Both the DelayTime and IMGap fields are in units of tenths of a microsecond, or LSB of 100 nanoseconds. Therefore a value of 1000 means 100.0 microseconds.
Additionally, both of these fields represent dead-bus time – the amount of time there is no signal on the 1553 bus. When things like intermessage gap time are expressed, this is the time measured from the mid-parity point of the last word of the previous message to the mid-sync point of the next command word. Half of the parity bit is 0.5us and half of the sync pattern is 1.5us, therefore you can convert dead-bus time by adding 2 microseconds – this gives you “conventional” time (from mid-parity to mid-sync).
To determine the correct value for the BCCB DelayTime from a desired conventional intermessage gap time:
DelayTime = (desiredGapTimeInMicroseconds – 2) * 10
For example, if you want an intermessage gap of 100 microseconds, first subtract 2 microseconds, then multiply by 10 – this gives you a DelayTime value of 980.
Note – there is a small amount of processing delay in the BC firmware that adds about 0.6 microseconds, you can correct for this by subtracting 2.6 microseconds rather than 2 microseconds – so here it is with this correction:
DelayTime = (desiredGapTimeInMicroseconds – 2.6) * 10
To apply this to our example, if you want an intermessage gap of 100 microseconds, first subtract 2.6 microseconds, then multiply by 10 – this gives you a DelayTime value of 974.
To convert the CDP IMGap value to a conventional intermessage gap time:
IntermessageGapTimeInMicroseconds = (IMGap / 10) + 2
For example, if you read a CDP that has an IMGap value of 0x03D4 (980 decimal), divide this by 10 and add 2 microseconds – this gives you an intermessage gap time of 100.0 microseconds.
