EN IEC 60839-11-5:2020

SLOVENSKI STANDARD
01-november-2020
Alarmni in elektronski varnostni sistemi - 11-5. del: Elektronski sistemi nadzora
dostopa - Odprti protokol nadzora naprav (OSDP)
Alarm and electronic security systems - Part 11-5: Electronic access control systems -
Open Supervised Device Protocol (OSDP)
Ta slovenski standard je istoveten z: EN IEC 60839-11-5:2020
ICS:
13.320 Alarmni in opozorilni sistemi Alarm and warning systems
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

EUROPEAN STANDARD EN IEC 60839-11-5

NORME EUROPÉENNE
EUROPÄISCHE NORM
September 2020
ICS 13.320
English Version
Alarm and electronic security systems - Part 11-5: Electronic
access control systems - Open Supervised Device Protocol
(OSDP)
(IEC 60839-11-5:2020)
Systèmes d'alarme et de sécurité électroniques - Partie 11- Alarmanlagen - Teil 11-5: Elektronische
5: Systèmes de contrôle d'accès électronique - Protocole Zutrittskontrollanlagen - Open Supervised Device Protocol
ouvert d'appareil supervisé (OSDP) (OSDP)
(IEC 60839-11-5:2020) (IEC 60839-11-5:2020)
This European Standard was approved by CENELEC on 2020-08-12. CENELEC members are bound to comply with the CEN/CENELEC
Internal Regulations which stipulate the conditions for giving this European Standard the status of a national standard without any alteration.
Up-to-date lists and bibliographical references concerning such national standards may be obtained on application to the CEN-CENELEC
Management Centre or to any CENELEC member.
This European Standard exists in three official versions (English, French, German). A version in any other language made by translation
under the responsibility of a CENELEC member into its own language and notified to the CEN-CENELEC Management Centre has the
same status as the official versions.
CENELEC members are the national electrotechnical committees of Austria, Belgium, Bulgaria, Croatia, Cyprus, the Czech Republic,
Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, the
Netherlands, Norway, Poland, Portugal, Republic of North Macedonia, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland,
Turkey and the United Kingdom.

European Committee for Electrotechnical Standardization
Comité Européen de Normalisation Electrotechnique
Europäisches Komitee für Elektrotechnische Normung
CEN-CENELEC Management Centre: Rue de la Science 23, B-1040 Brussels
© 2020 CENELEC All rights of exploitation in any form and by any means reserved worldwide for CENELEC Members.
Ref. No. EN IEC 60839-11-5:2020 E

European foreword
The text of document 79/634/FDIS, future edition 1 of IEC 60839-11-5, prepared by IEC/TC 79 "Alarm
and electronic security systems" was submitted to the IEC-CENELEC parallel vote and approved by
CENELEC as EN IEC 60839-11-5:2020.
The following dates are fixed:
• latest date by which the document has to be implemented at national (dop) 2021-05-12
level by publication of an identical national standard or by endorsement
• latest date by which the national standards conflicting with the (dow) 2023-08-12
document have to be withdrawn
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. CENELEC shall not be held responsible for identifying any or all such patent rights.
Endorsement notice
The text of the International Standard IEC 60839-11-5:2020 was approved by CENELEC as a
European Standard without any modification.
Annex ZA
(normative)
Normative references to international publications
with their corresponding European publications
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements of this document. For dated references, only the edition cited applies. For
undated references, the latest edition of the referenced document (including any amendments)
applies.
NOTE 1 Where an International Publication has been modified by common modifications, indicated by (mod), the
relevant EN/HD applies.
NOTE 2 Up-to-date information on the latest versions of the European Standards listed in this annex is available
here: www.cenelec.eu.
Publication Year Title EN/HD Year
IEC 60839-11-1 2013 Alarm and electronic security systems - EN 60839-11-1 2013
Part 11-1: Electronic access control
systems - System and components
requirements
- -  + AC 2015
IEC 60839-11-2 2014 Alarm and electronic security systems - EN 60839-11-2 2015
Part 11-2: Electronic access control
systems - Application guidelines
- -  + AC 2015
IEC 60839-11-5 ®
Edition 1.0 2020-07
INTERNATIONAL
STANDARD
Alarm and electronic security systems –

