ESPA 4.4.4; Serial Data Interface For Paging Equipment

Gordon Scott

gordon at gscott dot co dot uk


Revision History
Revision 0.22003-08-05Revised by: GLS
Fix typos, add some further notes, add BCC code clip, added EOT packet failure to TemporaryMaster SDL diagram. improve some grammar and layout.
Revision 0.12003-08-02Revised by: GLS
First publication of annotated xml version.

Serial Data Interface For Paging Equipment.

This annotated version is copyright July 2003 by G. L. Scott and is derived from the 1984 original.

ESPA 4.4.4 defines a serial protocol intended for the interconnection of paging equuipment with telephone exchanges or computer systems. The protocol used is message-based and is built upon the ISO1745 transport specification. The protocol is designed to support multi-drop communications on an RS485 data bus, however it is also often used point-to-point on V24/RS-232 connections. The general principle is that two equipment designed the standard should interwork satisfactorily and sanely without extensive configuration, despite the significant variations in operation of both PBX and paging systems. Gordon Scott.

Table of Contents
1. Introduction
2. Character Description
2.1. Character Structure
2.2. Character Set
2.3. Control Characters
2.3.1. Transmission Control Characters
2.3.2. Information Separator Characters
2.4. Transmission Control Prefixes
3. The Protocol
3.1. Polling
3.2. Selecting
3.3. Termination
3.4. Examples of Poll and Select Sequence
3.5. Transfer of Data Blocks
3.6. Example of Transfer of Data Blocks
4. Data Block Description
4.1. Block Structure
4.2. Headers
4.3. Records
4.4. Examples of Transactions
5. Hardware Specification
5.1. Definition of Interchange Circuits
5.2. Electrical Characteristics
5.3. Connector Specification
5.4. Audio Signals
5.5. Baud Rates
6. Conventions
7. Glossary
8. List Of References
9. Appendix A
10. Appendix B
10.1. SDL Diagrams
11. Appendix C

1. Introduction

Telephone systems and paging systems today [1984] are reaching levels of sophistication that were hardly to be considered just a few years ago. Far more information about subscribers is available in such systems, enabling more intelligent and more efficient use to be made of the facilities that are available to caller and subscriber alike.

This information requires a degree of inter systems control above that which has so far been required and will, as systems become more powerful, require yet more capability from inter-systems communications.

As higher degrees of machine intelligence become available and digital electronic designs replace the old electromechanical systems, so the methods of communication between systems become much more oriented towards the rapid transfer of information between such systems. To this end, ESPA have decided to recommend a serial data interface for paging systems, which is both powerful and flexible and opens an extensible path for the future.

The method recommended conforms to ISO1745 "Information Processing - Basic mode control procedures for data communication systems". It uses conventional stop-start character formats, a "handshake" protocol with error detection, is suitable for half duplex operation and may be used via modems if required.

Proposals for a Tie-line Type Interface and for a Subscriber Line Interface can be found in ESPA Publication 4.4.3.

N.B. For conventions and a glossary of terms used in this publication, see chapters 6 and 7.


If you have any issues with this document, including errors, omissions, typos and the like, please feel free to contact me. I'll address what I reasonably can using notes or whatever, however the original document probably must stand; Gordon.