Amstrad Serial Interface

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The Pace RS232 interface, as sold by Amstrad

Amstrad released two RS232 interfaces, both using the same input/output addresses - and, ultimately, therefore establishing the standard for serial interfaces - but with significantly different hardware.

Original serial interface

Amstrad's first serial interface was one of the original peripherals released for the CPC. It was remarkable (in a bad way) for the necessity for an external, brick-like power supply.

Software was supplied on ROM, mostly comprising utility commands for hobbyists rather than a comms application per se - though there was limited terminal emulation for connecting to the then-popular Prestel service]. The original, UK version included commands for file transfer entitled |SUCK and |BLOW, which used a proprietary Amstrad format similar to XModem-1k but, annoyingly, subtly incompatible. These commands were reputedly renamed for the US edition to avoid causing offence.

Because of the external power supply and the poor quality of the software, this model failed to establish itself as the "standard" serial interface in the way that (say) the DDI-1 did for disc drives. This allowed other serial interfaces to flourish, such as those sold by Cirkit and KDS.

Pace serial interface

As a result, Amstrad dropped their original design and instead chose to remanufacture a Pace design. This included significantly improved hardware (a more compact interface and no external PSU) and vastly superior software.

The software was a ROM version of Pace's terminal utility, Commstar. This was a user-friendly program with standard XModem file transfer functions. However, one foolish design decision was the idea of using a 24x80 screen with a single-line status display - though presenting a good user interface, this forced the use of software rather than hardware scrolling, making the interface unusable for terminal emulation at any speed above 2400 baud.

Like the original interface, the Pace design used an 8250 UART chip, which was reliable up to 9600 baud and could sometimes function at faster speeds. The possibility has been discussed of replacing this with a pin-compatible 16550 and gaining even faster transfer speeds, but this is not believed to have been tested.

Alternative software

The limitations of the Commstar software, and the desire to use more recently developed file transfer protocols (such as XModem-1k, YModem and ZModem) meant that many users chose to use their Amstrad serial interface with third-party software. This included:

  • Ansiterm - a fast AMSDOS utility (also known as ANSI.ARC)
  • Charley's Comms ROM - sideways ROM capable of 4800 baud terminal emulation
  • ZMP - ZModem file transfer under CP/M

A vast range of comms software was available for CP/M, including bulletin board hosts, offline readers (notably CRR - CP/M Read and Reply), and many terminals.