Balanced Capacitor Read-Only Storage

For the full details of Balanced Capacitor Read-Only Storage read the following document at

It is a PDF document.

But here is a simpler explanation. It is largely taken from [1].

BCROS consists of 8 bit-planes, 4 on each side of a 'gate' in the Central Processor Unit (CPU).
Four can be seen in this photograph above the person's back:


Each bit-plane is a copper clad laminate, approximately 203mm x 457mm, etched with a bit pattern.
Each bit-plane contains 352 words of 100 bits.
Mus.Cat. NEWUC:2003.31 Mnfctr: IBM Date: 1967 Ser. No: 993309 Part No: 5737338-67
Comp: BCROS Plane Length: 505 mm Width: 211 mm Thickness: 0.5 mm Weight: 72 g


A closer view is:

BCROS closeup

An even closer view shows the individual bits.

BCROS closeup

This type of read-only storage uses as a unit cell the capacitances formed at the intersections
of two groups of suitably shaped conductors,
the ones in the bit-plane are vertical and the others in the sense-plane are horizontal.

The selection of one data line causes voltage swing along that line,
which is capacitively coupled to the sense lines.
To ensure that the characteristic impedances and delays along the address and sense lines
are independant of the information patterns used, a balanced scheme is introduced.

Two Bits
Figure 2 "Flag" configuration showing two bits.

The address line has associated with it a balance line,
which contains a complementary pattern that couples it capacitively to the sense lines.
In other words, each bit is made up of the capacitive intersection of an data line
and a balance line with a pair of sense lines (Fig. 2).
Detection of stored data is accomplished by sensing differentially at the midpoint of the terminated sense lines.

Here are some photographs showing the individual bits of eight planes.
Take care they are 6 MBytes and will take a while to download on a slow Internet connection.
(5277312 and 5277313 are in fact the same part number, namely 5277312, and look like empty.)
(5737328 still has the rubbery sponge layer that distributed the pressure evenly.)
(5737332 has bits of the rubbery sponge still attached.)
5737330 5737332 5737335 5737336 5737338 5277312 5277313 5737328

[1] S.A.Abbas, J.K.Ayling, C.E.Gifford, R.G.Gladu, T.C.Kwel, W.J.Taren,
"A Balanced Capacitor Read-Only Storage," IBM J. RES. DEVELOP., July 1968.
The diagram on this page was reproduced by permission of IBM from an article:
S.A. Abbas, J.K.Ayling, C.E.Gifford, R.G. Gladu, T.C. Kwei, W.J. Taren,
"A Balanced Capacitor Read-Only Storage," IBM J. Res. Develop. Vol 12, No 4, Page 307 (1968)

Back to Firmware