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To modify the inputs of VMIVME-012536-00x for sensing the presence of 28VDC.
The -00x board is designed for an input voltage range of 0 to +5V.
1. Referring to VME2536 User Manual Fig 3.3-1 page 3-5, the value of Rvs is determined by "A" order code as described on the first page of product spec sheet. There should be a label on the board which indicates a three digit value for the order code implemented. The B field is reserved and not used. The C field indicates a one for CE compliance, otherwise a zero. The options are as follows:
ABC = 00x Rvs = 1.6Kohm. Max Input voltage = 5Vdc.
ABC = 10x Rvs = 5.1Kohm. Max Input voltage = 12Vdc.
ABC = 20x Rvs = 12Kohm. Max Input voltage = 24 to 28Vdc.
ABC = 30x Rvs = 27Kohm. Max Input voltage = 48Vdc.
ABC = 40x Rvs = 75Kohm. Max Input voltage = 125Vdc.
2. RVS is the channel input resistor. RTH is 3.3Kohm for all configurations. Resistor packs RP5 thru RP8 should not be populated for voltage sensing applications.
3. Normally, the board should be returned to GE-Fanuc for the necessary RVS resistor changes and protection of warranty status. However, this may not be practical due to time constraints. Instead of modifying the board, an external resistor can be added in series with the input to accommodate higher input voltages.
4. The following descibes the behavior of opto-isolator input and how to determine the value of an external resistor to allow a -00x board to safely sense a 28VDC input. The same process can be used for other input voltages and board options.
5. The Opto-Isolator input diode limits the voltage across it to 1.2v typical, 1.65v maximum at 50mA thru the LED. This part is an ISOCOM ILQ-2. According to the datasheet, the diode voltage will be between 0.8 and 1.2V typical for diode currents between 1 and 10mA. It's not possible to exactly analyze a circuit which contains a non-linear element like a diode using just ohms law, but an acceptable approximation can be made.
6. For VMIVME-012536-00x configuration, RVS = 1.6K ohm. Lets assume a diode current of 2mA which means there should be approximately a 1 volt drop across the diode, according to the ILQ-2 datasheet. This voltage drop is also across Rth, so the current thru RTH must be 1 / 3.3Kohms = 0.3mA. So, we have 2.3mA that will pass through RVS. RVS must drop 28 - 1 = 27v with 2.3mA passing through it. Using Ohms Law, this would be 27v / 2.3mA = 11.7K ohm. If a 10K ohm resistor (11.7 - 1.6 = 10.1) is connected in series with the high input to the board, the 28V source can be safely sensed. Power dissipation in the external 10K ohm resistor is (0.023A)^2*10K ohm = 52.9mW. A 1/8 watt resistor or greater should be used. The above technique can also be used to estimate the correct series resistance for a -10x board. For that option, the external resistance would be 6.6K ohm. Power dissipation in the resistor would be 35mW.
7. Note that if the external series resistor is too small by 20% or more, opto-isolator diode current may be excessive causing premature failure of the part. This event would not be covered under the warranty. External resistor tolerance should be +/-10% or less.