S-100 bus, connector pin row widths

This document last updated June 25 2023. (c) Copyright 2023 Herb Johnson. To return to my S-100 bus Web page click here.

For more details about the S-100 bus, check my S-100 Bus Web page.

S-100 backplane connector, row width spacing, brands

Summary: By Altair and S-100 and IEE-696 standards, the 100 edge-connector pins are .125 inches apart or a "pitch" of 8 pins per inch, on each side along the 50 pins per side. But the spacing between the rows of pins on the backplane PC board connectors was not standardized. Bottom line: the MITS Altair and some early Altair-bus computer backplanes, used a narrow .140 inches between the rows. But most subsequent S-100 and IEEE-696 systems used .250 inches between the rows. Check your S-100 backplane and replacement connectors to confirm they have the same row width.

Spacing between the connectors, is usually 3/4" - except, again, for the MITS Altair and possibly other brands. Check my check my S-100 Bus Web page.

Measurements and documents

What's the width between rows of pins, on S-100 connectors? Empirically, the bulk of S-100 backplanes by brand, appear to have connectors with row spacings of .25 inches (1/4 inch) apart. But the MITS Altair 8800 used backplane connectors which were more narrowly spaced. MITS documents don't specify the spacing, or the brand/model of connector they purchased. In Jan 2022, my visual inspection of an Altair backplanes and documents suggested a spacing less than .250. I could not make a physical measurement at the time.

In later discussion with Jon Chapman in January 2022, he identified the same MITS narrow spacing, for the Byte shop BYT-8 and the Polymorphic Poly-88 systems backplanes. That was in response to discussion from Jeff Galinat about the narrow spacing on the MITS Altair 8800 backplane.

I found online, a reference to "Sullins" brand edge connectors to replicate the MITS Altair. J M Precision's Web site, suggests a Sullins connector EBA50DRTH-S734, which by manufacturer specs has a pin width of .140 inches.

In the Processor Technology SOL-PC Systems manual, Chapter III, it describes a connector: "1 100—pin Edge Connector, TI H322150-0306A". Web search is futile.

I found a value in "The S-100 Bus Handbook" by Dave Bursky (Hayden Press, 1980). Chapter 4, "The Basic S-100 Bus", first page number 33. It says "The pin to pin spacing on the rear of [?] the connectors for the Altair and IMSAI motherboards is 0.125 in., while the row-to-row spacing for the Altair motherboards [connectors] is 0.14 in and 0.25 in for the IMSAI motherboard."

physical measurements of S-100 backplanes

I (Herb Johnson) personally measured a number of S-100 backplanes in Jan 2022, some again in June 2032, for row-spacing between the connector pin rows of 50. I added Jon Chapman's entries, to create the lists below.

The following all measure .250 inches :

California Digital	Northstar Horizon HRX		ITT Chernow
"P100MB-12", "P100MB-6" Jade Isobus			Morrow Wunderbuss
N8VEM Ver 4 2015	Syntech brand			IMSAI EXP-22
SD Systems 10-slot	Jade no model			IMS A950 12 slot
QR Silent Plus		TDL (1977 8 slot)		Compupro 12 slot
WMC / Wameco QMB-9 1978

The following measured .140 inches by Jonathan Chapman AKA Systems Glitch in Jan 2022:

MITS Altair 8800	Byte shop BYT-8			Polymorphic Poly-88
.. and Dave Bursky in the "Handbook" confirmed the .140 MITS spacing. 

IN June 2023, I checked most of these same backplanes, and found the spacing *between* connectors was .750 3/4". But I could not check the Altair, Byte, Northstar, SOL, or Polymorphic.

Odd fact about the MITS 680

Bill Degnan's vintagecomputing.net site has a MITS Altair 680B with the three-slot expansion board. I have more discussion with Bill about the Altair 680 on this linked page. The 680 bus signals are not S-100! but it has 100 pin connectors. They are brand-marked "1977 Sullins ESA50DRKH" Desperate searching of the Sullins catalog shows a ESA43DRKH (same features but 43 pins X 2, not 50 X 2) is a 43-pin connector with "RK = .140 X .225" DIP solder termination". On that MITS 680b Web page, I do more research. Oddly enough, while the MITS 680B uses MITS Altair 8800 connectors, the bus is not Altair bus compatible. Since the 680 uses a Motorola 6800 processor, the latter is not surprising.

EDAC 100 pin connectors and odd data sheets

EDAC is a long-standing brand of PC board edge connectors. Some S-100 companies used their connectors on their backplanes. In 2022 you can still order these connectors. But be sure the connector has 100 pins and the row spacing is what you expect (or may be different than you hope).

Mid-2022 resales of EDAC connectors include a model EDAC 346-100-520-802 that's still a current part. Some vendors described it as "100 pin dual .125 X .200 green", suggesting a .200 row spacing. It actually has a .250 row spacing. There were contradictory specification sheets from EDAC. The following links are EDAC Web links, but various distributors either link to EDAC or have copied EDAC documents. (The documents may have been corrected since, see following.)

General descriptions of the 346 series from EDAC describe "346 SERIES features: .125 (3.175mm) Contact Spacing x .250 (6.35mm) Row Spacing". But the EDAC CAD drawing of the 346-100-520-802 part doesn't specify the 520 (straight PC pin) spacing. But one document often-referenced by distributors and sellers is the 346/396 series EDAC ordering guide. It says "0.100” (2.54mm) contact spacing x 0.200” (5.08mm) row spacing". Other EDAC documents for the 346 series suggest a .250 inch row spacing.

In Sept 26 2022 I sent a contact-us email to EDAC on this situation, referencing their documentation. They replied and acknowldged the error, and were working to correct their current documents. If you (the reader of this Web page) have one of these EDAC connectors, please measure and confirm the row width and contact me. - Herb Johnson

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Herb Johnson
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Copyright © 2023 Herb Johnson