Part 11-5: Electronic access control systems – Open supervised device protocol

(OSDP)
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
ICS 13.320 ISBN 978-2-8322-8480-3

– 2 – IEC 60839-11-5:2020 © IEC 2020
CONTENTS
FOREWORD . 8
INTRODUCTION . 10
1 Scope . 11
2 Normative references . 11
3 Terms, definitions and abbreviated terms . 11
3.1 Terms and definitions . 11
3.2 Abbreviated terms . 12
4 Overview . 12
5 Communication settings. 13
5.1 Physical interface . 13
5.2 Signaling . 13
5.3 Character encoding . 13
5.4 Channel access . 13
5.5 Multi-byte data encoding . 13
5.6 Packet size limits . 14
5.7 Timing. 14
5.8 Message synchronization . 14
5.9 Packet format . 15
5.10 Multi-part messages . 17
5.10.1 General . 17
5.10.2 Multi-part message usage rules . 17
5.11 Smartcard handling . 18
6 Commands . 19
6.1 General . 19
6.2 Poll request (osdp_POLL) . 19
6.3 ID report request (osdp_ID) . 19
6.4 Peripheral device capabilities request (osdp_CAP) . 20
6.5 Local status report request (osdp_LSTAT) . 20
6.6 Input status report request (osdp_ISTAT). 20
6.7 Output status report request (osdp_OSTAT) . 21
6.8 Reader status report request (osdp_RSTAT) . 21
6.9 Output control command (osdp_OUT) . 21
6.10 Reader LED control command (osdp_LED) . 22
6.11 Reader buzzer control command (osdp_BUZ) . 24
6.12 Reader text output command (osdp_TEXT) . 25
6.13 Communication configuration command (osdp_COMSET) . 26
6.14 Scan and send biometric data (osdp_BIOREAD) . 27
6.15 Scan and match biometric template (osdp_BIOMATCH) . 28
6.16 Encryption key set (osdp_KEYSET) . 29
6.17 Challenge and secure session initialization request (osdp_CHLNG) . 29
6.18 Server's random number and server cryptogram (osdp_SCRYPT) . 29
6.19 Manufacturer specific command (osdp_MFG) . 29
6.20 ACU receive size (osdp_ACURXSIZE) . 30
6.21 Keep reader active (osdp_KEEPACTIVE). 30
6.22 Abort current operation (osdp_ABORT) . 31
6.23 Get PIV data (osdp_PIVDATA) . 31

IEC 60839-11-5:2020 © IEC 2020 – 3 –
6.24 General authenticate (osdp_GENAUTH) . 31
6.25 Authentication challenge (osdp_CRAUTH) . 32
6.26 File transfer command (osdp_FILETRANSFER) . 33
6.27 Extended write data (osdp_XWR) . 33
6.27.1 General . 33
6.27.2 Mode set command . 34
6.27.3 Mode-00 read setting . 35
6.27.4 Mode specific command codes for XRW_MODE=1 . 35
6.27.5 Mode-01 transparent content send request . 35
6.27.6 Mode-01 connection done . 35
6.27.7 Mode-01 request secure PIN entry command . 36
6.27.8 Mode-01 smartcard scan . 37
7 Replies . 37
7.1 General . 37
7.2 General acknowledge – Nothing to report (osdp_ACK) . 38
7.3 Negative acknowledge – Error response (osdp_NAK) . 38
7.4 Device identification report (osdp_PDID) . 39
7.5 Device capabilities report (osdp_PDCAP) . 40
7.6 Local status report (osdp_LSTATR) . 41
7.7 Input status report (osdp_ISTATR) . 41
7.8 Output status report (osdp_OSTATR) . 41
7.9 Reader tamper status report (osdp_RSTATR) . 42
7.10 Card data report, raw bit array (osdp_RAW) . 42
7.11 Card data report, character array (osdp_FMT) . 43
7.12 Keypad data report (osdp_KEYPAD) . 43
7.13 Communication configuration report (osdp_COM) . 44
7.14 Scan and send biometric data (osdp_BIOREADR) . 44
7.15 Scan and match biometric template (osdp_BIOMATCHR) . 45
7.16 Client's ID and client's random number (osdp_CCRYPT) . 45
7.17 Client cryptogram packet and the initial R-MAC (osdp_RMAC_I) . 46
7.18 Manufacturer specific reply (osdp_MFGREP) . 46
7.19 PD busy reply (osdp_BUSY) . 46
7.20 PIV data reply (osdp_PIVDATAR) . 46
7.21 osdp_GENAUTHR. 47
7.22 Response to challenge (osdp_CRAUTHR) . 47
7.23 Manufacturer specific status reply (osdp_MFGSTATR) . 48
7.24 Manufacturer specific error reply (osdp_MFGERRR) . 48
7.25 File transfer status (osdp_FTSTAT) . 48
7.26 Extended read reply (osdp_XRD) . 49
7.26.1 General . 49
7.26.2 Mode specific reply codes for XRW_MODE=0 . 50
7.26.3 Mode-00 error reply (osdp_PR00ERROR). 50
7.26.4 Mode setting report (osdp_PR00REQR). 50
7.26.5 Card information report (osdp_PR00CIRR) . 51
7.26.6 Mode specific reply codes for XRW_MODE=1 . 51
7.26.7 Mode-01 NAK or error reply (osdp_PR01ERROR) . 52
7.26.8 Card present notification reply (osdp_PR01PRES). 52
7.26.9 Transparent card data reply (osdp_PR01SCREP) . 52
7.26.10 Secure PIN entry complete reply (osdp_PR01SPER) . 53

