VMIVME-2128 Pull-up Resistor and External Voltage selection


To select the correct pull-up resistors for use with a 28VDC external source.  The load resistance on each output channel will be 50K ohms or higher.

The VMIVME-2128 Product Specification shows the various options available for this board. Normally, the selection of pull-up resistors is defined by the option dash number as shown in the bottom right portion of the first page of the spec. The -3xx option provides 6.8Kohm pullup resistors intended for use with a 24 to 30V external voltage. The -xx1 option provides SIP sockets for installation of the resistor packs for both pullup and warming resistors. The -xx0 option has no sockets. Depending on the option configuration, you could have pullup resistors soldered directly to the board, or no pullup resistors at all.

For the simplest case on a -xx1 board, with no pullup resistors installed, or the wrong values, you would just need to insert 6.8Kohm 10 pin resistor packs into the odd sockets beginning with RP1 and ending with RP31. Pin 1 of each network goes to the square solder pad. You can see this pad from the back side of the board. The even numbered sockets are for installation of warming (pull-down) resistors which are not used in this application.

The Bourns part number for a suitable 6.8K network is 4610X-101-682. With the output collector turned on, and using 28V, there will be 115mW dissipated in each resistor. These resistor networks are rated at 125mW per resistor. The collector-off output voltage with a 50Kohm load would be 24.6v.

If your application will have more than half of the drivers in each byte turned on continuously, a  higher pullup resistance value should be used to keep the power dissipation to a safe level in the resistor packs. i.e. 4610x-101-273 network provides 27Kohms. The power dissipation in this network with collector turned on would be only 30MW. The collector-off output voltage with a 50Kohm load would be 18v.  

Connection to the output pull-up resistor networks for the external voltage is provided on a byte-wide basis. The mapping between the P2 connector inputs and the actual output channels is as follows:

P2      Data Output Channels
C1       0 - 7
C2       8 - 15
C3       16 - 23
C4       24 - 31
C5       32 - 39
C6       40 - 47
C7       48 - 55
C8       56 - 63
C9       64 - 71
C10     72 - 79
C11     80 - 87
C12     88 - 95
C13     96 - 103
C14     104 - 111
C15     112 - 119
C16     120 - 127