Servo Controller Question

Lautarocondor

Hello,
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I would like to know what would be the maximum length of cable (power, ground and signal) that can be effectively used with the servo controller # 28023. Could I use this with lengths of 100ft or more? What could happen if I do?
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Thanks

allanlane5

Wow. Well, the Servo connection was designed to fit between an RF reciever and a servo inside a model airplane, maybe 10 inches away. Now, the PIC inside the BS2 was designed as a reasonably powerful I/O device -- so it can drive (and sink) about 20 mA max per pin, which is HUGE in TTL terms. However, it only goes to +5 and zero volts, which isn't a lot.

The problem you get with long distances in cable (like over 10 feet or so) is that the capacitance in the cable is directly related to its length. So a long cable has quite a bit of capacitance. And the effect of this capacitance is to 'low-pass filter' your signal -- meaning those nice sharp transitions you can see at the BS2 get 'rounded' over at the far end. Once the 'rounding' gets gradual enough, the BS2 will be commanding the 'drop' before the 'rise' ever fully gets to the Servo. So the Servo won't see the right size pulse (or see a pulse at all) and it won't move.

One way to get around this would be to put an RS-423 driver at the BS2 (good to 5,000 feet of cable) and an RS-423 reciever at the Servo. You'll need a power supply line for the '423 at the servo, with a 10 uF capacitor across the power supply line and ground at the servo. AND you'll probably have to use some impedance controlled twisted pair wiring for the signal -- CAT-5 cable would work really well for this.