– 4 – IEC 60839-11-5:2020 © IEC 2020
Annex A (normative) Command and reply code numbers commands . 54
A.1 Commands . 54
A.2 Replies . 55
Annex B (normative) Function code definitions list . 56
B.1 General . 56
B.2 Function code 1 – Contact status monitoring . 56
B.3 Function code 2 – Output control . 57
B.4 Function code 3 – Card data format . 57
B.5 Function code 4 – Reader LED control . 57
B.6 Function code 5 – Reader audible output . 58
B.7 Function code 6 – Reader text output . 58
B.8 Function code 7 – Time keeping . 58
B.9 Function code 8 – Check character support . 58
B.10 Function code 9 – Communication security . 59
B.11 Function code 10 – Receive bufferSize . 59
B.12 Function code 11 – Largest combined message size . 59
B.13 Function code 12 – Smart card support . 59
B.14 Function code 13 – Readers . 60
B.15 Function code 14 – Biometrics . 60
B.16 Function code 15 – Secure PIN entry support . 60
B.17 Function code 16 – OSDP version . 60
Annex C (normative) CRC definition . 61
Annex D (normative) Encryption . 64
D.1 Encryption method: OSDP-SC . 64
D.1.1 General . 64
D.1.2 Overview . 65
D.1.3 The process . 65
D.1.4 Secure channel session connection sequence (SCS-CS) . 65
D.1.5 Communication during a secure channel session . 67
D.1.6 SCS_16 PD->ACU . 67
D.1.7 SCS_17 ACU->PD . 67
D.1.8 SCS_18 PD->ACU . 67
D.2 Commands . 67
D.2.1 Encryption key set (osdp_KEYSET) . 67
D.2.2 Challenge and secure session initialization request (osdp_CHLNG) . 68
D.2.3 Server's random number and server cryptogram (osdp_SCRYPT) . 68
D.3 Replies . 68
D.3.1 Client's ID and client's random number (osdp_CCRYPT) . 68
D.3.2 Client cryptogram packet and the initial R-MAC (osdp_RMAC_I) . 69
D.4 Algorithms and support functions . 69
D.4.1 Session key derivation . 69
D.4.2 Key diversification . 69
D.4.3 Client cryptogram . 70
D.4.4 Server cryptogram . 70
D.4.5 Padding . 70
D.5 Message authentication code (MAC) generation . 70
D.5.1 General . 70
D.5.2 The wrap operation for security block types SCS_15, SCS-16, SCS_17,
and SCS_18 . 71

IEC 60839-11-5:2020 © IEC 2020 – 5 –
D.5.3 The unwrap operation . 72
D.6 Error recovery . 72
D.7 Field deployment and configuration . 72
Annex E (normative) Test vectors . 74
Annex F (informative) Mapping of mandatory functions in IEC 60839-11-1 . 75
Bibliography . 85

Figure 1 – Schematic overview of an OSDP connection . 12
Figure D.1 – MAC algorithm . 71

