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The SMV Max Delay parameter, found via menu path Configuration > System, defines how long the receiver waits for the SMV frames before activating the TxTR ALARM output. This setting also delays the local measurements of the receiver to keep them correctly time aligned. The SMV Max Delay values include sampling, processing and network delay.

Warning: For best performance, the SMV Max Delay value should not be set needlessly high since this delays the protection by an equal amount of time Table 1. Setting it too low can cause SMV samples to arrive after the set deadline and thus make SMV-based protection inoperable. To determine suitable SMV Max Delay values, see Table 2.

TxTR ALARM activates when two or more consecutive SMV frames are lost or late. A single loss of frame is corrected with a zero-order hold scheme. The effect on protection is considered negligible in this case and it does not activate the TxTR WARNING or ALARM outputs.

Table 1. Topology-dependent SMV max delay setting
Number of hops	Internal application delay (μs)		Internal switch latency (μs)¹	Store and forward latency (μs)¹⁾	Queue latency (μs) ^{1) ,}²	Additional tolerance (μs) ³	Theoretical max delay (μs)		Recommended max delay setting (ms)
Number of hops	50 Hz	60 Hz	Internal switch latency (μs)¹	Store and forward latency (μs)¹⁾	Queue latency (μs) ^{1) ,}²	Additional tolerance (μs) ³	50 Hz	60 Hz	50 Hz	60 Hz
2	1555	1296	20	24	240	80	1919	1660	2	2
5	1555	1296	50	60	600	200	2465	2206	3	3
10	1555	1296	100	120	1200	250	3225	2966	5	4
15	1555	1296	150	180	1800	300	3985	3726	5	4
20	1555	1296	200	240	2400	350	4745	4486	5	5
25	1555	1296	250	300	3000	400	5505	5246	6	6
30	1555	1296	300	360	3600	450	6265	6006	7	6

Table 2. Protection delays and network margins
System frequency (Hz)	SMV max delay setting (ms)	Ethernet network margin (ms)⁴^,⁵	Additional protection delay (ms)
50	2	0,25	0,94
	3	1,5	2,19
	5	2,75	3,44
	6	4,00	4,69
	7	5,25	5,94
60	2	0,21	0,78
	3	1,25	1,83
	4	2,29	2,87
	5	3,33	3,91
	6	4,38	4,95

¹ Latencies are totals over the number of hops and they are calculated for 100 Mbit full-duplex Ethernet network.

² Queue latency calculated when the port has started to send a full-sized frame (1500 bytes) before the SMV frame and the switch has been configured to prioritize SMV

³ Additional tolerance in case of long wires or disturbance in network

⁴ Average values, variation is ±0.17 ms

⁵ based on REX615 6.0 as sender without network and switch latencies