent connecting the bus lines 2, 3 to the termination resistor in response to a deactivating input from the control line. This causes deactivating termination of the bus by the BTCP via the control line.

[0022] As also illustrated in the example embodiment of Figure 2, bus termination by 5 the intermediate module(s) may be deactivated by ground connection of the BTCP 13 and the control line 16 thereof. Hence, the termination load on the bus may remain constant. The mounting base units 10, 12 may thus comprise an electromechanical connector assembly adapted such that adding a further I/O module (or mounting base unit 12) into the sequence causes grounding the control line 14 of the intermediate I/O module/mounting base 10. The BTCP may thus be adapted such that the termination resistor is activated and active as default and deactivated upon grounding of the control line.

[0023] Figure 3 illustrates a BTCP 11 (or 13) according to some embodiments, which may be comprised in an slave I/O mounting base unit, such as the slave I/O mounting base unit 10 or 12, and connectable to serial bus lines 2 and 3 (without limiting in any way to the — example of Figures 1 and 2). In an alternative embodiment, the BTCP 11 may be included in the communication module, which may be connected to the bus (typically by a mounting plate). The example BTCP 11 comprises: - a first FET 20 comprising or coupled with a first diode 21, - a second FET 30 comprising or coupled with a second diode 31, and - a termination resistor 40.

[0024] The FET 20, 30 may herein refer to a FET unit comprising (the FET element and) the respective diode. In some embodiments, the FETs 20, 30 are metal oxide = semiconductor FETs (MOSFETs) and the diodes are MOSFET body diodes. However, it N will be appreciated that other types of FETs may alternatively be applied, and the diode may S 25 — be external from and coupled to respective FET source and drain terminals. © - [0025] A source terminal 22 of the first FET 20 is connected to the first line 2 of the s data bus. A source terminal 32 of the second FET 30 is connected to the second line 3 of the A data bus. The termination resistor 40 is connected to a drain terminal 23 of the first FET and 5 to a drain terminal 33 of the second FET.

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[0026] Gates 24, 34 of the first FET 20 and the second FET 30, respectively, are connected to a control input or line 50. The control input 50 is adapted for activating the respective module and BTCP 11 to terminate the bus, by causing connecting the first line 2 and the second line 3 to the termination resistor 40.

[0027] The control line 50 may be a control input, in which an operating voltage, such as 5 V, or ground may be selectively connected the gates 24, 34. As further illustrated, the input line comprises a resistor 51. When the operating voltage is connected to the gates, the bus termination is activated by the BTCP 11.

[0028] The first diode 21 is adapted to connect the first line 2 to the termination resistor 40 when the bus termination is activated, e.g. by the input line 14 in the example of Figure 2, i.e. the bus termination by the respective bus termination circuit is in active (bus termination) state. The diode 21 connects the first line 2 to the termination resistor 40 when gate-to-source voltage (Ves) of the first FET 20 is below a threshold voltage value, or Vr, such as around 1.2 V for many FET types, and when the source voltage (Vs) of the first FET 20 is higher than drain voltage (Vp) of the first FET.

[0029] Similarly, the second diode 31 is adapted to connect the second line 3 to the termination resistor 40 in active bus termination state when gate-to-source voltage of the second FET 30 is below a threshold voltage value, such as around 1.2 V for many FET types, and when the source voltage of the second FET 30 is higher than the drain voltage of the second FET 30, i.e. Vps < 0.

[0030] When the BTCP 11 is activated for serial bus termination, the BTCP thus operates as below: - When differential signals of the bus are such that line 2 has higher voltage than that = of line 3: The second FET 30 connects line 3 to the termination resistor 40 via FET & channel between the drain and source (when VGS > VTH). The first diode 21 in the 3 first FET 20 connects the termination resistor 40 to the line 2 (when Vs> Vp). ©

N I 25 - When differential signals of the bus are such that line 2 has lower voltage than that a > of line 3: The first FET 20 connects line 2 to the termination resistor 40 via FET

O S channel between the drain and source (when VGS > VTH). The second diode 31 in

LO N the second FET 30 connects the termination resistor 40 to the line 3 (when Vs> Vp).

