2 RDM - Read Interface Module

Contents of this section

The modules have been tested with a test tape at 135~ips and the procedure ``testrdm'' presented in section autotest . The test reports can be found in the station-specific section of this documentation binder.

The data signals flow in a direct fashion from the rear panel connectors towards the components near and the connectors at the front panel. Thus with the NIM module side panel removed it is relatively easy to follow the signals with a normal oscilloscope.

Another test procedure is documented in VLBA memo #370 (which is also available as Mark~IV memo #166) and this is provided in the section ``RDM'' of the documentation binder.

The front panel of a RDM is presented in figure rdm.eps .

Figure file rdm.eps.ps: Front panel of RDM.

2.1 RDB - Read Board

RDB provides three major functions:

  1. Selecting 2 out of 36 possible track signals for monitoring. (This is used for both bypass monitoring in WRM and for ``RAW'' read-after-write/playback/reproduce signals in RDM.)
  2. Conditioning these two signals for decoding circuitry (clipping with a comparator outputting differential ECL) and buffering these for front panel BNC connectors ``A'' and ``B''. (This is done in both RDM and WRM.)
  3. Amplifying and equalizing 2 selected playback signal tracks from HAM according to playback speed. (This is needed in RDM only.)

The same PCB is used for providing full functionality for RDM and the bypass selection capability only for WRM. This ``write-only'' version of RDB is known as a ``RWB'' and it is presented in section rwb ---the amplifiers and equalizers of a full RDB are simply not populated in a RWB.

This is a standard double-sided PCB with plated-through holes and 35~μm of copper. RDB documentation can be found in the directory ftp://kurp-ftp.hut.fi/pub/mk4/rdm/rdb and printouts of these files are provided in the section ``RDM'' of the documentation binder.

In addition to the standard board documentation files discussed in Standard Files , the following files are provided: 

pub/mk4/rdm/rdb/rdeq.sch   -- Schematics for equalizer DIL carriers.
pub/mk4/rdm/rdb/rdeq01.ps  -- Printout of the previous in PostScript.
pub/mk4/rdm/rdb/rd09.ps    -- Same as "rdeq01.ps".

There are three selectable equalizers for both of the ``A'' and ``B'' channels. Two of these support standard playback speeds of 135~ips (4.5~MHz) and 270~ips (9~MHz). The equalizer circuits are built on DIL carriers to allow for easy replacement in case of new data rates and tape speeds.

Populating RDB

C91-C96 low pass capacitors are 390pF.

Solder GND test point lug to J8.

No modifications are required to prerouted jumper blocks J9 and J10.

U31 LM340T5/7805 +5V 1A, thermal pad (no insulation), M3x8mm DIN912 hex screw, lock spacer, nut.

U30 mylar insulation under heat sink (27x39mm), M3x8mm DIN912 hex screw, lock spacer, nut.

U32, U33 TO220 silicone insulation, M3x10 DIN 912 hex screw, lock spacer, nut.

Six No~4x1/4" self-tapping Supadriv screws are used to fasten the three ribbon cable connectors tightly to the board.

Wiring

Fasten an AMP 205841-1 CPC square flange receptable for pin contacts to the front plate location J1 with four M3x10mm hex screws (ISO 7380 low-profile), M3 lock spacers, and M3 nuts.

Although it is possible to populate J1 of RDB with either Weidmüller-Klippon or AMP 0.2" pitch connectors, this is not recommended due to decreased reliability. Instead, wire the AMP connector J1 at RDM front panel directly to the RDB as follows:

With small AMP 66506-9 HD-20 pin contacts (red dot) and 20AWG stranded copper wire (from Alpha cable): 

RDM front -- total length incl. contact + color -- RDB
J1-1 -- 90mm red     -- J1-1 (+12V)
J1-3 -- 95mm green   -- J1-2 (-12V)
J1-5 -- 100mm black  -- J1-3,4, or 5 (RETURN)

2.2 NIM/RDM

The enclosure for RDM is a standard 2/12 wide AEC/NIM cassette described in section NIM of this document. A RDM-specific drilling plan and front and rear panel silkscreening drawings are provided in the section ``RDM'' of the documentation binder.

NIM/RDM Fixings

The RDB is mounted in the NIM box with the following accessories. NIM rails have M3 threadings; mounting order is: NIM rail -- spacers -- RDB -- lock spacers -- screws.

2.3 Cable PWMRDM

This cable supplies power to the RDM. See the Haystack drawing 4712A104 for a reference description. Alpha cable 4x20AWG stranded copper wire type ``1896/4C'' is used, since only 3 conductors are needed. 

PWM front  -- 550mm cable --   RDM front
----------------------------------------
AMP 206434-1              --  AMP 205838-1  -- plugs
AMP 206062-1              --  AMP 206062-1  -- std. cable clamps
AMP 66506-9 (pin)         --  AMP 166291-7 (socket)  -- contacts
----------------------------------------
J5-1       -- red         -- J1-1 (+12V)
J5-3       -- green       -- J1-3 (-12V)
J5-5       -- black       -- J1-5 (+-12V RETURN)
J5-6 -- dummy pin for polarization
J5-7 -- dummy pin for polarization

2.4 Cable RDMWRMIOM

This is a standard (plain grey) 40-way ribbon cable that connects the monitor signals from IOM connector J3 to WRM~1 and further to RDM connector J3. Cable length is 165~cm and the spacing between RDM and WRM connectors is about 10~cm. All connectors have their pin 1 (marked with a triangle) aligned to the red wire.

Please note that while WRM #2 takes care of the read/write headstack #2 (see the block diagram figure block.eps ), the monitor signals are shared between RDM and WRM number 1 when using this cable.

RDM Front Panel, J4 (Monitor)
 -- WRM 1 Front Panel, J4 (Monitor) 
      -- IOM, J3 (Control Out)

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