The BT redcare alarm monitoring system provides continuous monitoring of a BT telephone line that links a professionally installed alarm system on a customer’s premises to an Alarm Receiving Centre (ARC).
Unlike other alarm monitoring services, redcare both alerts the Alarm Receiving Centre when it detects a line cut and provides encryption to prevent the substitution of equipment or "spoofing" (emulation) of line conditions.
Monitoring at the customer’s premises is performed by a small microprocessor-based device connected to the alarm system and known as the Subscriber's Terminal Unit, or "STU" for short.
Originally, communications between the STU and the rest of the redcare network relied entirely upon a BT telephone line. With changes in Police policy and insurance requirements, it became necessary to include an additional communication channel for backup, and to provide confirmation of alarm reports.
Various upgrade products appeared on the market to add a GSM backup channel to existing STUs, and the need for a new STU design with an integrated backup path was soon recognised.
In October 2000, Versus Technology Ltd was pursuing this new STU design, and approached Antronics Ltd for assistance in the embedded firmware development.
The previous generation of STU was based on an 8051, and the Triscend E505 was selected as the basis for the new "GSM STU".
The Tricend E505 integrates an 8051 processor, 16K of SRAM and 512 cells of configurable logic onto a single chip with up to 124 IO lines - thus maintaining code & processor architecture compatibility with the existing STU whilst adding the enhanced capabilities necessary to support the demands of the added GSM features.
The configurable logic allowed the implementation of a PWM generator and two UARTs, whilst having no external "glue" logic at all.
Antronics Ltd were able to assist Versus Technology in the following specific areas:
The existing Versus STU code used an obsolete Assembler, and was quite tightly constrained by the limited resources of the old processor.
For maintainability, to facilitate integration with the 3rd-party software required for the GSM interface, and to enhance the possibility of future re-use, this code was to be converted to use Keil C51 - an ANSI 'C' cross-compiler with extensions to target the 8051 architecture.
Fortunately, the Assembler code was quite well documented and well structured - resulting in a relatively painless conversion process.
Extensive use was made of the Simulator within the Keil µVision IDE (Integrated Development Environment) to create test harnesses for each module. Thus it was possible to test each module as it was converted - without the need for finished hardware.
Because the Triscend E5 contains both a processor and configurable logic in the same package, the ROM device must contain both the application firmware code and the logic configuration data. The application firmware code can be developed with any 8051 software tools (such as Keil C51), and the logic configuration is developed with Triscend's FastChip software.
Working in close collaboration with the Tool vendors, and within the requirements of the Versus Technology ISO9001 Quality System, Antronics Ltd defined an integrated build process allowing the entire firmware code and logic configuration to be built, downloaded, and debugged from within the Keil µVision IDE.
This work also formed the basis of Keil's Application Note 159
The GSM backup channel was to use the same proprietary BT redcare protocol as used in the redcare ISDN service, with modifications to accommodate the differences between the ISDN and GSM networks. The protocol software was provided as a Reference Implementation, intended to be portable to any platform.
Due to the extremely restricted nature of the 8051 architecture - especially in relation to 'C' programming - the protocol Reference Implementation, could not be directly implemented on the new GSM STU platform. Antronics Ltd were able to devise an implementation compatible with the 8051 target, while still compatible with the specified protocol, and not too dissimilar to the original Reference Implementation to compromise maintainability.
The GSM STU is now established in production, and in widespread use with the BT redcare gsm service.
We are grateful to BT redcare for their kind permission to feature their GSM STU in this article.
Please note that Antronics Ltd does not sell or
support BT redcare products or services - for details of BT products and services,
please contact BT: