64FDC terminal cable

[Mike Loewen]

   I don't see this mentioned anywhere, not even in the 64FDC manual or schematic. The manual for the 64FDC lists the RS-232 pinouts for the terminal connector (2, 3 and 7), but the pins on the connector are NOT labeled as what I expected. The standard for these header connectors is alternating pins (1,3,5,7 etc. on one side, 2,4,6,8 etc. on the other). On the 64FDC the connector pins go from 1-13 on one side and 14-26 on the other. My 64FDC didn't come with a terminal cable and this caused me much wasted time trying to get an RDOS prompt on my terminal.

   I hope this might save someone time in the future.

[Peter Higgins]

True, the numbering of serial port pins in the user manual and schematic do not follow convention for an IDC header. Instead they refer to the pin number on the DB25 connector it is cabled to. In the Cromemco 64FDC manual (under the heading of "cables") is mention of a ribbon cable connecting J4 and the DB25 serial connector, which suggests how Cromemco actually intended this cable to be manufactured. This removes the guesswork of figuring out which pin on J4 corresponds to which pin on the DB25 connector.

Regardless of the numbering on J4 connector, wiring a DB25 serial connector to it is designed to be super simple: crimp a 25 conductor ribbon cable to a DB25 female IDC connector on one end, and to a 26 pin female IDC connector on other end, lining up the stripe on the cable with pin 1 on both connectors. This leaves pin 26 on the 26-pin female IDC header not connected to the cable. You can also buy very inexpensive pre-made cables exactly like this on Amazon and eBay.

[Mike Stein]

I've probably made over a hundred cables like this, both for Cromemco systems and others, and unless you're just making a 3-wire cable Cromemco's way is by far the simplest.

You'll find the same thing in the PC world, for example: some manufacturers pinned their motherboard-to-rear-panel serial and parallel headers the same simple, non-standard DB-25 crimp-on way and others the 'standard' 'zigzag' way, which did indeed occasionally lead to confusion.

A similar situation existed in the world of ribbon cables plugging into IC sockets; some (most) were made with pin 1 in the top left corner with others having it in the top right corner; lots of fun...

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