Hi,
I'm new to MIDI, so this question may seem a little "noobish." I am trying to connect a keyboard to a 3.3V board using MIDI. I see in the MIDI DIN Electrical Specification that MIDI didn't always allow 3.3V, so I assume that there may have been an important hardware change with that. Do I need to use a newer keyboard or cable to correctly setup the connection, or do I only worry about using the resistor values and such as described in the specification?
Thanks
I'm new to MIDI, so this question may seem a little "noobish." I am trying to connect a keyboard to a 3.3V board using MIDI. I see in the MIDI DIN Electrical Specification that MIDI didn't always allow 3.3V, so I assume that there may have been an important hardware change with that. Do I need to use a newer keyboard or cable to correctly setup the connection, or do I only worry about using the resistor values and such as described in the specification?
Thanks
Accepted Answer
1
Undo
Votes
The MIDI specification specifies a 5 mA current loop, and shows three 220 Ω resistors to limit the current when used with a 5 V transmitter.
When you have a 3.3 V transmitter, the two resistors in the transmitter must be smaller to get the same current, as shown in the Electrical Specification Update.
The receiver does not need to know what voltage the transmitter uses; it always uses the same 220 Ω resistor. All transmitters are designed to work with that same receiver circuit. This means that all transmitters and receivers are compatible.
(The receiver can use either 5 V or 3.3 V as supply for the optoisolator; this part of the circuit is isolated from the current loop, and does not affect compatibility of the MIDI interface. But note that only the PC900/H11L1 optoisolators can run at 3.3 V; other common models like the 6N137 or 6N138 require 5 V.)
When you have a 3.3 V transmitter, the two resistors in the transmitter must be smaller to get the same current, as shown in the Electrical Specification Update.
The receiver does not need to know what voltage the transmitter uses; it always uses the same 220 Ω resistor. All transmitters are designed to work with that same receiver circuit. This means that all transmitters and receivers are compatible.
(The receiver can use either 5 V or 3.3 V as supply for the optoisolator; this part of the circuit is isolated from the current loop, and does not affect compatibility of the MIDI interface. But note that only the PC900/H11L1 optoisolators can run at 3.3 V; other common models like the 6N137 or 6N138 require 5 V.)
Accepted Answer
1
Undo
Votes
The MIDI specification specifies a 5 mA current loop, and shows three 220 Ω resistors to limit the current when used with a 5 V transmitter.
When you have a 3.3 V transmitter, the two resistors in the transmitter must be smaller to get the same current, as shown in the Electrical Specification Update.
The receiver does not need to know what voltage the transmitter uses; it always uses the same 220 Ω resistor. All transmitters are designed to work with that same receiver circuit. This means that all transmitters and receivers are compatible.
(The receiver can use either 5 V or 3.3 V as supply for the optoisolator; this part of the circuit is isolated from the current loop, and does not affect compatibility of the MIDI interface. But note that only the PC900/H11L1 optoisolators can run at 3.3 V; other common models like the 6N137 or 6N138 require 5 V.)
When you have a 3.3 V transmitter, the two resistors in the transmitter must be smaller to get the same current, as shown in the Electrical Specification Update.
The receiver does not need to know what voltage the transmitter uses; it always uses the same 220 Ω resistor. All transmitters are designed to work with that same receiver circuit. This means that all transmitters and receivers are compatible.
(The receiver can use either 5 V or 3.3 V as supply for the optoisolator; this part of the circuit is isolated from the current loop, and does not affect compatibility of the MIDI interface. But note that only the PC900/H11L1 optoisolators can run at 3.3 V; other common models like the 6N137 or 6N138 require 5 V.)
0
Undo
Votes
The 5 volt spec used 2 x 220 ohms. The 3.3 volt spec uses 10 & 33
That seems proportionally incorrect.
The voltage drop over the receiver (1.1 V over the 220 Ω resistor, and somewhere between 1.1 V and 1.7 V over the optocoupler's LED) should stay constant. The transmitter's resistors must drop the remaining voltage; in the worst case, this would be about
5.0 V − 1.1 V − 1.7 V = 2.2 V, or
3.3 V − 1.1 V − 1.7 V = 0.5 V.
(The section "Low-Voltage Signaling Technical Notes" of CA33 shows how to compute the resistor values. However, in practice, designers make less pessimistic assumptions about the LED forward voltage; many 3.3 V devices actually use 2× 47 Ω in the transmitter.)
- Page :
- 1
There are no replies made for this post yet.
