ZX-81 custom chip - watizit?

From: Pete Turnbull <pete_at_dunnington.u-net.com>
Date: Fri Jul 31 02:16:12 1998

On Jul 30, 21:54, Richard A. Cini, Jr. wrote:
> Subject: ZX-81 custom chip - watizit?
> I just got a ZX-81 kit (unassembled). I didn't know this before, but the
> uses a custom 40-pin IC to perform certain I/O functions. Is this chip
> "custom" or is it a relabeled Z80 support chip?

No, it's a ULA (uncomitted logic array, a sort of early ASIC) made by
Ferranti. The ULA replaces several TTL chips that were present in the
ZX80. There were two or three revisions of these, and a similar idea was
used in the Sinclair Spectrum. The BBC Micro also used Ferranti ULAs, one
for the serial controller and one for the video controller. IIRC, the ZX
and Spectrum ULAs are still available from CPC in Britain.

A ULA consists of lots of simple logic gates laid out in a square array,
but the basic design has no final metalisation layer - and hence no
interconnections between the gates. A designer using a ULA would translate
his circuit diagram into an interconnection diagram for the array, and hand
that over to Ferranti, who would then produce the mask for the final stage
of the manufacturing process. The end result is a bit like an FPGA, but
with simopler building blocks. Apart from that, the relationhip between a
ULA and an FPGA is bit like the relationship between a masked ROM and an

Pete						Peter Turnbull
						Dept. of Computer Science
						University of York
Received on Fri Jul 31 1998 - 02:16:12 BST

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