1 Introduction to Read/Write Electronics Upgrade Modules

Major changes since the March 1995 release of version 1.4 of this document include:

This upgrade package provides your Mark~IIIA tape recorder with increased recording capacity required by the new VLBA and Mark~IV VLBI recording formats. It consists of the following:

• A new head assembly module.
• One Mark~IV read module.
• Two Mark~IV write modules.
• One power distributor module.
• A new I/O board for the tape recorder.

The head assembly module replaces the old read and write interface boxes (behind the tape recorder reel panel) with new head interface and preamplifier boards fitted onto a new ``septum divider.'' The two headstacks of Mark~IIIA are reused and placed side-by-side in the upper head block position.

Read and write modules complement the head assembly to form a complete set of R/W electronics. A power distributor module is required to create supply voltages for these modules from the existing supply voltages. I/O board has to be swapped because the old one cannot control new read/write modules.

Some reasons for upgrading R/W electronics:

• Old electronics supports only 28 write and 14 read tracks.
• Old electronics writes at 8~Mbits/s maximum.
• Old electronics writes using only one headstack simultaneously.

All of these shortcomings are remedied in the upgraded electronics:

• New electronics supports 2x36 write and 36 read tracks. (Thus, the upgraded recorder could be used with a VLBA formatter because it is able support VLBA system tracks. However, Mark~IV formatters do not support system tracks, only 32 data tracks per headstack.)
• New electronics can write at 16~Mbits/s per track (18~Mbits/s with parity bits included).
• New electronics can simultaneously use two headstacks for writing.

Note that the new I/O board does not support selection of enabled write tracks. This is handled by the new Mark~IV formatter. Similarly, please note that reading from tape at 16~Mbits/s is not supported by the Haystack design.

A Mark~IV tape recorder upgraded in this way can record at the maximum rate of 1~Gbits/s (=2~heads x 32~tracks x 16~MBits/s). It can be further upgraded to accommodate four headstacks. This ``full upgrade'' requires:

• two additional narrow-track headstacks,
• additional mechanics i.e. the lower head block assembly,
• another head assembly module for the lower head block position, and
• two additional Mark~IV write modules.

The resulting ``full Mark~IV'' tape recorder is capable of recording at 2~Gbits/s.

The Metsähovi delivery consists of the following submodules, easily identified by their three-letter abbreviations. These abbreviations are present on the serial number labels fitted on the modules.

1. RDM - Read Interface Module
2. WRM - Write Driver Module (2 modules)
3. PWM - Power Distribution Module
4. IOM - Input/Output Module
5. HAM - Head Assembly Module
6. SPK - Spare Parts Kit

Each module is supplied with the cables required to connect it to the rest of the system. These cables are described in individual module descriptions.

Throughout this document and in the drawings, track numbers are referenced using the ``VLBA/MkIV'' track numbering. The 36 tracks available in a Metrum-style narrow-track headstack receive the numbers 0--35. So, the ``traditional'' Mark~IIIA track number 1 is track 4 in the new numbering scheme.

Please also note that as a consequence of this numbering change, the conventional Mark~IIIA ``even'' tracks are now found at connectors labeled ``odd'' and vice versa.

Write signal path

Intrinsically, installing the upgrade is a simple matter: you just replace all electronics in the current signal path from formatter to write headstack and from read headstack to decoder. The headstacks themselves and their adjustments are not touched.

In practice you need to remove first:

• Mark~IIIA reproduce electronics from backside of transport door (above heads).
• Mark~IIIA write electronics from backside of transport door (below heads).
• Mark~III (wire-wrapped) I/O board.
• 28 track write interface (adjoint write head in head assembly).
• 14 track read interface (adjoint read head in head assembly).

Then you can install the following:

• New NIM modules: PWM, RDM, 2xWRM.
• A new Mark~IV I/O board (a PC board, ``IOB'').
• New 36 track write interface to the write head.
• New 36 track write interface to the read/write head (former read-only head).

A Haystack reference drawing ``4712K003'' of the installation is provided in the documentation binder section ``Installation Instructions.''

For complete step-by-step installation instructions, please see the separate document ``Installing Mark~IV Read/Write Electronics Upgrade'' that is provided in the section ``Installation Instructions'' of the documentation binder.

This section presents strategies to system-level testing of the modules. Module-level test procedures are described in the installation manual and its checkout procedure section.

Automated procedures used at Metsähovi

Two automated SNAP procedures were developed for testing RDM and WRM modules. The procedures can be found in file ``mdextr.prc'' which has been printed out together with test log reports in the station-specific information section of the documentation binder.

Since these tests were conducted with Metsähovi VLBA DAR, the procedures make use of the VLBA formatter capability to embed track numbers in the auxiliary data field of each formatter frame. They also expect to find SNAP commands for manipulating VLBA features of the VLBA formatter, and this is provided in the NASA FS~9.1.0 and later.

The log files produced by applying these procedures to those particular RDMs and WRMs that have been delivered to a given station are available in the station-specific section of the documentation binder.

The procedure ``testwrm'' exercises the write signal paths by checking the bypass reproduce signal of each track. (This signal is not present unless WRM is driving a HAM, so the WRM was connected to Metsähovi MkIV/VLBA HAM.) The decoded track numbers must match the expected track numbers and the data quality analyzer must show zero parity errors.

The procedure ``testrdm'' relies on reading a test tape written with all 36 VLBA tracks enabled. Again, the decoded track numbers must match the expected track numbers. Please see the printouts of running these procedures on your hardware, included in the station-specific section of this documentation binder.

Suggested manual procedures

With the new Mark~IV formatter it should be possible to write a blank tape with one track enabled for a short period, say 200~feet. All tracks of the write head would be written in succession on the tape, only one track enabled at a time. Then the tape would be played back and the decoder used to if a given track is the only track decoding properly for a given 200 feet tape segment.

The process should be repeated (always on a blank tape) with 4~Mbits/s and 135~ips and 8~Mbits/s and 270~ips. Recording at 16~Mbits/s with 320~ips can be played back at 160~ips and verified in the same way as 8~Mbits/s. Neither the Mark~IV RDM nor the existing Mark~III decoder were designed for direct 16~Mbits/s playback.

This procedure can be written in SNAP schedules when the formatter control language can be tested in practice.

The fastest way to contact Metsähovi to get assistance with Mark~IV read/write electronics is to send email to:

vlbi@hut.fi

You can also reach us using the following fax number:

+358-0-264531

Our address is:

Metsähovi Radio Research Station

Mark~IV R/W

Metsähovintie 114

FIN-02540 KYLMÄLÄ

FINLAND

Please do not hesitate to contact us if you have questions about the upgrade. We can help you for instance in getting accessories for additional upgrades:

• Hard-to-find components.
• Components that need to be purchased in quantities.
• Printed circuit boards.
• Mechanical parts.
• Brimflyx ultra flexible wire!