VSI connector interface and its cable wiring (draft Proposal)

IVS-TDC group (JN), CRL, Kashima

Purpose of this document:

Selection idea of the VSI (VLBI Standard Interface) connector and cables.


The VSI connectors and associated metal cable assembly hardware which enable to handle 64MHz data and clock. In a telescope site or in a correlation site, the digital signals are transferred between DAB, DIB, DOB and correlator length with the maximum distance 20 meters.


D-sub 50 pin connector with pin assignment in Table-1 is proposed. Multi-purpose digital cable assembly consist from 17 set of twisted pairs are recommended for the cables. The D-sub connector shell should use ISO bolts and nuts.


This VSI-1 cable/connector combination is very common in digital HDTV products using 75MHz clock. It becomes popular as well as currently used VGA connectors and cables. The cable assembly is de-fact standard supplied very reasonable price. Above 32MHz data rate, long distance data connection is hardly reliable without independent shielded twisted pair.

Table 1. VSI-1 with popular cable assembly
Cable unitName(*) HOT COLD SHIELD**
1 CLK 1 34 18
13 DATA0(16) 2 35 19
14 DATA1(17) 3 36 20
2 DATA2(18) 4 37 21
3 DATA3(19) 5 38 22
4 DATA4(20) 6 39 23
5 DATA5(21) 7 40 24
6 DATA6(22) 8 41 25
7 DATA7(23) 9 42 26
8 DATA8(24) 10 43 27
9 DATA9(25) 11 44 28
10 DATA10(26) 12 45 29
11 DATA11(27) 13 46 30
12 DATA12(28) 14 47 31
15 DATA13(29) 15 48 32
16 DATA14(30) 16 49 33**
17 DATA15(31) 17 50 33**
*() labeled for second connector name
**each shield is independent without unit 16/17

Additional remarks:

Fig.1 Flat cable.Fig.2 ID1 cableFig.3 ID1 & VSI

VLBI data transfer had been using differential ECL signals and twisted pair cables. Figure 1 shows typical cable and an example of shielded cable. Figure 2 shows ID-1 connector and cables used in Japanese K-4 VLBI. The assembled cable is also consist from twisted pairs and an assembly shield. These cables are enough performance below 32MHz rate. But above the 32MHz and multi vendor environment expected in the VSI, exact ECL signal shape must be maintained between the unit. The IVS-TDC CRL group experienced parallel 128MHz ECL data transfer will cause terrible degrade and error with slight cable difference. We strongly propose to employ twisted pairs with shielded each for 64MHz expansion. Although this will increase the weight of cables, the de-fact standard cable as in Fig.3, the assembly employed AWG27 equivalent conductor is attractive size (15mm diameter) and reasonable price ($50/meter). If a special cable is designed for VSI, it will drastically increase the price and difficult to obtain replacement one. As for the connector, The connector shell and connector latch mechanizm expected be have stiffness to support the cable weight. The D-sub with screw bolt is the most preferred for the purpose. VLBI equipment are connected and disconnected very often on site. But If a top grade brand is employed less troubles are expected. Also it important the on site engineer can inspect signals directly. Although others are small and attractive, it is better to employ D-sub 50 for the VLBI purpose. The succeeded Japanese 1024Mbps VLBI system uses four ID1(256Mbps) cables currently. This is very confusing in cable connection work. We will adapt our interface to the VSI standard when they are determined.

Below 32MHz rate, it is possible to introduce another signal by special wiring as in Table2. This needs special wiring and not recommended.

Table 2. VSI-0 special cable assembly (example)
Cable unitName HOT COLD SHIELD*
1 CLK 1 34 18
13 DATA0 2 35 19
14 DATA1 3 36 19
2 DATA2 4 37 21
3 DATA3 5 38 21
4 DATA4 6 39 23
5 DATA5 7 40 23
6 DATA6 8 41 25
7 DATA7 9 42 25
8 DATA8 10 43 27
9 DATA9 11 44 27
10 DATA10 12 45 29
11 DATA11 13 46 29
12 DATA12 14 47 31
15 DATA13 15 48 31
16 DATA14 16 49 33
17 DATA15 17 50 33
18 AUX1 20 22 24
19 AUX2 26 28 30
20 AUX3 32 -- --
* shields are partially common

Fig.4 D-sub50 wiring in a shell.

Table 3. Investigated VSI Connector Candidate
Figure Approx size
Panel cross section
High speed long
cable combination
(20meters maximum)
Price in Akihabara Application,
reputation and
D-sub 65x14YES$12-$20 HDTV. Good reputation and long history. Price dynamic range. D-sub 25 is very popular and matured. Easy to check signal on site.
half pitch(50)
52x11Unknown,Latch seems poor to support shielded cable weight$10 SCSI, no long history.
80x15YES$15 Printer, computer,Share decreased. due to huge connector size and repeatability.
Half pitch(50)
40x10Unknown,Latch seems poor to support shielded cable weight$18 SCSI
$15 VME, cable connector available.
Flat cable
68x6Latch poor to support shielded cable weight$5 Computers, very popular cheap. Expensive discrete wiring.
*Commonly used connector name. May be differ by country.

March 31, 1999