DEC BC01V RS232 Cable

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DEC BC01V Cable

For the PDP-8, the KL8-E (M8650) and KL8-JA (M8655) connect to the outside world via a 40-pin “Berg” connector, arranged as 2 rows of 40 pins as shown here. These boards supports 20mA current loop or RS232 communications, depending on the cable that is connected to it.

This cable configuration was also subsequently used on some UNIBUS and QBUS PDP-11 systems as well.

To connect a KL8-E to an RS232 peripheral, DEC specified a DEC BC01V cable. This has a 40-pin Berg female housing on one end (with no mechanical strain relief) and a DB25M on the other end. Its wired as a DTE device, as are VT-series terminals and the serial ports on IBM PC compatibles. So you will usually need a DB25F-to-DB25F “Null Modem” cable to go between the BC01V and the device you want to connect to. The Null Modem cable will swap the relevant wires (eg pin 2 at one end goes to pin 3 at the other end).

Details of the BC01V cable can be found at Page 54 of this PDF of PDP-8/e Engineering Drawings on Bitsavers. A copy of that page is also shown here.

DEC’s BC01V cable is somewhat over-engineered for the KL8-E. It has 15 wires, when only 3 are required (plus one loop-back jumper wire which is also required). In fact many of the 15 wires are not supported by the KL8-E. Presumably DEC intended this cable would be used on a variety of other more capable Asychronous Interface boards in the future. The drawing referred to above says that the BC01V was first used on the PDP-8/e. I’ve also used a 24-wire DEC serial cable with the KL8-E. I’m not sure yet of the part number of this 24-wire cable, but it also seems to work fine with the KL8-E.

Note that the KL8-E creates fake (always asserted) DTR and RTS signals, and does not support any incoming handshaking or status lines such as DSR, CTS or RI. So it will generally be pointless running these wires in your own home made cables, unless you will be using the cable with a more advanced DEC serial interface board. Keep in mind that you may need to create local loopbacks on some signals at your peripheral, if they are required by the peripheral (eg for the DSR and/or CTS signals).

Minimal RS232 cable: KL8-E to 25-Pin DTE Device

You can easily make a cable to connect the KL8-E directly to a peripheral that has a male 25-Pin DTE RS232 connector on it. For example, this cable will connect the KL8-E directly to a VT100 terminal or to the 25-Pin RS232 connector on the back of an IBM compatible PC, without the need for any “null modem” cable.

The cable shown here uses the AMP 102387-9 housing as a substitute for the Berg connector (further details of this housing are provided below).

This cable is NOT a substitute for the BC01V. Instead, this cable swaps the TXD and RXD wires, so you can connect the cable directly to the peripheral.

Here are the connections required. In the list below: P1 is the Berg female housing, P2 is a female DB25 connector, and signal names are relative to the KL8-E.

  • GND: P1-VV (Pin 40) to P2-7
  • RXD: P1-J (Pin 8) to P2-2
  • TXD: P1-F (Pin 6) to P2-3
  • EIA loop: P1-E (Pin 5) to P1-M (Pin 11)

I find that a cable length of about 3.0m works well for many applications.

Minimal RS232 cable: KL8-E to 9-Pin DTE Device

You can easily make a cable to connect the KL8-E directly to a USB-to-RS232 adapter that has a male 9-Pin RS232 connector on it. This cable will connect the KL8-E directly to the USB-to-RS232 adapter, without the need for any “null modem” cable.

Here are the connections required. In the list below: P1 is the Berg female housing, P3 is a female DE9 connector, and signal names are relative to the KL8-E.

  • GND: P1-VV (Pin 40) to P3-5
  • RXD: P1-J (Pin 8) to P3-3
  • TXD: P1-F (Pin 6) to P3-2
  • EIA loop: P1-E (Pin 5) to P1-M (Pin 11)

Substitutes for the Berg housing

The Berg header used by DEC for the BC01V is in fact a 44-pin housing, as shown here. It consists of the 40 pins that are used for signals, plus 2 unused pins at either end of the housing.

It has overall external dimensions of 2.195 by 0.195 inch, and a pin spacing (in both dimensions) of 0.1 inch.

I suspect that genuine Berg connectors and pins are no longer available. So we have to find a suitable substitute when making a new cable.

Option 1: Generic 40-pin housing

A generic 40-pin housing (such as AMP 3-87456-6) will typically work just fine. But these typically have a nominal external dimension of approximately 2.0 by 0.2 inch, and they can easily be inserted into the Berg connector one pin too far left or right, which is a problem.

Option 2: “AMPMODU Mod IV” Series Connectors by TE Connectivity / AMP

A good substitute for the original Berg connector is the 44-pin housing with part number AMP 3-87456-7 or 3-87456-8. These are shown on TE Connectivity’s Drawing No C-87456. It has nominal dimensions of 2.205 x 0.240 inch. I haven’t yet tried this 44-pin connector, but it should be fine. The 0.240 inch thickness should be no problem (the 40 pin version has the same thickness, and that fits just fine). The problem is that it is somewhat difficult to obtain. Digikey only sell it in minimum of 100 pieces (US$911) and element14 don’t stock it at all. Mouser currently have a small number in stock, but their shipping to Australia is expensive.

So I have gone with the alternative of selecting the 50-pin version (AMP 3-87456-9 or 4-87456-0) and cut it down to make a 44-pin connector. This cut-down connector is what I used on the “KL8-E to 9-Pin DTE Device” cable shown above.

One nice thing about this option is that you can fabricate a loop between vacant holes at either end of the housing, and use that loop as a handle when disconnecting the housing from the board.

Option 3: Another “AMPMODU Mod IV” Series Connector – with different dimensions!

A good option is AMP 102387-9. It is shown on TE Connectivity’s Drawing No C-102387.

This is a 40-pin connector that has a thick wall at either end of the connector. The result is that the overall dimensions are 2.180 x 0.227 inch. It mates with the KL8-E without any risk of misalignment. Another nice thing about this housing is that because there are only 40 holes (rather than 44), there is less risk of mis-counting the holes and clicking the pins into the wrong holes.

This connector is used on the “KL8-E to 25-Pin DTE Device” cable shown above.

This part is currently available from Digikey and element14 in small quantities. I have used this connector and it works well for me. The only problem is that it comes with a polarising key on one side, which must be cut off.

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Revision History

17 January 2017: Posted initial version.