An Intelligent Multiport Serial Controller for the Semiconductor Industry

The LocSoft ImpacII board was designed in response to what is becoming a familiar request in the embedded hardware market. Our customer had employed a commercial off-the-shelf (COTS), ISA-based multiport serial board at the heart of their industrial machine. This machine sells for tens of thousands of dollars, but its viability was threatened by the board vendor's end of life (EOL) announcement. This EOL was due to the demise of the ISA bus as an industry standard, which in turn reflects the trend nature of the PC and illustrates the dangers of basing long-life industrial equipment around a PC product.

The customer came to LocSoft in response to our web-based description of a multiport ISA bus serial board with software support for iRMX, the real-time operating system around which their system was based. (Intel's protected mode real-time OS is still available from TenAsys Corporation--http://www.tenasys.com/.) LocSoft's Impac8 board was considered and indeed used as a replacement, but this involved significant redesign by the customer to accommodate the new form factor of the serial port interface. LocSoft therefore proposed a redesign of the Impac8 to allow it to be used as a form, fit and function replacement for the current COTS board.

The hardware design considerations for the replacement board were as follows:

Several different architectures were considered, and eventually an x86 architecture was selected as offering the best performance and shortest time to completed product. The need to keep down cost, restrict the PCB real estate requirement and support readily available code, without the need for excessive processing power and thermal dissipation, pointed toward a 486-based solution. At the time, the main contenders were the ST family of microprocessors and the then unannounced Elan SC520 processor. LocSoft's close relationship with AMD allowed us access to advance information on the Elan, and it quickly became apparent that this device offered the optimal solution. The single-chip footprint allowed the functionality to be easily designed into the AT form factor outline, while the PC-compatible architecture and readily available sample schematics provided a fast and proven route to prototypes. The only ``clean sheet design'' areas were the ISA bus slave interface (implemented easily into a CPLD) and the octal UART. This was provided in a single device through the Cirrus Logic CL-CD1865 that offered a small footprint, high functionality, low cost and readily available driver code. We decided to keep the basic PC architecture intact. This allowed the board to be debugged quickly using a standard BIOS and allowed for future variants of the board to be developed.

In order to minimize the effect of firmware bugs and support a variety of potential applications, the ImpacII was designed to house basic bootstrap ROM-based software. The board would download the operating system and application at boot time across the ISA bus, through a FIFO-based interface, providing a flexible and relatively fast data exchange mechanism. The control for this FIFO interface was implemented in a CPLD, which provided read/write control ports for either side of the interface. These ports provided FIFO status, direct data exchange and board reset capabilities.

In order to insulate the product from the future disappearance of the ISA bus, a 10/100 Ethernet port was added. This could allow the board to be controlled via a network or simply provide additional connectivity, thus enhancing the potential appeal of the product to the customer.

In addition, a 16MB paged StrataFlash ROM was added to allow the board to operate in a totally standalone mode. Finally, in order to aid debugging, field diagnosis and production testing, two extra features were added. The first made use of the extra serial ports located within the Elan to allow a ``programmers serial port'' to be implemented. The second allowed two hexadecimal LED displays to be connected to the board and be used to display code checkpoint information.

In order to reduce cost, all these additional features were configured as optional extras that could be omitted from the bill of materials and the board cost, as desired.

The long lifetime issue is a trickier problem to address. There are several design techniques that can be employed to minimize the exposure to EOL issues, for example, multifootprint CAD, device selection using suppliers' long-life road maps and selection of multivendor devices. In addition, careful monitoring of device availability and customer demand allows for last-time buys to be carried out when the worst finally happens. Ultimately, however, there will be devices that are single-sourced, and devices do go EOL. The approach adopted by LocSoft was to isolate the customer from this by defining a set of protocols to handle board control and data transfer across the ISA bus. This totally isolates the customer from the hardware. While it is envisioned that the board will be manufacturable for at least five years, by reducing the customer interface to the ISA bus interface and the protocols placed on top of this, LocSoft effectively maintained the ability to redesign (partially or even totally) the board to resolve supply issues now and in the future. Of course, this approach works only when the customer allows the supplier the freedom to take this approach, and when the system design is not constrained by regulation (e.g., medical equipment).

At the end of the system design phase, we proposed the following board specification. The ImpacII, an Elan SC520-based, ISA bus-compatible computer designed to provide eight asynchronous serial ports in an IBM PC compatible environment. The ImpacII will comprise a PC compatible form factor board (Host Board) and a remote custom form factor board (Remote Board). The inter-board interface will be TTL level RS-232 signals.

The feature set of the ImpacII Host Board (see Figure 1) includes an AMD Elan SC520 CPU running at 133MHz, DIMM-based SDRAM memory site, 16KB cache memory, 1MB Flash EPROM, 16MB StrataFlash memory expansion site, eight RS-232 TTL level serial ports, FIFO-based ISA (AT Bus) compatible interface and PCI-based Ethernet 10/100 Base-T interface with RJ45 connector AT Compatible form factor.

Figure 1. ImpacII Host Board

The feature set of the ImpacII Remote Board (see Figure 2) contains TTL to RS-232 level shifters for eight ports, eight RJ45 RS-232 connectors, opto-isolation on four ports, RTS reset on four ports, fiber Tx/Rx on two ports, full modem support on one port and a custom form factor to be defined by the customer.

Figure 2. ImpacII Remote Board

Figure 3. ImpacII Block Diagram