The BC01V RS232 Interface Cable
Part of
the PDP-8/E Hardware Documentation
|
The BC01V RS232 Interface Cable
Part of
the PDP-8/E Hardware Documentation
|
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.
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
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.