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The BC01V RS232 Interface Cable

Part of the PDP-8/E Hardware Documentation
by Douglas W. Jones
THE UNIVERSITY OF IOWA Department of Computer Science

The M8650 and M8655 boards use the BC01V cable to communicate with RS232 devices. These notes describe how to build this cable.

The BC01V cable supports most of the RS232 standard, but most devices don't use more than 9 of the 25 pins in the standard. The M8650 and M8655 only require three of these, transmitted data (txd), received data (rxd), and ground (gnd), but they provides support for request to send (rts) and data terminal ready (dtr), both of which are held high. The following cable should work with any DEC board that expects a BC01V cable, so it provides full support for all 9 commonly used signals.

Male DB25 connector                              Female 40 pin plug
 as seen from back                                as seen from back

   ---1                                          Board A   B  Free
  |       14            This jumper configures   side         side
  |   2-----------.     the interface for RS232        C   D
  |       15       \                            \
  |   3---------.   \                             -----E  ,F  <txd
  |       16     \   \                     ,-----|-------'
  |   4-------.   \   \                   /      |     H  ,J  >rxd
  |       17   \   \   \    ,--cts>---   /   ,---|-------'
  |   5---------\---\---\--'           \/   /    |     K   L
  |       18     `---\---\----<rts---. /\  /     |
  |   6-------.       \   \           X  \/       -----M   N
  |       19   \       \   `--<txd---' \ /\
   ---7---------\---.   \               X  \           P   R
          20--.  \   `---\-----gnd-----/-\--\--.
      8-----.  \  \       \           /   \  \  \      S  ,T  >cts
          21 \  \  \       `---rxd>--'     \  `--\-------'
      9       \  \  \                       \     \    U  ,V  <rts
          22   \  `--\--------<dtr-------.   `-----|-----'
      10        \     \                   \        |   W   X
          23     \     `-------dsr>--------\--.    |
      11          \                         \  \   |   Y  ,Z  >dsr
          24       `-----------rsd>------.   \  `--|-----'
      12                                  \   \    |   AA ,BB >rsd
          25          The cable should     `---\---|-----'
      13              be no more than           \  |   CC ,DD <dtr
                      25 feet long.              `-|-----'
                                                   |   EE  FF
                                                   |
                                                   |   HH  JJ
                                                   |
                                                   |   KK  LL
                                                   |
                                                   |   MM  NN
                                                   |
                                                   |   PP  RR
                                                   |
                                                   |   SS  SS
                                                   |
                                                   |   UU ,VV
                                                    -----'
8 conductor modular telephone cable works very well for this cable, assuming that you do not have a high noise environment.

The order of conductors shown in the cable above provides some noise immunity by routing the data lines adjacent to the ground line and by surrounding the data by other signal lines that rarely see much use but are likely to be terminated at one end or the other.

A DB25 solder cup connector, with individually placed wires works well for the 25 pin connector.

At the M8650 end, an insulation displacement connector such as the following will mate with the Berg connector on the board (DigiKey part numbers for AMP parts are given):

    ASC40G-ND  40 pin gold socket connector, no polarizing key
    ASSR40-ND  strain relief for above
    ASPT40-ND  pull tab to make it easy to unplug
This works well with modular telephone cable, as long as each wire is carefully pressed into the contact forks by hand (fingernails are useful here!) and as long as appropriate strain relief is used (the strain relief bar that comes with the connector, augmented by a cable tie holding the end of the sheathed cable in place).

In cross section, the connector, cable and pull tab are assembled as follows:

                                        Free side of connector
                             wires
       ___________________    ||    crimp bar
     |                 ___ \  ||    /  _______
      \        Strain |   | | || -- __|       | Socket
       \       relief |   | | |||  |__|       | body
        \              ---  | || -- |||_______|
          ------------------  ||____||
   Pull tab                    ------   Board side of connector


   \\        \\____//
    \\  ______||__||______
     \\| o  ____        o
      \\  _|_   |
  ---------| |  |
 |         | |  |
 |         H |  |
 |         H | B|
 |         H | E|
  ---------H | R|
       | EEH_| G|
       |  -|____|
This shows the finished cable on the M8650 or M8655 card. Note that the pull tab extends beyond the card edge, and note that the cable has been routed through the hollow of the pull tab to a cable-tie at location EE that binds the end of the cable sheath to the strain relief bar. The column of H characters shows the route of the unsheathed conductors up the side of the connector until they disappear between the strain relief and the crimp bar. Note that the strain relief has one slightly hollow side. This should face the PC board to allow clearance for the cable tie between the board and the connector.

Ommitted Wires

The following additional wires are needed to make complete BC01V cable. Of these, only RI (Ring indicator) is likely to be useful for full support of some modems:

   DB25     40 pin    Meaning

    11        FF
    12        JJ      SDCD Secondary data carrier detect
    15         N      TC   Transmitter clock
    17         R      RC   Receiver clock
    22         X      RI   Ring indicator (Useful!)
    24         L      (TC) External transmitter clock
    25         C

Null Modems

When used with the BC01V cable, the M8650 and M8655 conform to the RS232 specification by having a male 25 pin connector on the far end, configured as DTE (data terminal equipment). Assuming the correct baud rate, this may be plugged into any DCE (data communications equipment) connector, since these are supposed to be female connectors.

Modems are usually configured as DCE devices, but essentially all other RS232 devices such as terminals or other computers are usually configured as DTE devices. To connect one DTE device to another, for example, to connect your M8650 to a terminal or another M8650, a null modem is required.

DEC's documentation for the M8650 specifies the H312 null modem. INMAC sells an H312A equivalent as part number H298-2, for $25. You can build one as follows from a pair of female DB25 connectors, a pair of 1.5 inch 4-40 threaded standoffs, and a few scraps of wire and screws.

           1------------------1        Protective ground
      14                 14
           2-----. .----------2      < Transmitted data (txd)
      15          X      15
           3-----' `----------3      > Received data (rxd)
      16                 16
   --------4                  4---   < Request to send (rts)
  |   17                 17       |
   --------5----.   ,---------5---   > Clear to send (cts)
      18         \ /     18
   --------6      X           6---   > Data set ready (dsr)
  |   19         / \     19       |
  |        7----/---\---------7   |    Signal ground
  |   20-------/-. ,-\---20       |  < Data terminal ready (dtr)
  |        8--'   X   `-------8   |  > Received signal detect (rsd)
  |   21         / \     21       |
  |        9    /   \         9   |
   ---22-------'     `---22-------   > Ring indicator (rng)
           10                 10
      23                 23
           11                 11  (Note that in this wiring diagram,
      24                 24        all places where 4 wires seem to
           12                 12   meet are really places where two
      25                 25        wires cross without touching!)
           13                 13
The theory behind this is as follows:
 a) txd from one device becomes rxd to the other.
 b) rts from one device becomes cts for that device and rsd for the other.
 c) dtr from one device becomes dsr and rng for the other.
Parts b and c only really matter for devices that use EIA flow control. Such devices lower dtr to stop the flow of incoming data, and they await dsr before they send outgoing data. In addition, they raise rts when ready to transmit, and they await cts before transmitting.