P300H P300 Series Modem Installation and Operating Handbook Page 195
APPENDIX D: REMOTE M&CREMOTE M&C PROTOCOL
Summary
The P300 Series Modems support two separate remote M&C protocols, these are:
1. P aradise / FDC protocol, compatible with all previous Paradise equi pment.
2. S A -bus protocol (as defined by Scientific Atlanta).
The protocols should co-exist wit hout adverse effect on the same M&C bus (provided the unit address es
are unique !). Both protocols assume one `Master device` and multiple `Slave devices`. The Master device
initiates all communications and Slave devices only ever send a message in response to a request from
the Master.
Character Format / Baud Rate
The character format & Baud Rates are as follows:
Paradise / FDC: 8 bits, no parity , 1 stop bit, selectable 300-19200 Baud,
SA-bus: 7 bits, even parity, 1 stop bit, specified only at 1200 & 9600 Baud.
The SA-b us protocol definiti on specifies that the characters in the `Body` of the message should, as far as
possible, be limited to the printable ASCII characters. Whilst this is not specified for the Paradise / FDC
protocol, Paradise Datacom have attempted to achieve this wherever possible. For this reason, only 6 bits
of each character of the `Body` are utilised (bit s 0 to 5). Bit 6 is fixed at a one for bot h prot ocols, and for t he
Paradise / FDC protocol (wher e the character has 8 bits, 0-7), bit 7 is fixed at a zero. Note that this applies
on to `Body` characters, there are no such constraint s on the other characters in the message.
Electrical Interface
Both protocols require a 4 wire plus ground interconnection between equipments. Signals are at RS485
levels (effectively a tri-statable RS422) with Tx & Rx data being tra nsmitted as a se ries of async ch aracters
over a differential pair (labelled `A` and `B`). Lines ref erred to as Tx-A and Tx-B are outputs, and Rx-A and
Rx-B are inputs. The Paradise co nvention (as spec ified by RS422) is that the `B` lines represent true data
(ie the inactive state is `mark`, which is high), and the `A` lines inverse data (ie the inactive state is `space`,
which is lo w).
An async character then appears as:
RS 485 `B` Line mark space mark mark
(Paradise / FDC) idle start bit 0 bit 1 bit 2 bit 3 bit 4 bit 5 bit 6 bit 7 stop idle
(SA bus) idle start bit 0 bit 1 bit 2 bit 3 bit 4 bit 5 bit 6 parity stop idle
RS 485 `A` Line mark space mark mark
The differential pair from Mas ter to Slaves is t ypically driven all the time by the Master devi ce (ie never goes
high imped ance). The return pair (from Slaves to Master) is driven by a Slave device when sending a
message. All devices hold their output driv ers at a high impedance unless actually transmitting a message,
to allow other Slave devices on the bus access to the `return pair`.
It is possible to operat e a link to a single device using an RS232 interface inst ead of RS485. However as
RS232 cannot go high impedance, only one Slave device can be on the bus otherwise there is permanent
bus contention. Note that on RS232 systems, a `mark` (high) is defined as ` negative voltage and a space