ServoMaster: Observations on FerretTronics FT639


This simple serial controller was being manufactured by now defunct FerretTronics. I don't think it is possible to get a hold of this controller anymore, but in any case there may be some people who still need the driver. Even though it is available, I would not recommend using this controller - see below.

Supported Features
Silent Operation

Good Things

  • Cheap, as in inexpensive.
  • Simple. Takes an hour or less for somebody familiar with the craft to put it together.
  • The protocol has a relatively low overhead (for example, SSC has an overhead of 33.3% - every command (two bytes) has to start with the sync byte).
  • Has a single power source. For me, this might have been a deciding factor.

Bad Things

  • Cheap, as in having limited functionality.
  • Is a write-only device. It is not possible to even tell whether it is connected to the serial port, save read the status. Minimal functionality, like "controller ready" status reflected on hardware flow control pin, would be great.
  • I think the guys really, really underestimate the necessity of higher speed communication. Consider this: on 2400 baud, it takes about 10-11 milliseconds to transmit a command to the servo with the protocol currently used. It allows up to 100 steps per second when controlling a single servo.

    When controlling five servos simultaneously (maximum supported by FT639) this figure drops down to about 20 steps per second.

    When controlling twenty five servos simultaneously (maximum supported by FT649) this figure drops to about 4 (four) steps per second.

    Suppose, I have a requirement of controlling the servos in a smooth, silent manner, or smoothly and precisely. Does this requirement get satisfied when I have twenty five servos? No. Please note, that raising the communication speed to 9600 baud is hardly enough. 19200 seems to be more or less acceptable.

    Oh well.

  • More about communication speed (thanks, Chester): there are actully two separate issues: latency and bandwidth. I believe in this case the latency is negligible (more numbers later), because the mechanical delays in the apparatus connected to the servo may more or less be of comparable order with the protocol latency. Bandwidth, however, is an issue here.
  • Be careful with the header length support. The lower values for the header length make the controller issue really short pulses that make the servos go out of range. The indication of that (at least for Futaba S3003) is a high-pitch noise (ever been to the dentist? The kind of noise their tools have, but higher). I don't think it is very good for the servo, and it definitely makes the servo uncontrollable.

    The default value for the header length (doesn't seem to be 0, doesn't seem to be 15 (maximum), doesn't seem to be 7) works just fine. Use this feature only if everything else fails. Keep in mind that this is a controller-wide feature, not servo specific.

  • State transitions of the hardware controller when changing the header length and/or range (90/180 degrees) are really unpredictable. I've taken countermeasures against the range changes, and gave up on header length changes - caveat emptor.
  • Seems to be unreliable when a massive data transfer on the serial port is coming. When I install a crawl transition controller on a servo and set it to a different position, the controller gets flooded with data and seems to be misinterpreting the servo number once in a while. As a result, wrong servo gets positioned, and since the position of a different servo is unrelated, this wreaks havoc on the system.

    For quite a long time I thought that this is related to the noise in the serial cable, but now I tend to think that the controller is the culprit.


Usable. You won't find it any longer, however.