O N

[0031] Hence, when the BTCP 11 is activated for serial bus termination, the present — coupling arrangement of the FETs with the diodes enables connecting of the bus lines 2, 3 to the termination resistor 40 at all signal levels of the bus lines. When the bus line signal level is such that the FET threshold voltage Vu is not exceeded to set the FET in conducting state, the diode 21 or 31 connects the appropriate termination resistance to the bus.

[0032] When a BTCP like the BTCP 11 illustrated in Figure 3 is applied in the I/O modules or mounting units 10, 12, the modules or mounting units themselves may provide the bus termination resistor, and separate bus termination module is no longer needed. This enables to reduce hardware costs and reduce installation errors, since an assembling person no longer needs to remember to attach the termination module. It becomes easier to add and remove devices and I/O modules to the serial bus for field devices of an industrial automation system, for example. Mounting rail space can be saved within installation cabinet.

[0033] Various advantages are further achieved by the present circuit configuration, e.g. when compared to bus termination comprising an optocoupler, or photosensitive semiconductor switch, like in DE102009056563. The presently disclosed circuit configuration can tolerate substantially higher temperatures. Further, controller area network (CAN) systems have relatively lower speed (max. speed 1 Mbps, except for CAN-SD 5 Mbps), whereas the presently disclosed circuit can be efficiently used for buses with much higher speed (e.g. 24 Mbps). The system as in DE102009056563 would not operate, at least as well as the presently disclosed circuit, with such high-speed buses since the applied components would cause high capacitive strain on the bus. Capacitance of components — coupled to a bus with an optocoupler is high. Further, in DE102009056563 FETs are galvanically separated, and continuously in conductive state, and the coupling does not cut peaks outside operating point as the presently disclosed circuit. = [0034] When the BTCP 11 is provided by mounting base unit for the I/O modules, I/O N modules themselves do not need to be eguipped with the bus termination circuit. This enables = 25 — also to reduce number of bus termination couplings. For example, in case of an assembly N comprising 16 I/O modules, at minimum two bus termination couplings may be needed (or = at maximum 7). This enables to reduce the total capacitance of the bus termination S arrangement. SV supply, which may be provided by a mounting base as a default, can be 0 used since the diode of the second FET is operative (instead of the second FET connection). O 30 In an alternative example embodiment 10V supply is generated.

[0035] When the module (connected to the respective mounting base unit) is not the last one in the seguence, input to the control line and the BTCP is adapted to prevent current flow to the termination resistor 40. As indicated in connection with Figure 2, this may be arranged by grounding such BTCP 13 by the line 16. The first diode 21 and the second diode 31 are adapted to prevent, respectively, connecting the first line 2 and the second line 3 to the termination resistor 40 when the control line 50 deactivates termination of the bus (by the respective BTCP). In this deactivated bus termination state, the FET channels do not convey and either of the diodes at a time prevents current flow to the termination resistor 40.

[0036] In an example embodiment, when the gates 24 and 34 are connected to ground, e.g. the ground 15 as illustrated in Figure 2, the mounting base unit 10 and termination of the bus by the BTCP 11 is deactivated.

[0037] The diodes 21 and 31 may be included in respective FET (units). The diodes may be referred to as (FET) body diodes. The diodes may be external diodes.

[0038] Figure 4 illustrates in further detail an example of electronic circuit comprising a BTCP similar to that of Figure 3. Operating voltage is supplied via input line 50 comprising a pull-up resistor R4, and via resistors R2 and R3 to respective gates G of FETs V1 and V2. Resistors R2, R3, and R4 may be 1000 ohm resistors, for example. The use of R2 and R3 is for minimizing the capacitance through FET to the bus.

[0039] The bus termination resistor RI is selected according to properties of the applied bus. For example, 150-ohm resistor may be appropriate for RS485 bus.

[0040] Transient voltage suppression diode (TVD) devices 60, 61 connect respectively the first line 2 and the second line 3 to ground.

[0041] The FETs 20, 30 should be selected such that they have small internal N capacitance (e.g. a couple of or few picofarads can be considered as such small capacitance), N so that load to the bus caused by non-active termination couplings can be reduced or 2 minimized. Furthermore, drain current maximum needs to be high enough, and drain leakage - 25 — current should be small, e.g. drain leakage less than 100 HA may be acceptable. These S parameters and FET selection depends on the bus parameters, such as bus capacity/speed. S The FETs 20, 30 may be N-channel MOSFETs. In some embodiments, trench MOSFETs = are applied. An example of such N-channel trench MOSFET is NX3020NAK, in which type N Van is 1.2 V, Vps maximum is 30 V, Vas maximum is 20 V, drain current maximum is 200 mA (when Vgs is 10 V), and drain-source on-state resistance is 4.5 ohm (when Vas is 10 V and drain current is 100 mA), and source-drain diode source-drain voltage is 0.7V.