Table 1 – Packet format . 15
Table 2 – Message control information . 16
Table 3 – The security block (SB) . 17
Table 4 – Multi-part message structure . 17
Table 5 – Behaviour modes . 18
Table 6 – Poll request . 19
Table 7 – ID report request . 20
Table 8 – Peripheral device capabilities request . 20
Table 9 – Local status report request . 20
Table 10 – Input status report request. 20
Table 11 – Output status report request . 21
Table 12 – Reader status report request . 21
Table 13 – Output control command . 22
Table 14 – Control code values . 22
Table 15 – Reader LED control command . 23
Table 16 – Temporary control code values . 24
Table 17 – Permanent control code values . 24
Table 18 – Color values . 24
Table 19 – Reader buzzer control command (osdp_BUZ) . 25
Table 20 – Reader text output command (osdp_TEXT) . 26
Table 21 – Text command values . 26
Table 22 – Communication configuration command (osdp_COMSET) . 27
Table 23 – Scan and send biometric data (osdp_BIOREAD) . 27
Table 24 – Biometric types. 28
Table 25 – Fingerprint formats . 28
Table 26 – Command structure: 6-byte header followed by a variable length template . 29
Table 27 – Manufacturer specific commands (osdp_MFG) . 30
Table 28 – ACU receive size (osdp_ACURXSIZE) . 30
Table 29 – Keep reader active (osdp_KEEPACTIVE) . 30
Table 30 – Abort current operation (osdp_ABORT) . 31
Table 31 – Get PIV data (osdp_PIVDATA) . 31
Table 32 – General authenticate (osdp_GENAUTH) fragment . 32
Table 33 – Authentication challenge (osdp_CRAUTH) fragment . 32

– 6 – IEC 60839-11-5:2020 © IEC 2020
Table 34 – File transfer command . 33
Table 35 – Extended write command structure . 34
Table 36 – Mode set command . 34
Table 37 – Mode 0 configuration . 34
Table 38 – Mode 1 configuration . 34
Table 39 – Read setting request . 35
Table 40 – Mode specific command codes . 35
Table 41 – Transparent content send request . 35
Table 42 – Smartcard connection done . 36
Table 43 – Request secure PIN entry command . 36
Table 44 – Smartcard scan . 37
Table 45 – General acknowledge (osdp_ACK) . 38
Table 46 – Negative acknowledge (osdp_NAK) . 38
Table 47 – Error codes . 39
Table 48 – Device identification report (osdp_PDID) . 40
Table 49 – Device capabilities report (osdp_PDCAP) . 40
Table 50 – Local status report (osdp_LSTATR) . 41
Table 51 – Input status report (osdp_ISTATR) . 41
Table 52 – Output status report (osdp_OSTATR) . 42
Table 53 – Reader tamper status report (osdp_RSTATR) . 42
Table 54 – Card data report, raw bit array (osdp_RAW) . 43
Table 55 – Card data report, character array (osdp_FMT) . 43
Table 56 – Keypad data report (osdp_KEYPAD) . 44
Table 57 – Communication configuration report (osdp_COM) . 44
Table 58 – Scan and send biometric data (osdp_BIOREADR) . 45
Table 59 – Scan and match biometric template (osdp_BIOMATCHR) . 45
Table 60 – Manufacturer specific reply (osdp_MFGREP) . 46
Table 61 – PD busy reply (osdp_BUSY) . 46
Table 62 – PIV data reply (osdp_PIVDATAR) . 47
Table 63 – General authenticate response (osdp_GENAUTHR) . 47
Table 64 – Response to challenge (osdp_CRAUTHR) . 48
Table 65 – Manufacturer specific status reply (osdp_MFGSTATR) . 48
Table 66 – Manufacturer specific error reply (osdp_MFGERRR) . 48
Table 67 – File transfer status (osdp_FTSTAT) . 49
Table 68 – Extended read reply . 50
Table 69 – Mode specific reply codes . 50
Table 70 – Error reply . 50
Table 71 – Mode setting report . 51
Table 72 – Card information report . 51
Table 73 – Mode specific reply codes . 51
Table 74 – Error reply . 52
Table 75 – Card present notification reply . 52
Table 76 – Transparent card data reply . 52

