David, The original 1964 RS-232 specification for the Bell System 103A modem specified bipolar levels at least +-6 V. This was also the case for the 201A, 202A and others. However, the Bell System 113A modem used 0 V for logic 0 and +6 V or logic 1. There has been mass confusion ever since; sometimes TTL levels work, sometimes not. It's easy to wire a couple of transistors to match TTL levels to bipolar.
Dave Dave David Woolley wrote: > In article <[EMAIL PROTECTED]>, > Martin Burnicki <[EMAIL PROTECTED]> wrote: > > >>The reason for this is mostly historical. On-board logic levels used to be >>close to 0V for logical "low" and close to +5V for logical "high" signals. > > > Whilst it certainly is historical, I have doubts about this interpretation. > > In particular, it doesn't explain why control signals use positive logic. > > >>Those on-board signals were converted to RS232 levels using inverting >>driver chips, > > > That's definitely wrong, as the standard pre-dates that level of logic > integration. When it was developed, logic would have been ECL or DTL. > ECL uses very low voltage differentials and DTL doesn't naturally produce > the TTL 5 volts power supplies. At the low level (probably zero, > at that time) of integration for DTL logic, the distinction between > positive and negative logic would really depend on whether you wanted > a NOR or a NAND function. > > >> so you have negative voltages for a logical RS232 "high" >>level on the wire. > > > The 12 volts bipolar probably comes from using polarised relays with a 12 > volt (13.8V) standard coil voltage (12, 24 and 48 volts are standard lead > acid battery configurations, with 48 still used for telephone systems). > I suspect that the use of negative logic, for the data (but not the > control signals), came about as a result of not wanting teletype motors > to run when the connection is broken. With current loop teletypes, > the idle state is with current flow and is the marking state. When 7 > bit an 8 bit teletype codes were developed, they arbitrarily chose mark > to be binary one. > > The start of a current loop character is signalled by breaking the loop, > which means that a disconnected cable produces a continuous start signal > and the teletype motor keeps running, wearing out the machine. I suspect > it was decided that the default state for RS232 should be that the control > signals should be false and the teletype be stopped. That requires a logic > low on the control signals correspond to a marking state on the data, and > therefore requires the opposite polarity to that which produces a one in > the character codes. > > >>Normally a PC's serial port expects RS232 levels as inputs, so the PPS pulse >>which if normally fed to the Carrier Detect (DCD) pin should also be RS232 >>level. > > > Note that, as a control signal, DCD uses positive logic. > > >>However, in practice the RS23 chips built into standard PCs accept also >>voltage levels in the range of 0..+5V as input levels. > > > This is actually a very old characteristic of RS232. It ensures that > disconnected (0 volts) is treated as negative, so a false control signal > and a marking data signal, which is the correct fail safe configuration. > _______________________________________________ questions mailing list [email protected] https://lists.ntp.isc.org/mailman/listinfo/questions