[0042] Figure Sa illustrates an example of voltage variation in close to ideal case with varying signals on the bus. Figure 5b illustrates an example based on applying a BTCP as illustrated above, indicating that measured waveform shape 81 can effectively suppress transient voltages and remains quite close to the original waveform shape 80 even with maximum amount of slave I/O mounting bases attached to the bus. For example, up to 8 mounting bases (with the BTCP) may be applied. The number of I/O mounting bases may be adapted according to space available.

[0043] It is to be understood that the embodiments of the invention disclosed are not limited to the particular structures, process steps, or materials disclosed herein, but are extended to equivalents thereof as would be recognized by those ordinarily skilled in the relevant arts. It should also be understood that terminology employed herein is used for the purpose of describing particular embodiments only and is not intended to be limiting.

[0044] Various described features, structures, or characteristics may be combined in any suitable manner in one or more embodiments. While the forgoing examples are illustrative of the principles of the present invention in one or more particular applications, it will be apparent to those of ordinary skill in the art that numerous modifications in form, usage and details of implementation can be made without the exercise of inventive faculty, and without departing from the principles and concepts of the invention. Accordingly, it is not intended that the invention be limited, except as by the claims set forth below.

[0045] The verbs “to comprise” and “to include” are used in this document as open limitations that neither exclude nor require the existence of also un-recited features. The features recited in depending claims are mutually freely combinable unless otherwise — explicitly stated. Furthermore, it is to be understood that the use of "a" or "an", i.e. a singular

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Claims

CLAIMS:

1. An apparatus for connecting an electronic communication module (10) to a serial data bus, wherein the apparatus comprises a bus termination circuit (11) comprising: - a first field effect transistor (20) comprising or coupled with a first diode (21), - a second field effect transistor (30) comprising or coupled with a second diode (31), and - atermination resistor (40), wherein a source terminal (22) of the first field effect transistor is connected to a first line (2) of the bus, a source terminal (32) of the second field effect transistor is connected to a second line (3) of the bus, the termination resistor is connected to a drain terminal (23) of the first field effect transistor and to a drain terminal (33) of the second field effect transistor, the first field effect transistor and the second field effect transistor are connected to a control line (50) for activating the apparatus for terminating the bus and adapted to connect the first line and the second line to the termination resistor, wherein the first diode is adapted to connect the first line to the termination resistor when gate-to-source voltage of the first field effect transistor is below a threshold voltage value and while source voltage of the first field effect transistor is higher than drain voltage of the first field effect transistor, the second diode is adapted to connect the second line to the termination resistor when gate-to-source voltage of the second field effect transistor is below a threshold voltage value and while source voltage of the second field effect transistor is higher S 25 than drain voltage of the second field effect transistor, and ro the bus termination circuit is adapted to prevent connecting the first line and the © second line to the termination resistor in response to deactivating termination of the z bus by the input line. a O 30

2. The apparatus of claim 1, wherein the bus termination circuit is adapted to connect N the first line and the second line to the termination resistor in response to a supply N voltage in the control line to a gate (24) of the first field effect transistor (20) and to a gate (34) of the second field effect transistor to activate the apparatus to terminate the bus.

3. The apparatus of claim 1 or 2, wherein the first diode (21) and the second diode (31) of the bus termination circuit are adapted to prevent connecting the first line and the second line to the termination resistor in response to grounding the gate (24) of the first field effect transistor (20) and the gate (34) of the second field effect transistor via the input line (50).

4. The apparatus according to any preceding claim, wherein the first field effect transistor (20) and the second field effect transistor (30) are N-channel metal oxide semiconductor field effect transistors.

5. The apparatus of any preceding claim, wherein the apparatus comprises a slave I/O mounting base unit for connecting the communications module to the bus.

6. A communications module, comprising the apparatus of any preceding claim 1 to 4.

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