The problem is that most devices (in the ham world) expect the driver
device to provide voltage.
In the communications (non-ham) world, the expectation is that the
driver device produces either a logic low (short to common) or a logic
high ( open circuit).
Look at the data sheets for "line drivers" and "line receivers" to check
out what I am saying.
Open collector or open emitter does not make a difference in function,
it is only a circuit design decision. Yes, open Collector (or open
drain) is commonly use in logic where the active state is zero volts
(transistor or FET conducting to ground).
The open emitter design is the opposite. A conducting device will
provide a voltage on the line (or signal) being driven.
The point is that in a properly designed communications system, the
drivers provide either conduction to ground or an essentially open
circuit to the communications line (think of a relay being either open
or closed). The receiver provides the voltage to detect whether the
driver is in an open circuit or closed circuit state.
If there are multiple receivers in the system, only one can be "boss",
and that one determines the open circuit voltage and contains the pullup
resistors for the system. Other receivers work in listen mode and will
contain no pullup resistors or active drivers.
This whole situation goes back to the "one driver, one receiver"
condition.
Only one driver can exist on a communications system without conflict.
Multiple receivers are possible, but only one (at the far end of the
line) should provide the pullup resistors. All other receivers must be
only in the listen mode.
73,
Don W3FPR
On 2/27/2018 8:06 PM, Joe Subich, W4TV wrote:
> But many (most) ham devices do not do it that way, they expect the
> pullup resistors will be provided by the driver gear.
Actually, most devices that use BCD "band data" expect an open
emitter driver not an open collector driver. Open emitter will
*source* voltage for logic high and be open circuit for logic
low. This is the convention from the early Yaesu rigs which
were the first devices to support "band data" (it is the way
the FL-7000 and Quadra amplifiers operate).
You will find the W9XT BCD10/BCD14 decoders with their opto-isolator
inputs work just fine with the "open emitter" drivers. Other devices
designed with Yaesu transceivers in mind have appropriate current
limiting (series) on the input lines and "pull down" (parallel)
resistors on the logic gates. Some "standard" devices (Top Ten
BD-Y and the original microHAM Band Decoder) will provide both
current limiting resistors and internal pull-ups but I have not
seen any amateur product with series diodes in the band data lines.
73,
... Joe, W4TV
On 2/27/2018 4:17 PM, Don Wilhelm wrote:
Knut,
That is the way it *should* be, and was that way in the K3 originally.
But many (most) ham devices do not do it that way, they expect the
pullup resistors will be provided by the driver gear.
So, because of that, Elecraft added pullup resistors to the band data
outputs of the K3 long ago.
So yes, we are left with a situation that often requires steering diodes.
73,
Don W3FPR
On 2/27/2018 3:48 PM, ab2tc wrote:
Hi Don and all,
Hear,hear, Don. The receivers should have the pullup resistor to
whatever
the appropriate voltage needed (within reason) *and* a steering diode in
series with the input. This will prevent another device with a higher
voltage from feeding current back into the device which could damage
semiconductors. Without the steering diode all receivers must use the
same
pullup voltage. Of course a single receiver is not a problem either.
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