IEC 60839-11-5:2020 © IEC 2020 – 7 –
Table 77 – Transparent card data reply . 53
Table A.1 – Commands code numbers . 54
Table A.2 – Replies code numbers . 55
Table B.1 – Function codes . 56
Table D.1 – SEC_BLK_TYPE assignment . 64
Table D.2 – Command structure: 2-byte header followed by variable length data . 67
Table D.3 – Command structure: 8-byte random number as the “challenge” . 68
Table D.4 – Command structure: 16-byte server cryptogram . 68
Table D.5 – Command structure: 32-byte structure . 69
Table D.6 – Command structure: 16-byte structure . 69
Table F.1 – Access point interface requirements . 76
Table F.2 – Indication and annunciation requirements . 77
Table F.3 – Recognition requirements . 80
Table F.4 – Duress signalling requirements . 81
Table F.5 – Overriding requirements . 81
Table F.6 – System self-protection requirements . 82

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INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
ALARM AND ELECTRONIC SECURITY SYSTEMS –

Part 11-5: Electronic access control systems –
Open supervised device protocol (OSDP)

FOREWORD
1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising
all national electrotechnical committees (IEC National Committees). The object of IEC is to promote
international co-operation on all questions concerning standardization in the electrical and electronic fields. To
this end and in addition to other activities, IEC publishes International Standards, Technical Specifications,
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Publication(s)”). Their preparation is entrusted to technical committees; any IEC National Committee interested
in the subject dealt with may participate in this preparatory work. International, governmental and non-
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with the International Organization for Standardization (ISO) in accordance with conditions determined by
agreement between the two organizations.
2) The formal decisions or agreements of IEC on technical matters express, as nearly as possible, an international
consensus of opinion on the relevant subjects since each technical committee has representation from all
interested IEC National Committees.
3) IEC Publications have the form of recommendations for international use and are accepted by IEC National
Committees in that sense. While all reasonable efforts are made to ensure that the technical content of IEC
Publications is accurate, IEC cannot be held responsible for the way in which they are used or for any
misinterpretation by any end user.
4) In order to promote international uniformity, IEC National Committees undertake to apply IEC Publications
transparently to the maximum extent possible in their national and regional publications. Any divergence
between any IEC Publication and the corresponding national or regional publication shall be clearly indicated in
the latter.
5) IEC itself does not provide any attestation of conformity. Independent certification bodies provide conformity
assessment services and, in some areas, access to IEC marks of conformity. IEC is not responsible for any
services carried out by independent certification bodies.
6) All users should ensure that they have the latest edition of this publication.
7) No liability shall attach to IEC or its directors, employees, servants or agents including individual experts and
members of its technical committees and IEC National Committees for any personal injury, property damage or
other damage of any nature whatsoever, whether direct or indirect, or for costs (including legal fees) and
expenses arising out of the publication, use of, or reliance upon, this IEC Publication or any other IEC
Publications.
8) Attention is drawn to the Normative references cited in this publication. Use of the referenced publications is
indispensable for the correct application of this publication.
9) Attention is drawn to the possibility that some of the elements of this IEC Publication may be the subject of
patent rights. IEC shall not be held responsible for identifying any or all such patent rights.
International Standard IEC 60839-11-5 has been prepared by IEC technical committee 79:
Alarm and electronic security systems.
The text of this International Standard is based on the following documents:
FDIS Report on voting
79/634/FDIS 79/636/RVD
Full information on the voting for the approval of this International Standard can be found in
the report on voting indicated in the above table.
This document has been drafted in accordance with the ISO/IEC Directives, Part 2.

IEC 60839-11-5:2020 © IEC 2020 – 9 –
A list of all parts in the IEC 60839 series, published under the general title Alarm and
electronic security systems, can be found on the IEC website.
The committee has decided that the contents of this document will remain unchanged until the
stability date indicated on the IEC website under "http://webstore.iec.ch" in the data related to
the specific document. At this date, the document will be
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
• amended.
– 10 – IEC 60839-11-5:2020 © IEC 2020
INTRODUCTION
This document describes the communication protocol for interfacing one or more Peripheral
Devices (PD) to an Access Control Unit (ACU). This document specifies the protocol
implementation over a two-wire RS-485 multi-drop serial communication channel.
This document is based upon the work done by the Security Industry Association OSDP
Working Group.
IEC 60839-11-5:2020 © IEC 2020 – 11 –
ALARM AND ELECTRONIC SECURITY SYSTEMS –

