RFL SMX3/40 Page 5 of 24 Issue 2.02
User Manual
07 May 2009
5. Channel Characteristics
SMX3/40 aggregates four point to point bi-directional V24 data channels across a
single X21 service (64 kbits/s)
Channels A, B and C sample at 16kbits/s
Channel D samples at 8kbits/s
DIP switch settings permit each V24 port to be allocated to any channel. Additionally
each V24 data channel may be selected to either ‘Transparent’ mode or one of two
‘Async’ modes.
Transparent mode uses a minimal delay protocol for applications where end-to-end
delay is critical (eg MPR LZPK102 protection relays operating at 600 baud). It may
also be used for applications where asynchronous character formats are not
specifically supported. See table for maximum supported baud rates.
In Async mode (8-bit or 9-bit characters) the channel baud rates may be set within
the range shown in the table to match the connected equipment.
V24 Channel A B C D
Sampling Rate ( 64kbits/s aggregate)
kbits/s
16 8
Transparent mode – max baud rate
supported
2400 1200
Async mode – baud rates supported 1200 - 9600 1200 - 4800
6. Performance
When two SMX3/40 sub-MUXs are interconnected on a direct 64kbit/s X21 link, the
following maximum delays will be achieved for a V24 channel set to Transparent
mode:
V24 Channel in Transparent mode A B C D
Max delay ms 0.5 1
When used for interconnecting MPR LZPK102 relays, the end-to end transmission
delay on the circuit is required to be less than 5ms. The delay introduced by the BT
X21 link is unknown. (BT SIN355 specifies a max. delay of 1.25ms for an Analogue
Standard data circuit between 1000 and 2600Hz. For digital circuits SIN355 states
“With the advent of the wholly digital BT core network, relative group delay has
become less of a significant parameter in relation to the transmission of voiceband
data”). If it is assumed that transmission delay within the BT element will not exceed
2.5ms, using SMX3/40 in transparent mode enables total end to end transmission
times in the order of 3 to 3.5ms to be achieved, depending on the channel used.
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