Part 11-5: Electronic access control systems –
Open supervised device protocol (OSDP)

1 Scope
This part of IEC 60839 specifies the Open supervised device protocol (OSDP) for electronic
access control systems. This includes communication settings, commands and replies
between the ACU and the peripheral devices. It also includes a mapping of mandatory and
optional requirements as per IEC 60839-11-1:2013 as covered by Annex F.
This document applies to physical security only. Physical security prevents unauthorized
personnel, attackers or accidental intruders from physically accessing a building, room, etc.
This document does not in any way limit a manufacturer to add other commands to the
protocol defined here.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their
content constitutes requirements of this document. For dated references, only the edition
cited applies. For undated references, the latest edition of the referenced document (including
any amendments) applies.
IEC 60839-11-1:2013, Alarm and electronic security systems – Part 11-1: Electronic access
control systems – System and components requirements
IEC 60839-11-2:2014, Alarm and electronic security systems – Part 11-2: Electronic access
control systems – Application guidelines
3 Terms, definitions and abbreviated terms
3.1 Terms and definitions
For the purposes of this document the terms and definitions given in IEC 60839-11-1 and
IEC 60839-11-2, as well as the following, apply:
ISO and IEC maintain terminological databases for use in standardization at the following
addresses:
• IEC Electropedia: available at http://www.electropedia.org/
• ISO Online browsing platform: available at http://www.iso.org/obp
3.1.1
client
service requester
EXAMPLE User interface, etc.
3.1.2
server
service provider
– 12 – IEC 60839-11-5:2020 © IEC 2020
EXAMPLE Access control unit, etc.
3.1.3
peripheral device
I/O device connected via OSDP to the access control unit
EXAMPLE Token reader, card reader, biometric reader, client, etc.
3.2 Abbreviated terms
For the purposes of this document, the abbreviated terms given in IEC 60839-11-1 and
IEC 60839-11-2, as well as the following apply.
ACU Access Control Unit
AES Advanced Encryption Standard
APDU Application Protocol Data Unit
CBC Cypher Block Chaining
C-MAC Command MAC (for packets from ACU to PD)
cUID Client's Unique Identifier
ICV Initial Chaining Vector
MAC Message Authentication Code
MK Master Key
PD Peripheral Device
PGM Portable Grey Map
R-MAC Reply MAC (for packets from PD to ACU)
SCBK Secure Channel Base Key
SCS Secure Channel Session
S-ENC Session Key for ensuring data confidentiality (message encryption)
S-MAC1 Session Key for Message Authentication, key 1
S-MAC2 Session Key for Message Authentication, key 2
SPE Secure PIN Entry
4 Overview
This document defines the protocol for connecting an ACU to peripheral devices including
communication settings, commands and replies as shown in Figure 1.

Figure 1 – Schematic overview of an OSDP connection

IEC 60839-11-5:2020 © IEC 2020 – 13 –
5 Communication settings
5.1 Physical interface
The physical interface shall be a half-duplex RS-485 communication bus using one twisted
pair of wires with shield/signal ground.
5.2 Signaling
The signaling port shall be half-duplex asynchronous serial with following settings:
• 8 data bits,
• 1 stop bit,
• no parity bits,
• at least one of 9 600, 19 200,38 400, 57 600, 115 200, 230 400 baud.
5.3 Character encoding
Data encoding shall be compliant with UTF-8 (ISO/IEC 10646:2017).
5.4 Channel access
The communication channel is used in the “interrogation/reply” mode. Only the ACU may
spontaneously send a message. Each message sent by the ACU is addressed to one and only
one PD. The “broadcast message”, as described in Table 1, assumes that there is only one
PD connected to the ACU.
The PD shall send a single reply message to each message addressed to it within the
specified MAX_REPLY_DELAY, as defined in 5.7.
Special case: if the PD is unable to accept the command for processing due to temporary
unavailability of a resource required to process the command, then the PD shall send the
osdp_BUSY reply as defined in 7.19. When the ACU receives the osdp_BUSY reply, it may, at
its discretion, choose to re-send the same command as it would if the command delivery
timed out. If the ACU elects to re-send the command that caused the osdp_BUSY reply
...